Solenopsis richteri (black imported fire ant)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Economic Impact
- Social Impact
- Risk and Impact Factors
- Uses List
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Solenopsis richteri Forel, 1909
Preferred Common Name
- black imported fire ant
Other Scientific Names
- Solenopsis pylades var. richteri Forel
- Solenopsis pylades var. tricuspis Forel
- Solenopsis saevissima st. oblongiceps Santschi
Local Common Names
- Argentina: hormiga de fuego; hormiga marron
- SOLERI (Solenopsis richteri)
Summary of InvasivenessTop of page
S. richteri is native to southeastern Brazil, central Argentina and parts of Uruguay. After its accidental introduction into the USA around 1918, it expanded its range into much of the southeastern USA and became a ubiquitous presence in a variety of urban and agricultural settings, as well as an important economic and environmental pest. However, the red imported fire ant, S. invicta, after its introduction through Mobile around 1930, gradually took over most of the range of S. richteri, and now occupies around 1,100,000 km2, primarily in the coastal plains from N. Carolina to Texas (Porter and Briano, 2000). Currently, S. richteri is restricted to approximately 30,000 km2 in northwestern Alabama, northeastern Mississippi and in parts of southern Tennessee, including a relatively recent introduction into Memphis (Jones et al., 1997). A broad band of hybridization zone between S.richteri and S. invicta exists between the two populations, occupying around 130,000 km2 (Shoemaker et al., 1994). Comprehensive reviews of imported fire ants can be found in Lofgren et al. (1975), Taber (2000) and Tschinkel (2006). S. richteri is apparently more cold-hardy than S. invicta and thus has some potential to expand farther north, including possibly the southern Great Plains of the USA, which are similar to the South American Pampas, to which S. richteri is native. However, given human control efforts combined with intrusions of S. invicta and the S.richteri/S. invicta hybrid, it does not seem likely S. richteri will expand its range significantly in the future in the USA (Taber, 2000).
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Hymenoptera
- Family: Formicidae
- Genus: Solenopsis
- Species: Solenopsis richteri
Notes on Taxonomy and NomenclatureTop of page
Fire ants belong to the genus Solenopsis, which contains 185 described species. Classification of Solenopsis species is problematical, due primarily to the morphological similarity of workers among species. Most authorities now recognize three natural groupings within the genus. The first includes about 20 New World species known as “fire ants”, which have polymorphic (highly size variable) workers; this group includes Solenopsis richteri (black imported fire ant, BIFA) and Solenopsis invicta (red imported fire ant, RIFA). The second group includes those small and widespread species often called “thief ants”. The third and least known group includes species that are social parasites (Tschinkel, 2006). Current taxonomic understanding of fire ant species is provided by Trager (1991) and a phylogenetic analysis by Pitts (2002). Fire ants are best represented in warm regions of the new world where rainfall is not extreme (Tschinkel, 2006). S. richteri and S. invicta hybridize in the southern USA, producing an intermediate form that can produce fertile offspring (Shoemaker et al., 1994).
DescriptionTop of page
S. richteri (Order: Hymenoptera, Family: Formicidae), is a social insect that lives in colonies, usually associated with a mound. Most individuals are sterile female workers that perform a variety of functions, including care of the queen and brood, foraging, defense and nest building. The worker caste is polymorphic, ranging from small (minor) through intermediate (media) to large (major) individuals. Additionally, immature stages (eggs, larvae and pupae, or brood), winged reproductives and at least one queen will be present.
Colonies take approximately 2 years to mature and, on average contain, 200,000-400,000 individuals. Mature S. richteri colonies produce conspicuous mounds similar to those of S. invicta, averaging 30-50 cm in height and width, but they may be larger, reaching 90 x 90 cm. In hot dry conditions of late summer, S. richteri mounds may flatten out or disappear as the colony moves entirely underground. Mound building activity is stimulated by rainfall (Rhoades and Davis, 1967), and outbreaks have been found to be correlated with heavy precipitation, due to the queen’s needs for moist soil to excavate a nest (Green, 1962; Lofgren et al., 1975). Foraging worker ants enter and exit the colony through tunnels radiating up to 5-10m away from the mound. Colonies extend into the ground below the mound as interconnecting galleries, as much as 30-40 cm below ground level. In the USA, S. richteri colonies are usually found in open areas associated with some type of disturbance, e.g., lawns, hayfields, pastures, roadsides and highway medians, athletics fields, school grounds, etc. In their native Argentina, ideal habitats for S. richteri include the Pampas grasslands, as well as pastures of varying water content and seasonally waterlogged grassland (Taber, 2000). The disturbance of mounds results in a rapid defensive response by the worker ants, which will climb vertical objects in large numbers to bite and sting.
General characteristics of adult worker fire ants include: a 2-segmented waist region; worker antennae with 10 segments each, with 2 of these forming a club at the tip; no spines on the propodeum (a region on the ant’s back); and a long hair or seta arising from the middle of the anterior margin of the clypeus (a region just above the mandibles) (Bolton, 1987; 1994). S. richteri and S. invicta are very similar and can be distinguished from other fire ants in having a conspicuous median tooth on the front or anterior edge of the clypeus, flanked by a lateral tooth on each side, for a total of 3 teeth; fire ants native to the USA and the tropical fire ant (Solenopsis geminata) lack the median tooth.
Worker ants are wingless and dark reddish brown to predominantly black, ranging in size from 1.5-5 mm. They are polymorphic, with major, media and minor castes, majors being most useful in identification.
Eggs are spherical and creamy-white and the larvae are legless, cream-colored 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. The eggs, larvae and pupae are referred to as brood.
Females are winged and reddish-brown, while males are blackish, with a smaller head. These ants stay in the colony until the conditions exist for their nuptial flight. Mated female reproductives become queens; they are larger than worker ants (9 mm) and remove their wings following the mating flight.
Distribution TableTop of page
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.Last updated: 17 Feb 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|United States||Present||Present based on regional distribution.|
|-Alabama||Present, Localized||Introduced||1918||Invasive||Localized to northwestern corner of state; previously widespread|
|-Mississippi||Present, Localized||Introduced||Invasive||Localised in 22 countries in northern MS|
|-Tennessee||Present, Localized||Introduced||Invasive||Restricted to southern TN, recently introduced to Memphis|
|Argentina||Present, Widespread||Native||Central, inc. Buenos Aires province|
|Brazil||Present, Localized||Native||Restricted to extreme southern part of country|
History of Introduction and SpreadTop of page
S. richteri was first reported in Mobile, Alabama in 1930 (Creighton, 1930). By this time, it had spread approximately 11 km from the port of Mobile and is thought to have actually arrived about 1918. Within a couple of decades it had become established over several thousand hectares (Lofgren et al., 1975) and subsequently spread over much of Alabama and Mississippi (Trager, 1991). Since then, it has been excluded from most of its US range by S. invicta which is thought to have arrived through the port of Mobile around 1930. Interestingly, S. richteri was initially considered a variant of S. saevissima (later invicta), namely S. saevissima richteri. The two species were not formally separated until 1972 (Buren, 1972).
Risk of IntroductionTop of page
S. richteri was introduced by ship through the port of Mobile; there may have been multiple introductions. The ants were possibly concealed in cargo (probably produce) or ballast soil taken from a South American riverbank (Taber, 2000). The potential for accidental introduction is high; S. richteri is small and easily concealed in soil, produce and other commodities, and a single fertilized queen could produce a new colony. Even with quarantine regulations requiring insecticide treatments, the potential for spread of S. richteri to other regions in the USA is significant, particularly because of its ability to escape detection in produce, soils and turf, nursery plants, hay, honey bee equipment, etc., as well as natural dispersal of mated females, often with the aid of vehicles and other modes of transportation upon which queens alight. This potential is, of course, much greater for S. invicta because it is so much more widespread, and explains recent introductions of this latter species to Arizona and California.
HabitatTop of page
S.richteri has the capability of dominating disturbed habitat and monopolizing food and space therein, in some cases excluding native ant species. This can occur both in the USA and S. richteri's native Argentina in grassland habitat that is susceptible to flooding (Folgarait et al., 2004). In the USA, S. richteri’s impact is limited, since it has been excluded from much of its former range by S. invicta, although the S.richteri/S. invicta hybrid is widespread.
Habitat ListTop of page
|Terrestrial||Managed||Cultivated / agricultural land||Principal habitat||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed forests, plantations and orchards||Secondary/tolerated habitat||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed grasslands (grazing systems)||Principal habitat||Harmful (pest or invasive)|
|Terrestrial||Managed||Industrial / intensive livestock production systems||Principal habitat||Harmful (pest or invasive)|
|Terrestrial||Managed||Disturbed areas||Principal habitat||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural forests||Secondary/tolerated habitat||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural grasslands||Secondary/tolerated habitat||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Riverbanks||Secondary/tolerated habitat||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
Neither S.richteri nor S. invicta are considered major pests of crops although S. invicta is documented to feed on several crops, at times causing minor damage. S. invicta is well known to feed, and S. richteri workers probably feed, on honeydew produced by certain sternorrhyncan hemiptera (e.g., aphids, scale insects, mealybugs, etc.). Since the ants may protect these insects, their numbers may increase on some horticultural crops, especially if their natural enemies are reduced by fire ants.
SymptomsTop of page
Information on crop hosts and feeding by S. richteri is limited, although S. richteri is known as a potato pest in Brazil (Taber, 2000). It is reasonable to expect similarities to S. invicta. S. invicta is omnivorous and foraging fire ants may be found in or on plants when they are preying on phytophagous arthropods associated with those crops. 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 maize and sorghum seeds. 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 soyabeans can interfere with mechanized cutting and harvesting operations.
Affected plant stages include flowering stage, fruiting stage, post-harvest, pre-emergence, seedling stage and vegetative growing stage.
There is little or no specific information available on symptoms occurring in crops as a result of S. richteri feeding. In S. invicta, the following types of damage may be observed:
- Fruits/pods: internal feeding; external feeding.
- Leaves: wilting.
- Roots: internal feeding; external feeding.
- Seeds: internal feeding; external feeding.
- Vegetative organs: internal feeding; external feeding.
- Whole plant: plant dead; dieback; uprooted or toppled; internal feeding; external feeding
Biology and EcologyTop of page
In all ants, sex is determined by fertilization; unfertilized eggs produce males and fertilized eggs become females. Males occur only in the reproductive form, while females may become sterile workers or fertile reproductives (incipient queens). Whether a female becomes a worker or reproductive depends on its feeding and chemical (juvenile hormone and pheromones) environment (Tschinkel, 2006).
The S. richteri life cycle is similar to that of S. invicta (Taber, 2000). Winged reproductives form mating swarms and mating occurs in the air, after which the queen lands and sheds her wings; males die soon after mating. Several hundred virgin males and females may leave a colony at any one time. Mating flights can occur year round, especially in the native range in South America, but in North America often occur between April and August, usually on a warm, sunny day following rain. Following wing removal, queens establish colonies and start laying eggs. S. richteri queens establish their nests within approximately 3 cm of the soil surface, which is shallower than for S. invicta queens (Lofgren et al., 1975); however, the vast majority of queens perish before they can establish nests.
Physiology and Phenology
S. richteri is an adaptable species in a variety of ways, which contributes to its success. It is primarily a creature of disturbed habitats, both natural and manmade, in both its adopted and native countries (Tschinkel, 2006). It can aggressively exploit such areas, which is even more evident in S. invicta, allowing it to colonize and exclude other species that are potential competitors. The good fortunes of imported fire ants are closely tied to human activities, especially since the arrival of Europeans in the New World and the accompanying huge areas of ecological disturbance that resulted (Tschinkel, 2006). Fire ants are perhaps best viewed as pioneer species, evolved to exploit relatively rare and short-lived habitat patches derived from disturbance. They evolved high reproductive output as a response to dealing with such rare and unpredictable optimum habitat; effective dispersal mechanisms were also required to exploit habitats unpredictable in space. Additional adaptations to such habitats include rapid colony growth and early reproduction over a long season. Thus, fire ants have successfully exploited the highly disturbed landscape of the southeastern USA (Tschinkel, 2006).
S. richteri’s colony populations, foraging behaviours, diets and feeding behaviours are similar to those of S. invicta, which has been studied much more intensively (Taber, 2000; Tschinkel, 2006). Ants communicate through vision (sight), vibration (sound), touch and chemicals (pheromones), including a queen pheromone that attracts workers and a trail pheromone associated with the worker ant stinger. Upon locating food resources, a pheromone trail is produced which directs other worker ants to the site. Fire ants are omnivorous, consuming primarily other arthropods and honeydew produced by aphids and related insects (primarily Order Hemiptera, Suborder Sternorrhynca), but also seeds and other plant parts like developing or ripening fruit, and dead plant and animal tissues (Vinson, 1997). Living prey may be subdued by stinging. Foraging ants may bring solid or liquid food back to the colony; however, only certain larvae can process solid foods. Workers store liquid food in their crops, from where it can be regurgitated for nest mates (trophallaxis) (Glancey et al., 1981). Optimum ambient foraging temperatures range between 70 and 85°F (Rhoades and Davis, 1967).
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Mean annual temperature (ºC)||15.2||20.4|
|Mean minimum temperature of coldest month (ºC)||0.8||9.3|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Mean annual rainfall||1011||1680||mm; lower/upper limits|
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Beauveria bassiana||Pathogen||All Stages||not specific|
|Caenocholax fenyesi||Adults||not specific|
|Kneallhazia solenopsae||Pathogen||All Stages||to genus||Argentina|
|Pseudacteon||Adults||to genus||Argentine, Brazil, Uruguay, USA (introduced)|
|Pyemotes tritici||Predator||All Stages||not specific|
|Steinernema||Pathogen||Arthropods|Larvae; Arthropods|Pupae||not specific|
|Vairimorpha invictae||Pathogen||All Stages||to genus|
Notes on Natural EnemiesTop of page
There is a great deal of information available on imported fire ant natural enemies, including S. richteri, due primarily to the potential for biological control in areas where fire ants are invasive. The socially parasitic fire ant, Solenopsis daguerrei, was first discovered in S. richteri mounds in South America (Santschi, 1930). Colony parasitization rates in a given area can be as high as 31% and mound densities in affected areas are lower than densities where the parasite is not found (Calcaterra etal., 1999). Phorid flies in the genus Pseudacteon and several related genera produce larvae that decapitate worker ants and pupate inside their empty heads (Porter, 1998). Each species of fly parasitizes a characteristic size range of ants (Morrison et al., 1997; Morrison and Gilbert, 1999). Species that attack fire ants appear to be specific to fire ants (Porter, 1998). In addition to mortality, phorids appear to affect fire ant worker behaviour in important ways. Once flies are recognized, most ant workers seek cover, others curl into a stereotypical c-shaped defensive posture, and yet others freeze their posture (Porter, 1998). These behaviours generally result in reduced foraging rates; the presence of a single fly can stop or greatly inhibit the foraging of hundreds of workers within 2-3 minutes (Feener and Brown, 1992; Orr et al., 1995; Porter et al., 1995). In Argentina, the presence of six phorid species that attack S. richteri reduced the number of ants at food resources in the field, as well as foraging activity in general (Folgarait and Gilbert, 1999).
Means of Movement and DispersalTop of page
Natural dispersal of S. richteri is similar to that of S. invicta (Taber, 2000). Incipient queens disperse by wind and air currents after mating flights, which tend to occur on warm days after precipitation. Wind direction can have a major influence on the distribution of new S. richteri colonies after mating flights (Rhoades and Davis, 1967). Fire ant colonies or portions thereof may also be transported by floodwaters, as mounds, or portions thereof containing ants, can float.
S. richteri, similarly to S. invicta, can be transported both short and long distances via such materials as turf, sod, hay and nursery containers. The long-distance movement of such articles has 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 (USDA-APHIS, 1999b). More recently, the transport of beehive support pallets infested with S. invicta from the southeastern USA is thought to have resulted in the infestation of California's central valley almond orchards (USDA-APHIS, 1999a; Weeks and Drees, 2002).
The long-distance transport of S. invicta or S. richteri 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. This was demonstrated by the accidental introduction of S. richteri and S. invicta into the USA through the port of Mobile, Alabama in the early twentieth century.
Pathway CausesTop of page
Pathway VectorsTop of page
Economic ImpactTop of page
In the USA, imported fire ants (S. invicta,S.richteri and S. invicta/S.richteri hybrids) are serious pests affecting humans, wildlife, crops and livestock (Vinson, 1997). The economic impact of imported fire ants extends to everyone living in infested areas. Fire ants are a multifaceted pest impacting (a) urban (e.g., damage to roadways and electrical equipment; recreational impacts, pet injury, medical issues, etc.), (b) agricultural (e.g., direct crop damage; livestock nuisance; movement in nursery stock, hay, soil or earth-moving equipment; damage to farm equipment; reduction of beneficial arthropods, etc.) and (c) ecological settings (e.g., decline of native species; damage to wildlife and endangered species; alteration of plant communities) (Vinson, 1997; Colby and Prowell, 2000; Myers et al., 2000; Wojcik etal., 2001). However, virtually all of the economic impact is due to S. invicta and the S. invicta/ S. richteri hybrid, due to the small area still occupied by S. richteri in the USA.
Some information on specific impacts of S. richteri exists. S. richteri mounds have been documented to cause damage to farm machinery, at least before S. invicta became prevalent in the south-eastern USA (Green, 1952). Experiments to estimate the impact of S. richteri ingestion on fish indicated no harmful effects (Ferguson, 1962). In southern Tennessee, where only S. richteri and the hybrid occur, economic impacts on the nursery industry are important, primarily because of quarantine regulations (Title 7, Code of Federal Regulations, Part 301.81 [CFR301.81]). All nursery shipments transported from regions infested by imported fire ants are required to apply treatments if plants are sold with roots and soil attached, even if the pest is absent. Market destinations for plants sold from southern nurseries requires approximately 80% of plants to be treated for fire ants. Treatment costs are considerable (US $635-2043 per hectare of trees), which must be absorbed by producers or passed on to consumers.
In Sao Paulo, Brazil, S. richteri is well known as a potato pest, eating the tubers and branches (Taber, 2000). In Argentina, S. richteri is not considered a pest either in fields or among livestock (Hays, 1958). However, it is reported to monopolize space and food in grassland habitat in Argentina that is susceptible to flooding (Folgarait et al., 2004).
Social ImpactTop of page
Most social impacts of S. richteri involve medical issues associated with its sting. S. richteri’s sting is similar to that of S. invicta and the hybrid, and causes similar reactions; most information on impacts and reactions is for S. invicta. The sting evokes a painful burning sensation, hence the name ‘fire ant’ (Taber, 2000). A white pustule usually forms at the sting site within 24 hours; this reaction is diagnostic. The pustule can remain for several days and may rupture and leave a crust or scar. In severe cases with multiple stings, the condition resembles smallpox (Taber, 2000). In rare instances stings can elicit an allergic reaction; in the most severe form, anaphylactic shock occurs, which can result in death. Symptoms of anaphylactic shock include fever, headache, dizziness, nausea, vomiting, perspiration, loss of consciousness, coughing, hoarseness, reduced heart rate, hives, a swollen larynx and low blood pressure (Prahlow and Barnard, 1998). Anaphylaxis occurs in 0.6 to 6% of persons who are stung (deShazo et al., 1999). Fire ant stings are the most common insect venom allergy in the south eastern USA. A survey of 1286 health practitioners in South Carolina estimated that over 33,000 people (0.94%) seek medical attention for imported fire ant stings. Interestingly, the venom has antiseptic properties, such that the sting site seldom becomes infected (Taber, 2000). There are records of massive fire ant stings resulting in limb amputations and skin grafts and there have been several fatalities among the elderly and infants indoors, who were not able to escape the ants (Adams, 1986).
S. richteri and S. invicta venoms are similar and alkaloid in nature. S.richteri venom contains at least three allergenic proteins; S. invicta and the S. richteri/S. invicta hybrid have four. S. invicta immunotherapy using venom, which is useful in treating allergy problems, is also expected to work for S. richteri and the hybrid (Hoffman et al., 1990).
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Has a broad native range
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Pioneering in disturbed areas
- Capable of securing and ingesting a wide range of food
- Benefits from human association (i.e. it is a human commensal)
- Fast growing
- Has high reproductive potential
- Host damage
- Increases vulnerability to invasions
- Infrastructure damage
- Negatively impacts agriculture
- Negatively impacts human health
- Negatively impacts animal health
- Reduced native biodiversity
- Threat to/ loss of endangered species
- Threat to/ loss of native species
- Causes allergic responses
- Competition - monopolizing resources
- Induces hypersensitivity
- Interaction with other invasive species
- Rapid growth
- Highly likely to be transported internationally accidentally
- Difficult to identify/detect as a commodity contaminant
- Difficult/costly to control
UsesTop of page
Uses ListTop of page
- Laboratory use
- Research model
DiagnosisTop of page
S. richteri shares several general characteristics with other fire ant species that can be used for recognition in the field, including large mounds (up to 90 x 90 cm and averaging larger than S. invicta, but similar to hybrid S. richteri/S. invicta) in open areas with no entry or exit holes for workers, small (2-5 mm), brownish-black workers of varying size (darker than S. invicta but readily confused with the hybrid), aggressiveness of workers when the mound is disturbed, including crawling up vertical surfaces to sting and bite (not seen in most ants), and a burning sting that produces a small white pustule a day or two after the sting.
Detection and InspectionTop of page
Methods for detection of S. richteri are the same as those for S. invicta (see datasheet on Solenopsis invicta).
Soil that is associated with any articles of trade or shipping equipment from areas known to be infested with S. richteri should be carefully inspected for the presence of ants. This could include various types of produce, turf and other nursery materials, honey bee equipment, hay, etc.
Baits are commonly used to survey for foraging activities of fire ant workers. A variety of food materials can be used, including sugar water, hot dogs, cookies, tuna, moistened pet food, etc. Baits are placed on or in such containers as petri dishes, plastic vials or test tubes, cardboard or laminated paper squares, etc. Under optimum conditions, fire ant workers will quickly find the baits and recruit other workers to them via trail pheromones. Baiting may be used by researchers to study ant behaviour, document impact of fire ants on other ant species, determine effectiveness of different control methods, time control applications, etc.
Monitoring fire ant mounds
Estimating the density of fire ant mounds in a given area is an easy way to quantify populations and monitor changes in population size in response to suppression measures. In addition to numbers, mound sizes and brood presence/absence can be used to further assess populations (e.g., see USDA mound rating system, Harlan et al., 1981). Some limitations to these methods include disappearance of mound structure in hot, dry weather, making detection more difficult ease of missing small, young colonies, location of fire ant colonies in areas not associated with a mound or hard to observe (e.g., tree stumps, hay bales), etc. Changes in populations through the year with changes in season usually necessitate sampling more than once to obtain reasonably accurate information.
Similarities to Other Species/ConditionsTop of page
S. richteri most closely resembles S. invicta, in appearance, behaviour and ecology. Morphologically, S. richteri workers can be distinguished by a darker head and first antennal segment, the presence of a yellowish spot or stripe on the dorsum (upper surface) of the basal segment of the gaster (just behind the waist region; spot is brownish-red in S. invicta), and a concavity in the middle of the dorsum of the pronotum (anterior segment of thorax; concavity lacking in S. invicta; Taber (2000) provides keys for separation of all fire ant species native or introduced to the USA). S. richteri mounds average slightly larger than S. invicta, although there is a great deal of overlap (Taber, 2000).
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Because fire ants are easily transported in nursery stock and soil, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (USDA, APHIS) developed a quarantine program for this pest in the 1950s. The USDA Imported Fire Ant Quarantine program is administered by state regulatory agencies. The purpose of the quarantine program is to minimize the spread of imported fire ants (S. invicta/S. richteri and hybrid) by requiring proper inspection and treatment of all nursery stock, turf grass, hay and other articles shipped out of quarantined areas. Inspectors also survey non-quarantined counties for fire ants and occasionally treat small, isolated infestations. More information can be found in ‘Imported Fire Ant 2005: Quarantine Treatments for Nursery Stock and other Regulated Articles’ (http://www.aphis.usda.gov/publications/plant_health/content/printable_version/IFA2007.pdf).
Cultural control and sanitary measures
The following were based on the recommendations from Texas Cooperative Extension Fire Ant Plan Fact Sheet #26 (http://fireant.tamu.edu/materials/factsheets_Pubs/pdf/fapfs026.2002rev.pdf) for S. invicta. However, these will also apply for S. richteri. It should be noted that cultural and sanitary practices do not eliminate fire ants; they can only serve as deterrents.
Increased habitat diversity encourages competitor ant species which can compete with fire ants for food and nesting sites; competitor ants may also kill new fire ant queens.
Insecticide treatments of ant competitors can be avoided by refraining from using broadcast applications in areas with less than 50 mounds per hectare and where native ants are prevalent.
Fire ants feed on other insects and their secretions (e.g., honeydew produced by sap-sucking insects such as aphids), thus pest-free plants do not offer food sources to sustain fire ants.
Good hygiene practices render food sources unavailable to fire ants.
Fire ants need water; lack of water sources may force fire ants to abandon habitats and deters ants from getting established.
Some mulches encourage ants to nest; thus the use of gravel may prevent nesting.
Mowing and disturbing areas where fire ant mounds are found will induce ants to move their mounds.
(See Managing Imported Fire Ants in Urban Areas; http://pubs.caes.uga.edu/caespubs/pubcd/B1191.htm). Individual mound treatment with two to three gallons of hot or boiling water is a fairly effective control, especially when much of the colony is located near the top of the mound, such as on a cool, sunny morning. However, this method, which works only about 60% of the time, requires careful handling and can destroy surrounding vegetation. To be effective, this treatment must result in the death of the queen. The use of barriers such as talcum powder, Teflon™ tape, tangle foot or hot plates or wires (to about 140°F) can prevent fire ants from crawling vertical structures and protect sensitive areas such as duck nesting boxes or greenhouse benches. There are various mechanical and electrical products on the market but these have not been scientifically evaluated or proven to be effective in controlling imported fire ants. Examples are electric devices that can electrocute worker ants but that are ineffective against queen and brood, and vibrating and sound-producing units that are designed to repel fire ants. There are also tools that employ microwaves or explosive units to heat or blow up mounds.
Imported fire ants, including S. richteri, are excellent candidates for biological control efforts where they have been introduced, including the USA (Porter et al., 1997). They were introduced without most of their natural enemies, which is probably the major reason that their population densities in the USA are several times greater those in their native South America. Several publications provide comprehensive accounts of the biological control efforts against imported fire ants (Jouvenaz, 1990; Williams et al., 2003). Williams et al. (2003) published a review of the status of biological pesticides, biocontrol agents and management strategies against imported fire ants. Potential biocontrol agents that have been imported, evaluated or discovered in the USA include bacteria (Bacillus thuringiensis, Bacillus spahericus, Serratia marcescens, Pseudomonas aeruginosa, Pseudomonas chloroaphis), fungi (Beauveria bassiana, Metarrhizium anisopliae), microsporidia (Kneallhazia solenopsae, Vairimorpha invictae), nematodes (Neoaplectana carpocapsae, Steinernemasp., Heterorhabditis heliothidis), phorid flies (Pseudacteon curvatus, Pseudacteontricuspis, Pseudacteon litoralis), straw itch mite (Pyemotis tritici), viruses (SINV-1) and social parasites (Solenopsis daguerrei) (Jouvenaz, 1990; Williams et al., 2003; Valles et al., 2007).
Historical perspectives of chemical control of imported fire ants are provided in Williams et al. (2001) and Tschinkel, (2006), primarily with respect to S. invicta. The first coordinated effort to control S. richteri used calcium cyanide dust applied on 800 ha of vegetable cropland in Baldwin County, Alabama, resulting in 80% control of fire ant mounds (Eden and Arant, 1949). Early mound drenches employed chlorinated hydrocarbons, such as heptachlor, dieldrin and chlordane. These insecticides were later evaluated in bait formulations before their use was restricted in the 1960s. The first toxic bait formulation was mirex, which consisted of corn cob grits impregnated with mirex dissolved in soyabean oil. The negative impact of mirex bait on non-target organisms led to its loss and the development of other classes and formulations of insecticides for fire ant control. Insecticides are sold as dusts, granules, liquid drenches or baits. Most conventional bait formulations combine pesticide ingredients with soyabean oil, which is absorbed onto processed corn grit. Soyabean oil is an attractive food for ants that is important to the success of the bait. Baits may be broadcast or placed around the mound periphery; workers pick up bait granules and take them back to the colony, eventually killing or sterilizing the queen (depending on product and mode of action). Baits are relatively safe and target specific, although they may heavily impact native, non-target ant species. Many contact insecticide products are available in a variety of formulations, which tend to act more quickly than baits but are often more toxic and less target specific. More information on chemical control of imported fire ants and related information can be found at http://www.extension.org/fire+ants.
ReferencesTop of page
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ContributorsTop of page
14/05/08 Original text by:
Ken Ward, Alabama A&M University, Forestry, Ecology & Wildlife Program, ARC 142, PO Box 1927, Normal, AL 35762, USA
Rufina Ward, Consultant, USA
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