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Datasheet

Solenopsis invicta (red imported fire ant)

Summary

  • Last modified
  • 11 October 2017
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Natural Enemy
  • Preferred Scientific Name
  • Solenopsis invicta
  • Preferred Common Name
  • red imported fire ant
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Arthropoda
  •       Subphylum: Uniramia
  •         Class: Insecta
  • Summary of Invasiveness
  • S. invicta is an ant native to South America. It is an aggressive generalist forager that occurs in high densities and can thus dominate most potential food sources. It breeds and spreads rapidly and, if distur...

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Pictures

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PictureTitleCaptionCopyright
Solenopsis invicta (red imported fire ant); worker ants, showing size variability.
TitleWorker caste size variability
CaptionSolenopsis invicta (red imported fire ant); worker ants, showing size variability.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); worker ants, showing size variability.
Worker caste size variabilitySolenopsis invicta (red imported fire ant); worker ants, showing size variability.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); laboratory multiple queen or polygyne colony showing brood (larvae, pupae), worker ants and several queens (large wingless ants).
TitlePolygyne colony
CaptionSolenopsis invicta (red imported fire ant); laboratory multiple queen or polygyne colony showing brood (larvae, pupae), worker ants and several queens (large wingless ants).
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); laboratory multiple queen or polygyne colony showing brood (larvae, pupae), worker ants and several queens (large wingless ants).
Polygyne colonySolenopsis invicta (red imported fire ant); laboratory multiple queen or polygyne colony showing brood (larvae, pupae), worker ants and several queens (large wingless ants).©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); worker ants in laboratory colony.
TitleWorker caste
CaptionSolenopsis invicta (red imported fire ant); worker ants in laboratory colony.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); worker ants in laboratory colony.
Worker casteSolenopsis invicta (red imported fire ant); worker ants in laboratory colony.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); close-up of worker ant.
TitleWorker caste
CaptionSolenopsis invicta (red imported fire ant); close-up of worker ant.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); close-up of worker ant.
Worker casteSolenopsis invicta (red imported fire ant); close-up of worker ant.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); development of worker ant from eggs (a) larval stages or instars (b-d), pupa (e), and adult (f).
TitleLife stages
CaptionSolenopsis invicta (red imported fire ant); development of worker ant from eggs (a) larval stages or instars (b-d), pupa (e), and adult (f).
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); development of worker ant from eggs (a) larval stages or instars (b-d), pupa (e), and adult (f).
Life stagesSolenopsis invicta (red imported fire ant); development of worker ant from eggs (a) larval stages or instars (b-d), pupa (e), and adult (f).©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); larvae (row a) and winged sexual pupae (row b).
TitleLarvae and pupae
CaptionSolenopsis invicta (red imported fire ant); larvae (row a) and winged sexual pupae (row b).
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); larvae (row a) and winged sexual pupae (row b).
Larvae and pupaeSolenopsis invicta (red imported fire ant); larvae (row a) and winged sexual pupae (row b).©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); pasture with ant nests or mounds. Texas, USA.
TitleAnt mounds
CaptionSolenopsis invicta (red imported fire ant); pasture with ant nests or mounds. Texas, USA.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); pasture with ant nests or mounds. Texas, USA.
Ant moundsSolenopsis invicta (red imported fire ant); pasture with ant nests or mounds. Texas, USA.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); mound in turfgrass.
TitleAnt mound
CaptionSolenopsis invicta (red imported fire ant); mound in turfgrass.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); mound in turfgrass.
Ant moundSolenopsis invicta (red imported fire ant); mound in turfgrass.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); mound next to footpath.
TitleAnt mound
CaptionSolenopsis invicta (red imported fire ant); mound next to footpath.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); mound next to footpath.
Ant moundSolenopsis invicta (red imported fire ant); mound next to footpath.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); mound in pasture, showing cross-section. Note scale.
TitleCross section of ant mound
CaptionSolenopsis invicta (red imported fire ant); mound in pasture, showing cross-section. Note scale.
Copyright©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); mound in pasture, showing cross-section. Note scale.
Cross section of ant moundSolenopsis invicta (red imported fire ant); mound in pasture, showing cross-section. Note scale.©Bastiaan M. Drees/Texas A&M University
Solenopsis invicta (red imported fire ant); natural enemy. A phorid fly (Diptera; Phoridae) emerges from the head of a red imported fire ant that has been parasitized and killed.
TitleNatural enemy
CaptionSolenopsis invicta (red imported fire ant); natural enemy. A phorid fly (Diptera; Phoridae) emerges from the head of a red imported fire ant that has been parasitized and killed.
CopyrightPublic Domain - Released by the USDA-ARS/orignal photograph by Sanford Porter
Solenopsis invicta (red imported fire ant); natural enemy. A phorid fly (Diptera; Phoridae) emerges from the head of a red imported fire ant that has been parasitized and killed.
Natural enemySolenopsis invicta (red imported fire ant); natural enemy. A phorid fly (Diptera; Phoridae) emerges from the head of a red imported fire ant that has been parasitized and killed.Public Domain - Released by the USDA-ARS/orignal photograph by Sanford Porter

Identity

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Preferred Scientific Name

  • Solenopsis invicta Buren, 1972

Preferred Common Name

  • red imported fire ant

Other Scientific Names

  • Solenopsis saevissima var. wagneri Santschi
  • Solenopsis wagneri Santschi, 1916

International Common Names

  • English: fire ant, red imported; RIFA
  • Spanish: hormiga brava; Hormiga colorada
  • French: fourmi de feu

Local Common Names

  • Germany: rote importierte Feuerameise

EPPO code

  • SOLEIN (Solenopsis invicta)

Summary of Invasiveness

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S. invicta is an ant native to South America. It is an aggressive generalist forager that occurs in high densities and can thus dominate most potential food sources. It breeds and spreads rapidly and, if disturbed, can relocate quickly, ensuring the survival of the colony. S. invicta is highly invasive because of its high reproductive capacity, large colony size, ability to exploit human disturbances, wide food range and ability to sting. Its stinging ability allows it to subdue prey and even repel larger vertebrate competitors from resources (ISSG, 2014). S. invicta has become widespread in the southern USA and Caribbean after its introduction in the 1930s (Morrison et al., 2004). It has also been introduced to parts of Australia and New Zealand. S. invicta is one of the most notorious invasive ants and has been nominated for the 100 World’s Worst Invaders list compiled by the Invasive Species Specialist Group (ISSG, 2014).

 

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Arthropoda
  •             Subphylum: Uniramia
  •                 Class: Insecta
  •                     Order: Hymenoptera
  •                         Family: Formicidae
  •                             Genus: Solenopsis
  •                                 Species: Solenopsis invicta

Notes on Taxonomy and Nomenclature

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Taber (2000) provided a history of the taxonomic status of S. invicta. The fire ants comprise a group of 18 to 20 species native to the New World within the genus Solenopsis. This genus contains approximately 200 species including 'thief ants', which nest close to other ant nests and rob them of stored food and brood. Most thief ants are incapable of stinging humans.

In 1916, Santschi (in Shattuck et al., 1999) described and named Solenopsis saevissima wagneri from Argentina, and later from Paraguay and Bolivia. In 1952 Wilson examined the Solenopsis saevissima species complex and placed wagneri as junior synonym of S. saevissima saevissima.Buren (1972) recognized two distinct species in southern USA, Solenopsis richteriForel (1909) and an undescribed species for which he proposed the name S. invicta. In 1991, Trager examined the S. geminata species group, which included S. invicta, S. saevissima and related species, and concluded that S. wagneri was conspecific with S. invicta, and not S. saevissima as previously believed. In 1995, Bolton  recognized S. wagneri as an available name and treated S. invicta as a junior synonym of S. wagneri (Shattuck et al., 1999). Because the use of the name S. wagneri would cause considerable confusion and disrupt the non-taxonomic scientific literature concerning this species, Shattuck et al. (1999) proposed that the use of S. invicta should be maintained because of its extensive use in the scientific literature.

Description

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Worker Ants

Worker ants are wingless, dark reddish-brown with black abdomens, and range from 1.5 to 5 mm long. Workers in the genus Solenopsis are polymorphic, meaning they are physically differentiated into more than two different body-forms (Holway, 2002 in ISSG, 2014). A new colony’s first workers, called minims, are smaller than those of later generations. Minors are slightly larger, and medias larger still (Taber, 2000). The largest  is the major worker, which (in later generations) can reach lengths of up to 5 mm. The sting of S. invicta ants can be found at the tip of the abdomen under close examination.

Developmental Stages

The eggs are spherical and creamy-white and the larvae are legless, cream-coloured and grub-like with a distinct head capsule. The pupae resemble the worker ants and are initially creamy-white turning darker before the adult ants emerge (eclose). The eggs, larvae and pupae are referred to as a brood.

Sexuals or Winged Reproductives

The females are reddish-brown whereas the males are shiny and black with a smaller head. These ants stay in the colony until the conditions exist for their nuptial flight. The queen ants are mated female reproductives, are larger than the worker ants (9 mm) and remove their wings following a nuptial flight.

Distribution

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S. invicta is native to central South America. In the USA, S. invicta was first introduced from Brazil into either Mobile, Alabama, or Pensacola, Florida, between 1933 and 1945. S. invicta currently occupies 128 million hectares in nine south-eastern states in the USA (Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Texas), with limited infestations in Arizona, Oklahoma, Tennessee, New Mexico and California.

Infestations also occur in a number of island countries in the Caribbean, including Puerto Rico, the Bahamas, the British and USA Virgin Islands, Antigua and Trinidad (Davis et al., 2001).

S. invicta has been reported in Cayman Islands, Malaysia, Singapore, Trinidad and Tobago, and the Turks and Caicos Islands (ISSG 2014). It has also been detected in New Zealand and Australia (Brisbane) (Korzukhin et al., 2001), Hong Kong (Hong Kong Government Information Centre, 2005), Taiwan (Chen et al., 2005) and China (IPPC, 2014). New Zealand incursions have been eradicated and mainland China populations are either eradicated or currently undergoing eradication (ISSG, 2014).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

ChinaRestricted distributionIntroduced2004 Invasive IPPC, 2008; CABI/EPPO, 2010; EPPO, 2014; ISSG, 2014
-FujianPresentIntroduced2005 Invasive IPPC, 2008; CABI/EPPO, 2010; EPPO, 2014; ISSG, 2014
-GuangdongPresentIntroduced Invasive IPPC, 2008; CABI/EPPO, 2010; EPPO, 2014; ISSG, 2014
-GuangxiPresentIntroduced2005 Invasive IPPC, 2008; CABI/EPPO, 2010; EPPO, 2014; ISSG, 2014
-Hong KongRestricted distributionIntroducedHong Kong Government Information Centre, 2005; CABI/EPPO, 2010; EPPO, 2014
-HunanPresentIPPC, 2008; CABI/EPPO, 2010; EPPO, 2014
-JiangsuPresentYang and Zhong, 2006
MalaysiaPresentIntroduced Invasive ISSG, 2014
SingaporePresentIntroducedISSG, 2014
TaiwanPresentIntroduced Invasive Chen et al., 2005; CABI/EPPO, 2010; EPPO, 2014; ISSG, 2014

North America

MexicoPresentSánchez-Peña et al., 2005; CABI/EPPO, 2010; Salas-Araiza et al., 2011; EPPO, 2014
USARestricted distributionCABI/EPPO, 2010; EPPO, 2014
-AlabamaWidespreadIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-ArizonaPresentCABI/EPPO, 2010; EPPO, 2014
-ArkansasRestricted distributionIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-CaliforniaPresentIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-ColoradoPresentCABI/EPPO, 2010; EPPO, 2014
-FloridaWidespreadIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-GeorgiaWidespreadIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-IllinoisPresentCABI/EPPO, 2010; EPPO, 2014
-LouisianaWidespreadIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-MarylandPresentCABI/EPPO, 2010; EPPO, 2014
-MississippiWidespreadIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-New MexicoRestricted distributionIntroducedAnon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-North CarolinaRestricted distributionIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-OklahomaRestricted distributionIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-South CarolinaWidespreadIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-TennesseeRestricted distributionIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-TexasRestricted distributionIntroduced Invasive Anon., 1999a; CABI/EPPO, 2010; EPPO, 2014
-VirginiaPresentCABI/EPPO, 2010; EPPO, 2014

Central America and Caribbean

AnguillaPresentWetterer and Davis, 2010
Antigua and BarbudaPresentIntroduced Invasive Davis et al., 2001; CABI/EPPO, 2010; Wetterer and Davis, 2010; EPPO, 2014
BahamasPresentIntroduced Invasive Davis et al., 2001; CABI/EPPO, 2010; EPPO, 2014
British Virgin IslandsPresentIntroduced Invasive Davis et al., 2001; CABI/EPPO, 2010; EPPO, 2014
Cayman IslandsPresentIntroducedISSG, 2009
Costa RicaPresentCABI/EPPO, 2010; EPPO, 2014
MontserratPresentWetterer and Davis, 2010
PanamaPresentCABI/EPPO, 2010; EPPO, 2014
Puerto RicoWidespreadIntroduced Invasive Davis et al., 2001; CABI/EPPO, 2010; EPPO, 2014
Saint Kitts and NevisPresentWetterer and Davis, 2010
Sint MaartenPresentWetterer and Davis, 2010
Trinidad and TobagoPresentIntroduced Invasive Davis et al., 2001; CABI/EPPO, 2010; EPPO, 2014
Turks and Caicos IslandsPresentIntroducedISSG, 2009; CABI/EPPO, 2010; EPPO, 2014
United States Virgin IslandsPresentIntroduced Invasive Davis et al., 2001; CABI/EPPO, 2010; EPPO, 2014

South America

ArgentinaRestricted distributionNative Not invasive Vinson and Sorenson, 1986; CABI/EPPO, 2010; EPPO, 2014
BoliviaPresentNative Not invasive Pitts, 2002
BrazilRestricted distributionNative Not invasive Vinson and Sorenson, 1986; CABI/EPPO, 2010; EPPO, 2014
-GoiasPresentPesquero and Penteado-Dias, 2004
-Mato GrossoPresentCABI/EPPO, 2010; EPPO, 2014
-Mato Grosso do SulPresentCABI/EPPO, 2010; EPPO, 2014
-Minas GeraisPresentConceição et al., 2006
-Rio Grande do SulPresentDiehl et al., 2005; CABI/EPPO, 2010; EPPO, 2014
-RondoniaPresentCABI/EPPO, 2010; EPPO, 2014
-Sao PauloPresentCABI/EPPO, 2010; EPPO, 2014
ParaguayRestricted distributionNative Not invasive Vinson and Sorenson, 1986; Pitts, 2002; CABI/EPPO, 2010; EPPO, 2014
PeruPresentNative Not invasive Pitts, 2002
UruguayPresentNative Not invasive Pitts, 2002

Oceania

AustraliaPresentIntroducedISSG, 2009; CABI/EPPO, 2010; EPPO, 2014
-QueenslandRestricted distributionIntroduced Invasive Drees and Gold, 2003; CABI/EPPO, 2010; IPPC, 2010; EPPO, 2014
New ZealandEradicatedIntroduced Invasive Drees and Gold, 2003; CABI/EPPO, 2010; EPPO, 2014

History of Introduction and Spread

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S. invicta is believed to have been introduced from northern Argentina or southern Brazil to the US port of Mobile, Alabama, between 1930 and 1945 (Taber, 2000). S. invicta then spread westward at about 198 km per year.

In Australia, S. invicta were first discovered at the Port of Brisbane and at Richlands in February 2001. Genetic analysis revealed there have been four different incursions of fire ants into Australia: the first two were discovered in 2001, one in the southwestern suburbs of Brisbane and the other at the Port of Brisbane. The third and fourth incursions were found in Yarwun, Central Queensland in 2006 and 2013. Both the Port of Brisbane and the 2006 Yarwun incursions have been successfully eradicated. Spread from the initial Brisbane infestation led to a number of small outlying infestations around the greater Brisbane area, including Ipswich Logan and Redlands. Isolated infestations have also been found in the Scenic Rim, Gold coast and Lockyer Valley areas of southeastern Queensland (Queensland Government, 2003).

Introductions

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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
Australia USA 2001 Yes Davis et al., 2001 Accidental introduction. Localised.
China USA 2005 Yes Accidental introduction. Localised/controlled.
USA Brazil 1933-1945 Yes Davis et al., 2001 Accidental introduction

Risk of Introduction

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The spread of S. invicta has been aided by humans via the shipment of infested articles such as nursery potting media, sod, bales of hay or soil. Objects contaminated with soil pose a high risk as soil is the natural nesting material for S. invicta.

Used electrical equipment that has been used outdoors or has been in contact with the ground has frequently been cited as containing S. invicta infestations. Worker ants are attracted to heat or pheromones released when other workers are electrocuted. The housing of electrical equipment also provides warmth and shelter for colonies, thus attracting infestation. Examples of infested equipment include air conditioning units, power company transformers, traffic signal control cabinets, electrical pumps and car electrical systems.

After mating, it appears queens are attracted to reflective surfaces such as trains and trucks, where hundreds of mated queens have been collected (Vinson and Greenberg, 1986). The association of S. invicta colonies with beehives has also been reported (Harris et al., 2005).

S. invicta has been introduced into parts of Australia, North America and onto some vulnerable island ecosystems including islands in the Caribbean (Puerto Rico and the Virgin Islands) and the Pacific (New Zealand) (McGlynn, 1999; Korzukhin et al., 2001; ISSG, 2010) and has the potential to colonize numerous other regions. Based on precipitation and temperature data and temperature-based predictions, Morrison et al. (2004) determined the potential global range expansion of S. invicta and concluded that large areas in Mexico, northern South America and Central America and many Caribbean islands are at great risk of invasion by S. invicta. Much of the region immediately surrounding the Mediterranean Sea, as well as some areas near the Black and Caspian Sea, are also at risk (Morrison et al., 2004).

Habitat

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S. invicta is a social insect that forms colonies in soil or other suitable media. However, under laboratory conditions, they can be cultured with no media (Banks et al., 1981). The colonies can occasionally occur in wall voids and other areas such as utility housings.

S. invicta can live in a wide range of habitats and is able to dominate altered habitats. It also does well in the extreme conditions of the South American rain forest (Trager, 1991). In disturbed and developed forested areas, S. invicta nests are abundant along roadsides and trails near buildings. Newly-mated S. invicta queens often move to pastures (Taber, 2000).

In infested areas, colonies are common in lawns, gardens, school yards, parks, roadsides and golf courses. Nests generally occur in sunny open areas and are especially common in disturbed and irrigated soil.

S. invicta mounds are 10-60 cm tall and have no visible surface entrance. Mounds are accessed through subterranean tunnels that spoke out from the central mound (Texas Invasive Species Institute, 2014). S. invicta build their mounds in a particular orientation such that the long sides of the mound are exposed to the sun early in the morning and before sunset (Bluthgen and Feldhaar, 2010).

Habitat List

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CategoryHabitatPresenceStatus
Freshwater
Irrigation channels Present, no further details Harmful (pest or invasive)
Lakes Present, no further details Harmful (pest or invasive)
Ponds Present, no further details Harmful (pest or invasive)
Reservoirs Present, no further details Harmful (pest or invasive)
Rivers / streams Present, no further details Harmful (pest or invasive)
Terrestrial-managed
Buildings Present, no further details Harmful (pest or invasive)
Cultivated / agricultural land Present, no further details Harmful (pest or invasive)
Cultivated / agricultural land Present, no further details Natural
Disturbed areas Present, no further details Harmful (pest or invasive)
Disturbed areas Present, no further details Productive/non-natural
Industrial / intensive livestock production systems Secondary/tolerated habitat Harmful (pest or invasive)
Industrial / intensive livestock production systems Secondary/tolerated habitat Productive/non-natural
Managed forests, plantations and orchards Present, no further details Harmful (pest or invasive)
Managed forests, plantations and orchards Present, no further details Productive/non-natural
Managed grasslands (grazing systems) Present, no further details Harmful (pest or invasive)
Managed grasslands (grazing systems) Present, no further details Productive/non-natural
Protected agriculture (e.g. glasshouse production) Secondary/tolerated habitat Harmful (pest or invasive)
Protected agriculture (e.g. glasshouse production) Secondary/tolerated habitat Natural
Rail / roadsides Present, no further details Harmful (pest or invasive)
Urban / peri-urban areas Present, no further details Harmful (pest or invasive)
Terrestrial-natural/semi-natural
Arid regions Secondary/tolerated habitat Harmful (pest or invasive)
Natural forests Present, no further details Harmful (pest or invasive)
Natural grasslands Present, no further details Harmful (pest or invasive)
Riverbanks Present, no further details Harmful (pest or invasive)
Scrub / shrublands Present, no further details Harmful (pest or invasive)
Wetlands Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

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S. invicta is omnivorous and foraging fire ants may be found in or on plants because they are preying on phytophagous arthropods associated with those crops. In addition, fire ant workers feed on certain plants and are attracted to nectaries on plants such as cotton and passion vine. Plant-feeding appears to be aggravated by dry or drought conditions. On other plants, the ants seem attracted to oil-containing plant parts such as the embryo portion of corn and sorghum seeds.

Livestock are affected, particularly those that cannot quickly move away from the threat (such as very young, old or confined animals). The ants move to moist areas of the body (eyes, genitals), the yolk of hatching birds and wounds, and begin stinging when disturbed. The stings result in injury such as blindness, swelling or death.

Foraging workers on plants can become a hazard to field workers and tall, hardened mounds harbouring ant colonies in certain crops such as hay pastures or soybeans can interfere with mechanized cutting and harvesting operations.

Host Animals

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Animal nameContextLife stageSystem
Bos indicus (zebu)
Bos taurus (cattle)

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Abelmoschus esculentus (okra)MalvaceaeOther
Arachis hypogaea (groundnut)FabaceaeOther
Brassica oleracea var. capitata (cabbage)BrassicaceaeHabitat/association
Carya illinoinensis (pecan)JuglandaceaeOther
Citrullus lanatus (watermelon)CucurbitaceaeOther
CitrusRutaceaeOther
Cucumis sativus (cucumber)CucurbitaceaeOther
Cynodon dactylon (Bermuda grass)PoaceaeHabitat/association
Fragaria ananassa (strawberry)RosaceaeOther
Glycine max (soyabean)FabaceaeOther
Helianthus annuus (sunflower)AsteraceaeOther
Ipomoea batatas (sweet potato)ConvolvulaceaeOther
Medicago falcata (yellow alfalfa)FabaceaeHabitat/association
Pinus (pines)PinaceaeOther
Solanum (nightshade)SolanaceaeOther
Solanum melongena (aubergine)SolanaceaeOther
Sorghum bicolor (sorghum)PoaceaeOther
Stenotaphrum secundatum (buffalo grass)PoaceaeHabitat/association
Trifolium (clovers)FabaceaeHabitat/association
Zea mays (maize)PoaceaeOther

Growth Stages

Top of page Flowering stage, Fruiting stage, Post-harvest, Pre-emergence, Seedling stage, Vegetative growing stage

Symptoms

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S. invicta tunnels through roots and tubers, feeds on plants, fruit and seeds and can girdle young citrus trees (Stewart and Vinson, 1991). As well as causing direct damage to plants, S. invicta also aggravates populations of other insect plant pests such as Homoptera (e.g. aphids, scale insects and mealybugs). The ants consume the sugary honeydew produced by these pests and protects them from natural enemies.

Worker ants bite (with mandibles) and sting (with stingers) aggressively and repeatedly. The sting feels like being burned. A day or so later, S. invicta venom forms a white fluid-filled pustule or blister at the red sting site, a symptom characteristic only of fire ants.

List of Symptoms/Signs

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Fruit

  • external feeding
  • internal feeding

Leaves

  • wilting

Roots

  • external feeding
  • internal feeding

Seeds

  • external feeding
  • internal feeding

Vegetative organs

  • external feeding
  • internal feeding

Whole plant

  • cut at stem base
  • external feeding
  • internal feeding
  • plant dead; dieback

General Signs

  • Swelling skin or subcutaneous, mass, lump, nodule

Ophthalmology Signs

  • Chemosis, conjunctival, scleral edema, swelling
  • Conjunctival, scleral, injection, abnormal vasculature
  • Conjunctival, scleral, laceration, cut, tear, injury
  • Conjunctival, scleral, papules
  • Conjunctival, scleral, redness
  • Corneal edema, opacity
  • Corneal injury, cut, tear
  • Corneal neovascularization, pannus
  • Corneal ulcer, erosion
  • Lacrimation, tearing, serous ocular discharge, watery eyes

Pain

  • Discomfort Signs/Skin pain

Skin

  • Integumentary Signs/Parasite visible, skin, hair, feathers
  • Integumentary Signs/Skin edema
  • Integumentary Signs/Skin erythema, inflammation, redness
  • Integumentary Signs/Skin papules
  • Integumentary Signs/Skin plaque
  • Integumentary Signs/Skin pustules
  • Integumentary Signs/Skin wheal, welt

Biology and Ecology

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A general overview of the biology and ecology of S. invicta can be found in Vinson (1997). Not all primary sources of information can be cited due to space limitations and the high volume of available references.

S. invicta is a social insect. The colonies usually produce hills, nests or mounds where they reside. The colonies are most common in open, sunny areas, but occasionally occur indoors and within structures such as utility housings or tree trunks. In clay-type soils, mounds may reach 30 to 40 cm high and 30 to 50 cm in diameter. The mounds have no entrance holes or central entrance hole on the surface. Inside, the mounds have interconnecting galleries that may extend 30 to 40 cm deep, although some tunnels can penetrate to the water table. Under extremely hot, dry conditions, colonies may live underground and not develop surface nests or mounds.

A fully developed colony can contain over 200,000 to 400,000 ants. In response to solar radiation and ambient conditions, the fire ants move within the mound seeking an optimum temperature for the development of the brood (eggs, larvae and pupae), a process called thermoregulation. Foraging worker ants enter and exit through tunnels radiating up to 5 to 10 m away from the mound. The disturbance of mounds results in a rapid defensive response by the worker ants, which quickly run up the vertical surfaces to bite and sting any objects that are encountered.

Genetics

The winged reproductive male ants develop from eggs that are not fertilized. The fertilized eggs can develop into either sterile female worker ants or winged reproductive females, depending on the nutrition provided to the larval stages and chemical signals (juvenile hormone level and pheromones) within the colony.

Two forms of S. invicta are recognized: monogyne or single-queen colonies and polygyne or multiple-queen colonies. Polygyne colonies contain two or more inseminated, reproductively-active queen ants. The worker ants in single-queen colonies respond defensively to neighbouring colonies to maintain territories, whereas multiple-queen colony worker ants do not display territorial behaviour. Consequently, they can produce three to ten times as many ant mounds in a given area of land infested with the polygyne form, which results in 500 mounds and 50 million ants per hectare. The areas infested with the single queen form normally have 50 to 75 mounds per hectare. These forms can be separated by multiplex PCR of Gp-9 alleles (Valles and Porter, 2003). S. invicta has hybridized with S. richteri, the black imported fire ant, in parts of southeastern USA (Alabama, Mississippi, Georgia and Tennessee) and produces sexually active offspring (Vinson, 1997).

Physiology and Phenology

S. invicta communicates through vision (sight), vibration (sound), touch and chemicals (pheromones), including a queen pheromone that attracts workers and may suppress dealation and reproduction, and a trail pheromone produced by the Dufour's gland that is associated with the worker ant stinger. Upon locating food resources, ants develop a trail using a pheromone that directs other worker ants to the site.

Activity Patterns

S. invicta forages when soil surface temperatures range from 12°C to 51°C. Maximum worker ants foraging occurs between 22°C (72°F) to 36°C (96°F) (Porter and Tschinkel, 1987).

Nutrition

Fire ants are omnivores that consume sugars (carbohydrates), certain amino acids, ions in solution and some oils, containing polyunsaturated fatty acids. Although they primarily consume other arthropods and honeydew produced by some types of sucking insects (Homoptera), they will also consume seeds and other plant parts like developing or ripening fruit, and dead plant and animal tissues.

While sugar is utilised primarily by the workers, amino acids are preferentially consumed by the queen and larvae. Plant oil is consumed equally among workers and larvae (Taber, 2000).

The worker ants’ mouthparts are used for biting and sipping liquids. The worker ants only consume liquids and very small particles, storing food in their crops and postpharyngeal gland (oils only) until they feed it to other worker ants and ultimately to the larvae and queen(s). This process is called trophallaxis. The young larval stages (instars) are only fed regurgitated liquid food. However, the final (fourth) larval stage can digest solid food particles. The worker ants place bits of solid food in a small depression (called a food basket) just in front of and beneath the larva's mouth (an area called the presternum). They externally digest the proteins (extra-orally) by secreting enzymes, and by chewing and swallowing smaller particles.

Reproductive Biology

Claustral colony foundation

Newly mated females that survive nuptial flights and reach suitable nesting habitats (estimated to be about 1% due to predation and other mortality factors) remove their wings and burrow into the ground.  They are sealed in a chamber and they begin to lay eggs Newly mated polygyne queens lay 20-30 eggs per day and monogyne queens lay around 200 eggs per day. Mature monogyne queen ant can produce 800-1000 eggs per day (Taber, 2000). Larvae hatch from eggs 6 to 10 days after laying.

The larvae are fed from energy produced from the breakdown of flight muscles, infertile (trophic) eggs, young larvae and oil reserves. The first worker ants that emerge are uniformly small and are called nanitics or minims.

Queens from polygyne colonies are not as successful at founding new colonies as monogyne queens. Polygyne queens are smaller, produce fewer workers, and are less likely to be inseminated due to a higher propotion of sterile males in polygyne colonies. Therefore, most new colonies in polygyne dominated areas are formed by budding.

Life Cycle

After hatching, larvae develop through four stages (instars) over 12 to 15 days before pupating for a period of 9 to 16 days. Development requires 22 to 37 days, depending on temperature. Most worker ants live for 60 to 150 days with the larger ants living longer, but during cooler weather the workers can survive for 8 months or more. The newly established colonies develop winged reproductive ants after about 6 to 8 months and can produce 4000 to 6000 alates (sexuals) per year. Queen ants live and lay eggs for for about 7 years.

The worker ants build up the colony, care for the queen and brood, defend the colony and forage for food. Their function within the colony is determined by the size and needs of the colony, and by the age of the worker ants. The younger workers serve as nurse ants that tend and move the queen and brood. The older workers serve as reserves to defend the colony, and construct and maintain the mound. The oldest worker ants become foragers. The propotion of larger workers increases continuously with increasing colony age, facilitating an increase of labour efficiency (Tschinkel, 1988).

Environmental Requirements

Climatic variables, especially temperature and rainfall, are known to play a substantial role in determining the distribution of S. invicta (Ward, 2009). In broad terms, continental areas receiving more than 510 mm of precipitation per year could support S. invicta, whereas continental areas receiving less than 510 mm of precipitation per year would probably only support S. invicta near sources of permanent water (such as lakes, rivers, springs) or regularly irrigated areas (such as fields and lawns) (Vinson, 1997; Morrison et al., 2004).

Temperature is a key variable in determining foraging in S. invicta. Porter and Tschinkel (1987) found soil temperature at 2 cm depth was the best predictor of foraging; in contrast, Vogt et al. (2003) found soil temperature at 15 cm depth was the best predictor. However, Ward (2009) observed that different measures of temperature were highly correlated and that temperature in the general sense is important. Foraging activity of S. invicta is also dependant on season, with a drop in activity over cooler months, and on habitat, with less activity in shadier (cooler) habitats (Markin et al., 1974; Porter andTschinkel 1987; Vogt et al., 2003).

Colony growth is also sensitive to temperature, with a reported lower threshold temperature for colony growth of 24oC (Porter, 1988, in Sutherst and Maywald, 2005).

Low temperatures appear to be key limiting factor for  northward expansion of the range of S. invicta in the USA. Ecological models predict that the species will fail to become established in regions where the soil stays at near-freezing temperatures for more than a couple of weeks (Smithsonian Marine Station, 2007). Predictions suggest that the ant is unable to survive where the minimum yearly temperatures are less than -12.3°C (10°F) to -17.8°C.

Associations

Solenopsis fire ants are predators of numerous arthropods, but similar to some other ant species, they are attracted to sap sucking insects such as aphids and scale insects (e.g. Homoptera) that excrete a sugary substance called honeydew. Ants prey on the predators of these sucking insects and eliminate those that are parasitized, diseased or otherwise unhealthy. The control of these sucking insect pests is often improved when the ants are eliminated.

Climate

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ClimateStatusDescriptionRemark
A - Tropical/Megathermal climate Preferred Average temp. of coolest month > 18°C, > 1500mm precipitation annually
Af - Tropical rainforest climate Preferred > 60mm precipitation per month
Am - Tropical monsoon climate Preferred Tropical monsoon climate ( < 60mm precipitation driest month but > (100 - [total annual precipitation(mm}/25]))
As - Tropical savanna climate with dry summer Tolerated < 60mm precipitation driest month (in summer) and < (100 - [total annual precipitation{mm}/25])
Aw - Tropical wet and dry savanna climate Tolerated < 60mm precipitation driest month (in winter) and < (100 - [total annual precipitation{mm}/25])
B - Dry (arid and semi-arid) Tolerated < 860mm precipitation annually
C - Temperate/Mesothermal climate Tolerated Average temp. of coldest month > 0°C and < 18°C, mean warmest month > 10°C
Cf - Warm temperate climate, wet all year Tolerated Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Argiope aurantia Predator Taber, 2000
Beauveria bassiana Pathogen Drees and Knutson, 2002 Florida
Beauveria brongniartii Pathogen
Burenella dimorpha Pathogen
Caenocholax fenyesi Parasite
Conidiobolus macrosporus Pathogen
Cyclosa turbinata Predator
Dorymyrmex insanus Predator
Kneallhazia solenopsae Pathogen Adults/Larvae/Pupae
Latrodectus mactans Predator Taber, 2000
Leptotyphlops dulcis Predator
Metarhizium anisopliae Pathogen
Oxyopes salticus Predator Taber, 2000
Paratrechina melanderi Predator
Phidippus audax Predator USA; Texas Croton capitatus
Pseudacteon curvatus Parasite Adults
Pseudacteon obtusus Parasite
Pseudacteon tricuspis Parasite Adults
Pyemotes tritici Parasite Florida; Georgia; USA; Florida pasture plants
Solenopsis molesta Predator
Steatoda triangulosa Predator
Steinernema carpocapsae Parasite Taber, 2000 Texas
Vairimorpha invictae Pathogen Drees and Knutson, 2002

Notes on Natural Enemies

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Numerous natural enemies of S. invicta have been reported and investigated (Williams et al., 2003). However, few of these show promise for use in classical or importation biological control programmes (see 'Prevention and Control').

Means of Movement and Dispersal

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Natural Dispersal

Spread occurs through mating flights, ground migration and floating colonies on water during floods. Mating flights occur between 10.00 h and 14.00 h during the first sunny days after a period of rainy weather when temperatures are suitable (24°C), at any time of the year but mainly in spring and autumn.

Under suitable conditions (warmth, high humidity, low wind), most often in the spring, mature monogyne colonies of S. invicta release reproductive males and, a short time later, the reproductive females. The winged male and female reproductive ants (also called reproductives, sexuales or alates) couple in the air (usually 50-100 m high) (Markin et al., 1971). The males die after mating, and the females land within a mile or two (unless carried by the wind) and are attracted to shiny surfaces (such as water surfaces and shiny truck beds) when seeking a suitable nesting site. The dispersal distances are typically on the order of hundreds of meters or, occasionally, a few kilometres. Nuptial flight can also occur with the polygyne form, typically occuring closer to the ground (2-3 m elevation). Newly mated polygyne queens do not seem to disperse as far as monogyne queens do (Markin et al., 1971).

Agricultural Practices

The long-distance movement of articles such as sod, bales of hay and nursery containers have resulted in the spread of S. invicta colonies. Generally, these colonies are nesting in soil, potting media, straw or other suitable nesting material associated with these articles. Quarantine treatments, developed to prevent the spread of colonies in these articles, are detailed in a publication by the USDA Animal and Plant Heath Inspection Service (Anon., 1999a). More recently, the transport of ant-infested bee hive support pallets from the south-eastern USA is thought to have resulted in the infestation of California's Central Valley almond orchards (Anon., 1999b; Weeks and Drees, 2002).

Movement in Trade

The long-distance transport of S. invicta can result when mated queen ants or colonies are shipped from one location to another on virtually any article of commerce, particularly those contaminated with soil that often clings to the bottoms of support pallets. S. invicta is thought to have been inadvertently transported to the port of Mobile, Alabama (USA), around the 1930s, in the ballast of cargo ships from the Paraguay River drainage in South America (Vinson, 1997).

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Animal productionEquipment - accidental Yes ISSG, 2014
AquacultureAssociated trade industries - accidental Yes ISSG, 2014
Botanical gardens and zoosPot plants or planting material - accidental Yes
Crop productionEquipment or associated planting material-accidental Yes ISSG, 2014
DisturbanceWorker ant could move queen ant/s to another location with brood - accidental Yes Morrison et al., 2004
Flooding and other natural disastersRaft of ants could float, accidental Yes Taber, 2000
Garden waste disposalA whole nest could move - accidental Yes Taber, 2000
HitchhikerElectrical equipment, vehicles - accidental Yes ISSG, 2014
HorticultureEquipment, soil and planting material Yes ISSG, 2014
Landscape improvementEquipment - accidental Yes ISSG, 2014
Nursery tradeEquipment, planting material - accidental Yes ISSG, 2014

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Containers and packaging - woodQueen, workers, eggs, larvae and pupae Yes ISSG, 2014
Debris and waste associated with human activitiesQueen, workers, eggs, larvae and pupae Yes Taber, 2000
Floating vegetation and debrisQueen, workers Yes Taber, 2000
Land vehiclesAll types Yes
Machinery and equipmentQueen, workers and brood if soil attached Yes Taber, 2000
Soil, sand and gravelWhole nest Yes Taber, 2000

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Growing medium accompanying plants adults; eggs; larvae; pupae Yes Yes

Wood Packaging

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Wood Packaging liable to carry the pest in trade/transportTimber typeUsed as packing
Non-wood Containers Yes
Solid wood packing material without bark Pallets Yes

Impact Summary

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CategoryImpact
Animal/plant collections Negative
Animal/plant products Negative
Biodiversity (generally) Negative
Crop production Negative
Cultural/amenity Negative
Economic/livelihood Negative
Environment (generally) Negative
Fisheries / aquaculture Negative
Forestry production None
Human health Negative
Livestock production Negative
Native fauna Negative
Native flora None
Rare/protected species Negative
Tourism Negative
Trade/international relations Negative
Transport/travel Negative

Impact

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Although the exact economic costs of fire ant damage and control are unknown, estimates for southeastern USA have been more than half a billion to several billion dollars per year (Williams et al., 2001). The costs associated with S. invicta accross all the USA have been estimated at US $1 billion per year. In 1998, the average household cost for imported fire ant problems per Texas household in urban areas was US $150.79, with US $9.40 spent on medical care. The total annual metroplex (Austin, Dallas, Ft. Worth, Houston and San Antonio) expenditures for medical care costs was 9% or US $47.1 million of the US $526 million total expenditure cost due to S. invicta (Lard et al., 2002).

The Australian Bureau of Agriculture Resources Economics has estimated that losses procured in rural industries to have amounted to more than AU$ 6.7 billion over 30 years (ISSG, 2014). In the case of introduction and establishment of S. invicta in Hawaii, researchers have estimated the impact on various economic sectors would be around US $211 million per year (Gutrich et al., 2007).

Equipment Damage

S .invicta may infest electrical equipment (such as computers, swimming pool pumps, cars or washing machines), thereby becoming a nuisance or even a danger to people. The ants or entire colonies move into buildings or vehicles seeking favourable nesting sites, particularly during flooding and very hot, dry conditions. Fire ant foraging and nesting activities can result in the failure of many types of mechanical (such as hay harvesting machinery and sprinkler systems) and electrical equipment (including air conditioner units and traffic box switching mechanisms).

Agricultural and Ecological Impact

In addition to being considered medically important pests of people, pets, livestock and wildlife, imported fire ants can also damage crops such as corn, sorghum, okra, potatoes and sunflowers by feeding on the seeds, seedlings and developing fruit. Teal et al. (1999) documented the impact to cattle production systems. The predatory activities of fire ants suppress populations of ticks, chiggers, caterpillars and other insects. This predatory activity reduces the wildlife in some areas.

Seed is generally unaffected by S. invicta in storage. However, once planted, the seeds of many plant species become vulnerable to S. invicta predation and the foraging worker ants consume all or parts of the seed and/or move the seed to new locations. Drees et al. (1991) investigated the factors affecting sorghum and maize seed predation by foraging S. invicta

These stinging ants also affect many animals, particularly those that can not quickly move away from the threat (e.g. very young, old or confined animals). The ants move to moist areas of the body (eyes, genitals), the yolk of hatching birds and wounds, and begin stinging when disturbed. The stings result in injury such as blindness, swelling or death. Indirectly, the animals avoid infested food, water and nesting areas. The ants can reduce the food sources for some insect-eating (insectivorous) animals such as some birds and lizards, and they may compete with seed-feeding (gramnivorous) animals for food or alter the distribution and composition of plant communities.

In addition to direct damage to plants, S. invicta also aggravates populations of other insect plant pests such as Homoptera (e.g. aphids, scale insects and mealybugs). The ants consume the sugary honeydew produced by these pests and protect them from natural enemies. However, the ants primarily prey on arthropods such as some species of ticks, many caterpillars and others often considered pests. This behaviour can provide benefits to producers of cotton, sugarcane and other commodities.

Environmental Impact

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In addition to the impact of these ants due to their nest-building and foraging activities, attempts to control them generally involve the use of insecticides. Non-target effects such as elimination or reduction of native ant species and the environmental impact of pesticides, including surface run-off water contamination, are more likely in these situations.

S. invicta reduces bio-diversity in newly-infested areas, particularly of other ant species. However, the impact on the diversity and density of ants and other arthropods remains controversial (Morrison and Porter, 2003). In Florida, USA, S. invicta introduction caused richness decline of five ant species (King and Tschinkel, 2008).

S. invicta interrupts and reduces the dispersal of ant-dispersed plants by competing with native ant dispersers. It may also reduce numbers of insects that benefit the such plants, such as plant mutualists that protect the plant. In addition, the preference of S. invicta for a protein-rich diet has a negative effect on plant mutualist insects (Ness and Bronstein, 2004, in ISSG, 2014).

In the USA, S. invicta has been found to negatively impact at least fourteen bird species, thirteen reptile species, one fish species and two small mammal species (through predation, competition and/or stinging) (Holway et al., 2002, in ISSG, 2014). In Texas, S. invicta reduces nesting areas of the least tern (Sternula antillarum) and can kill its chicks (Campomizzi, 2008). In southern USA, S. invicta attacks northern bobwhite (Colinus virginianus) chicks and nests and predates on intact and hatching eggs (Seymour, 2007; Staller et al., 2005). In Florida, USA, S. invicta predates on turtle eggs and hatchlings (Parris et al., 2002)

Social Impact

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Soil-dwelling, S. invicta colonies are most commonly found in urban, agricultural and non-agricultural areas and affect people, livestock, wildlife and the flora and fauna of infested lands. Where they are abundant, people and animals have modified their behaviour to avoid contact with these ants and their nests. The loss of enjoyment of the outdoors is a major social impact that cannot be assessed in economic terms. 

Medical Importance

Over 40 million people live in areas infested by S. invicta in the south-eastern USA. An estimated 14 million people are stung annually. According to The Scripps Howard Texas Poll (Drees, 2000), 79% of Texans have been stung by fire ants in the year of the survey, while 20% of Texans report not ever having been stung. West Texans were least likely to have been stung by fire ants: 61%, compared with 90% in central Texas, USA, 89% in the east, 86% in the gulf, 78% in the south and 72% in north Texas. A survey of 1286 health practitioners in South Carolina, USA, estimated that over 33,000 of people (94 per 10,000 or 0.94%) seek medical attention for imported fire ant stings and of these, 660 people (1.9 per 10,000 or 0.02%) are treated for anaphylaxis (Caldwell et al., 1999). Anaphylaxis occurs in 0.6 to 6% of persons who are stung and it has been reported these reactions caused more than 80 deaths (deShazo et al., 1999).

The venom of imported fire ants is produced in a gland connected to the stinger and contains two major components: alkaloids and proteins. The oily aliphatic substituted alkaloids (i.e. the piperidine alkaloid, Solenopsin A) are toxic to cells and cause a pustule to form by killing the cells at the site of the injection. These dead cells attract the body's defensive white blood cells, which accumulate at the venom site and form a pustule. If the skin is broken (by scratching), bacteria may enter and an infection is caused. The venom also contains a protein component (less than 1%), which has little or no effect on most people. However, some people are sensitive to these proteins and a sting can lead to a major allergic reaction called anaphylactic shock (Baer et al., 1979; Vinson and Sorenson 1986; Hoffman et al., 1988).

Risk and Impact Factors

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Impact mechanisms

  • Causes allergic responses
  • Hybridization
  • Interaction with other invasive species
  • Parasitism (incl. parasitoid)
  • Predation
  • Rapid growth
  • Trampling

Impact outcomes

  • Damaged ecosystem services
  • Damages animal/plant products
  • Ecosystem change/ habitat alteration
  • Host damage
  • Increases vulnerability to invasions
  • Infrastructure damage
  • Negatively impacts agriculture
  • Negatively impacts animal health
  • Negatively impacts animal/plant collections
  • Negatively impacts aquaculture/fisheries
  • Negatively impacts cultural/traditional practices
  • Negatively impacts forestry
  • Negatively impacts human health
  • Negatively impacts livelihoods
  • Negatively impacts tourism
  • Reduced amenity values
  • Reduced native biodiversity
  • Threat to/ loss of endangered species
  • Threat to/ loss of native species

Invasiveness

  • Benefits from human association (i.e. it is a human commensal)
  • Capable of securing and ingesting a wide range of food
  • Fast growing
  • Gregarious
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
  • Highly adaptable to different environments
  • Highly mobile locally
  • Is a habitat generalist
  • Pioneering in disturbed areas
  • Proved invasive outside its native range
  • Tolerant of shade
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc

Likelihood of entry/control

  • Difficult to identify/detect as a commodity contaminant
  • Difficult to identify/detect in the field
  • Difficult/costly to control
  • Highly likely to be transported internationally accidentally
  • Highly likely to be transported internationally deliberately
  • Highly likely to be transported internationally illegally

Uses

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Red imported fire ant workers are major predators of some other arthropods such as ticks and caterpillars. Nest-building activities and the construction of foraging tunnels can reduce soil compaction. Beneficial aspects of S. invicta are occasionally exploited such as in sugarcane production where the preservation of the ants reduces sugarcane borer population levels.

Uses List

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Environmental

  • Biological control

Diagnosis

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The diagnostic features of S. invicta worker ants include:

- Two-segmented waist region (the petiole and postpetiole)

- 10-segmented antennae with last two segments forming a club

- No spines on the propodeum (metanotum)

- A long seta (hair) arising from the middle of anterior margin of the clypeus, a region just above the mandibles (Bolton, 1994)

- A median tooth on the front of anterior edge of the clypeus, flanked by a lateral tooth on each side, giving a total of three teeth (Taber, 2000)

Variation in size is one distinguishing characteristic of imported fire ants. Many other ant species are uniform in size.

The following characters can help to identify S. invicta presence in the field:

- The occurrence of fluffy 'worked' soil, particularly a few days after heavy rain. Undisturbed mounds in pastures can reach 45 cm high, but most mounds in turf grass areas are usually just several centimetres tall.
- The ant mound or nest has no opening in the centre like most ant mounds. S. invicta leave and enter the mound housing the colony through underground tunnels.
- Upon disturbing or shovelling into the mound, white objects are observed. This is the brood: eggs, larvae and pupae of developing ants.
- When the mound is disturbed, dozens to hundreds of reddish-brown worker ants crawl up the vertical surfaces (like grasses and other objects) on and around the mound.
- The sting feels like being burned. A day or so later, S. invicta venom forms a white fluid-filled pustule or blister at the red sting site, a symptom characteristic only of fire ants.
- Worker ants bite (with mandibles) and sting (with stingers) aggressively and repeatedly.

The following keys can be used to identify S. invicta:

The Pacific Invasive Ant Key (PIAKey), http://itp.lucidcentral.org/id/ant/pia/PIAkey_v2.html

The Pacific Invasive Ants Taxonomy Workshop Manual, http://www.issg.org/cii/Electronic%20references/pii/project_docs/papp/pacific_ants_taxonomy_workshop_2009.pdf

Denmark HA, 1962. A Key and illustrations of Solenopsis geminata and Solenopsis seavissima richteri. Entomology circular No.3, Fla. Dept of Agr

Wojcik DP, Buren WF, GRissell EE, Carlysle T, 1976. The Fire ants (Solenopsis) of Florida (Hymenoptera: Formicidae). Entomology circular No 173. Fla. Dept. Agr. and Consumer survey Division of of plant industry.

Trager JC, 1991. A revision of the fire ants ,Solenopsis geminata group (Hymenoptera: Formicidae: Myrmecinae). Journal of New York Entomological Society. 99: 141-98.

Detection and Inspection

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Visual inspections

Soil that is associated with any articles of trade or shipping equipment from areas known to be infested with S. invicta should be carefully inspected for the presence of ants. Specimens can be collected in vials of 70% alcohol for preservation and identified by a specialist.

Visual inspection should be conducted in ant favourable habitats during fine, dry warm days when air temperature is over 20oC. Inspection is unsuitable on excessively windy days, when ant activity is minimal.

Favourable ant habitats

Low vegetation (including grass), soil, disturbed sites, rubbish piles, road margins, shiny surfaces, building edges and foundations, north facing banks, concrete slab edges, cracked concrete, drains and culverts, electrical equipment, exposed rocks, fence palings, grass areas, verges, hot water pipes, isolated weeds, logs and log stoppers, loose gravel, plant pot bases, base of flowering tree, shrubs, poles, watertraps, tree crotches and happows, vertical surfaces, weed and olant re-growth, wooden structures and pallets.

Surveying for ant foraging activity

USA

The worker ants of almost all ant species forage away from the colony for food and water at certain times of the day and year. Some ant species only forage underground. Although most ants consume a wide variety of food (they are omnivorous), certain species prefer some types of food and some even change their preferences over time. The foraging workers of some ant species, such as fire ants, establish temporary chemical (pheromone) trails that allow nest mates to locate food and water resources. These species can quickly recruit many other ants to a resource. Normally, collected food is brought back to the colony and is fed communally among the other members of the colony, including the queen(s) and brood. Foraging ant surveys can be useful in detecting S. invicta, allow managers to target appropriate management efforts better and document the impact of suppression actions. A simple way to survey for fire ant foraging and recruitment is to establish a pattern of 'bait stations'.

Bait stations are made using attractants such as moistened, dry cat food, Vienna sausage pieces, tuna fish or other attractive substances. These are placed in some type of small containers (e.g. 2-dram vial, scintillation vial or Petri dish) that can be capped upon removal. The vials are placed in a field on their sides in a grid pattern (e.g. 9 m to 15 m grid) or transect line with similar spacing. The bait stations are placed when the ants are foraging, usually when the temperatures range from 18 to 32°C. The containers are capped after 45 to 60 minutes and the collected ants are identified and counted later. The capped vials can be frozen or filled with alcohol (after the bait is removed) for storage until the sample is analyzed. The ants from each station are identified and counted (or estimated) to provide an indication of the abundance and location of fire ants and other species in the field map.

Survey methodology used in New Zealand to detect exotic ants in risk sites (adapted from Sarty, 2007; Stringer et al., 2010; Gunawardana et al., 2013):

Separate protein and sugar based baits should be composed and laid as follows:

a) Protein based bait: prepare a protein based bait station (~60 ml clear plastic container with a lid) by smearing in a line the blended peanut butter and soybean oil (the size of half a pea) to the inner side of each bait container on the side corresponding with the back of the label. In addition, a slice of processed sausage or smear of raw sausage meat is to be placed inside each pot.

b) Sugar based bait composition: Prepare a sugar based bait station by placing a plug of cotton wool soaked in 30% sugar solution inside the container, being sure to squeeze out the cotton ball to ensure ants can feed on the liquid without drowning. Alternatively a smear of sugary jam may be used.

Each bait container (station) should be labelled with at least the date and the site name. Ants typically locate these baits with their natural foraging behaviour and will recruit additional worker ants to feed upon the bait.

Where the ant habitat is continuous, baits are to be laid in rows 7-8 m apart, alternating between protein and sugar baits. Where the ant habitat is isolated, at least one of each bait type should be laid (minimum of one protein and one sugar bait stations within 15 m2), ensuring they are at least 1 m apart. Ground locations of the bait stations should be marked with a crayon or spray paint spot during the course of surveillance. Bait stations should be collected after approximately two hours when temperature is between 20 and 24.9oC, or after 30 minutes when the temperature is between 25 and 36oC. In the tropics, bait stations should be placed in the shade where practical when temperatures are above 28oC, as these baits are known to rapidly dry out, reducing their attractiveness to ants. Lids are put on the bait containers (station) to prevent ants escaping when they are collected. All bait stations should be sent to an entomologist to identify all samples in the bait station (as more than one species could be found in a bait station), and identification may require assistance from an expert.

Monitoring fire ant mounds

Counting the number of active fire ant mounds in an area is a simple and easy way to document the population of fire ant colonies. This method assumes that each mound is evidence of a fire ant colony. The results can be used to detect colonies, determine the approach most suitable for the management of ants in the area and monitor the effectiveness of the treatment(s) applied. However, this approach has several disadvantages:

- During, hot, dry periods of the year, fire ants dwell deeper in the soil and do not make a tall, observable mound. Also, the mounds may have already been treated.

- Some fire ant colonies are located in tree stumps, compost piles or other structures where their colony may not be readily observed or not associated with a mound.

- This method does not allow for the detection of native ant species, which may not build mounds at all.

For research purposes, a method has been developed to rate fire ant mounds and calculate a population index value (Harlan et al., 1981; Lofgren, 1982).

Similarities to Other Species/Conditions

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S. invicta is one of a number of species commonly referred to as fire ants. Taber (2000) discussed additional species within this group from Trager (1991). Several native fire ant species were present in the USA before S. invicta and S. richteri (black imported fire ant) arrived: S. xyloni (the southern fire ant), S. geminata (the tropical fire ant), and two desert-adapted species, S. aurea and S. amblychila. S. xyloni and S. geminata have also been considered as pest ants, but they have largely been displaced in southeastern USA by S. invicta and S. richteri. In the USA, S. richteri occurs in northern areas of Alabama, Mississippi, Georgia and southern Tennessee, with a sexually reproductive hybrid population of S. invicta and S. richteri throughout the remainder of these states and part of Georgia.

The characteristic that definitively saperates S. invicta from other fire ants is the number of clypeal teeth on its head, a feature that is best viewed under a microscope. S. invicta has 3 clypeal teeth whereas the other fire ants has two lateral teeth. A quicker but less precise way to recognise this species is to observe its aggressive behaviour when its nest is disturbed (Greenberg and Kabashima, 2013).

Prevention and Control

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Efforts to eradicate spot infestations of this species (e.g. in Brisbane, Australia, and parts of California, USA) are ongoing. In southeastern USA, eradication efforts have been abandoned and integrated pest management approaches have been developed to eliminate problems caused by this species where and when they occur.

Phytosanitary Measures

The inspection and treatment or elimination of the soil comprises phytosanitary methods that are designed to prevent or mitigate the further spread of S. invicta.

Cultural Control and Sanitary Methods

Some cultural management approaches in cattle production systems, such as the use of disc-type cutters, the quick removal of hay bales from the field and the scheduling of cow fertility programmes to avoid calving during hot, dry summer months have been promoted. Some landscape elements have been evaluated that are believed to repel or be less attractive to these ants.

Biological Control

A thorough review of the biological control of S. invicta can be found in Williams et al. (2003). Current efforts are focusing on the use of several species of parasitic flies (Diptera: Phoridae). Pseudacteon tricuspis, P. curvatus, P. litoralis, P. obstusus and P. cultellatus have been introduced and have established in the USA (Graham et al., 2003).

Other species are currently being studied as candidates for future releases. The microsporidium Kneallhazia solenopsae (synonym: Thelohania solenopsae) has been detected in southeastern USA and has been used to inoculate healthy colonies in the field and laboratory. Another microsporidium, Vairimorpha invictae, has been studied in Argentina (Briano and Williams, 2002; Valles and Briano, 2004; Briano, 2005; Birano et al., 2006) and the USA (Oi et al., 2005)

Many other natural enemies of S. invicta and related fire ant species have been identified and studied. Few are candidates for classical or importation biological control programmes because they do not spread between colonies in the field and do not sustain themselves in the environment. Native and exotic competitor ants, which prey on newly mated fire ant queens, raid or disrupt small colonies and/or compete for nesting sites and resources, are considered important biological resistance factors. They could help prevent high population levels of S.invicta and should benefit from the establishment of classical biological control agents that will reduce the exotic species' capacity to out-compete these species (Porter, 1998).

Chemical Control

Collins (1992), Williams et al. (2001) and Drees and Gold (2003) provided reviews of the history of control developments, efforts and strategies. Chemical control options include broadcast application of bait-formulated insecticide products, the treatment of individual ant colonies in mounds and surface or barrier treatments using contact insecticides. Regardless of method used, the objective is to kill not only the workers but also the queen, because she is the only ant capable of laying eggs. Common insecticides that can be used for fire ant control were given in Greenberg and Kabashima (2013).

IPM

Drees and Gold (2003) presented an account of the development of integrated pest management (IPM) approaches or programmes to suppress S. invicta population levels where they are needed or justified. Quarantine treatments of nursery stock and other regulated items are detailed in a publication by the USDA Animal and Plant Health Inspection Service (Anon, 1999a).

References

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Links to Websites

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WebsiteURLComment
Ant web (for pictures and identification assistance)http://antweb.org
Global Invasive Species Database (GISD)http://www.issg.org/database/

Contributors

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23/06/14 datasheet updated by:

Disna Gunawardana, Ministry for Primary Industries, Auckland, New Zealand

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