Wasmannia auropunctata
(little fire ant)

Wasmannia auropunctata (the little fire ant) is responsible for reducing species diversity, reducing overall abundance of flying and tree-dwelling insects, and eliminating arachnid populations. It is also known for its painful stings. On the Galapagos, it eats the hatchlings of tortoises and attacks the eyes and cloacae of the adult tortoises. It is considered to be perhaps the greatest ant species threat in the Pacific region. This species has been nominated among 100 of the 'World's Worst' invaders.

W. auropunctata is native to Central and South America (Holway et al., 2002; Wetterer and Porter, 2003). It has been introduced to parts of Africa (including Gabon and Cameroon), the Middle East (Israel) (Vonshak et al., 2009), Europe (Espadaler et al., 2020), North America (including Canada) and South America. It has been introduced to some islands in the Caribbean and the Pacific Ocean (including New Caledonia, Vanuatu, Tahiti, the Galapagos, Hawaii and Solomon Islands) (McGlynn, 1999; Roque-Albelo and Causton, 1999; Holway et al., 2002; JK Wetterer, Florida Atlantic University, Jupiter, personal communication, 2003; E Loeve, Fenua Animalia, Tahiti, personal communication, 2004).

Risk of introduction is high as it is likely to be transported in soil and on plant parts in tropical parts of the world. However, cold climates appear to be unsuitable for the successful invasion and establishment of W. auropunctata in native ecosystems. However, it is able to colonize and become abundant in man-made habitats such as cities (Foucaud et al., 2009). In the southern limit of its distribution in central eastern Argentina, the little fire ant is common and abundant in most cities (LA Calcaterra, USDA-ARS South American Biological Control Laboratory, Buenos Aires, unpublished data). In its introduced range, it may survive in human habitations or infrastructures including climate-controlled buildings and greenhouses. For example, W. auropunctata is a greenhouse pest in temperate regions, such as England and Canada. Although local spread is restricted in such cases, the population may act as a 'stepping stone' for the colonization of more suitable locations (McGlynn, 1999; Holway et al., 2002; JK Wetterer, Florida Atlantic University, Jupiter, personal communication, 2003).

W. auropunctata has been described as a true generalist in its choice of nest sites and habitats (Harris et al., 2005). It occurs in a range of habitats from urban settlements (Delabie et al., 1994) and fields through to undisturbed forest (Harris et al., 2005). Generally, W. auropunctata nests in unstable microhabitats favouring species that can cope with frequent migrations (Holldobler and Wilson, 1990). It can occur in habitats that are wet or dry and it will nest on the ground or in trees. Unlike Solenopsis geminata, it does not colonize disturbed habitats rapidly (Harris et al., 2005).

In natural environments W. auropunctata efficiently exploits twigs and leaf litter, and for its nesting substrate, while in houses, it may infest beds, furniture and food (Smith, 1965;  Brooks and Nickerson, 2000; Armbrecht and Ulloa-Chacón, 2003). In some regions, nests are frequently found behind the sheaths of palms or palmettos. During heavy rains, nests may be moved into buildings or trees to escape flooding (Hedges, 1998;  Brooks and Nickerson, 2000).

Favourable ant habitats include the following:

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

Nest Characteristics

Both multiple queened (polygyne) and single queened (monogyne) colonies occur (Wetterer and Porter, 2003). Colonies show low intraspecific aggression (unicolonial) and high interspecific aggression. Queens typically live about a year (Passera, 1994). Sexuals are produced throughout most of the year (Passera, 1994). On Santa Cruz Island, Galapagos, where W. auropunctata was abundant, estimated densities were 1000-5000 workers per square metre (Clark et al., 1982). They do not have structured nests, but use any available space: under leaf debris, rotten tree limbs, stones, in the crotches of trees or clumps of grass or in space between plants and soil (Harris et al., 2005).

W. auropunctata has two types of ecologically distinct populations: 1) dominant populations with a high density of nests and workers, associated with highly disturbed environments (prone to anthropic or natural disturbance) and low ant species richness; 2) non-dominant populations (low density of nests and workers) associated with undisturbed environments (Orivel et al., 2009). The similarity of the native and introduced populations encouraged the study of the transition from non-dominant to dominant populations in their native range as a key factor in the study of biological invasions by this species (Foucaud et al., 2009).

The success of this species as an invasive is associated with its unicolonial social organization in which individuals from different nests mix freely, forming a large, highly competitive supercolony, which can reach hundreds of kilometres in invaded areas (Le Breton et al., 2004). This type of organization has also been observed in some areas of its native range (Le Breton et al., 2004), although they form much smaller colonies.

It was recently discovered that populations of this species have two types of reproductive system: 1) clonal populations in which males and females reproduce independently by clonation, and workers reproduce sexually; and 2) classical sexual population (workers and females reproduce sexually and males by arrhenotokous parthenogenesis) (Fournier et al., 2005; Foucaud et al., 2007). Clonal populations are associated with human or man-made habitats, whereas sexual populations occur in natural habitats (Orivel et al., 2009). Clonality is believed to be responsible for the ability to colonize and persist in extreme environments or highly modified habitats, maintaining high levels of heterozygosis (Foucaud et al., 2009). Although there are different hypotheses, neither the origin nor the mechanism of clonality is well understood in this species.

Nutrition

W. auropunctata is a generalist feeder, preferring invertebrates, seeds and other plant parts (Clark et al., 1982; Romanski, 2001). When honeydew-producing Homoptera are present, a large part of its diet is likely to consist of the carbohydrate-rich residues produced by these insects (JK Wetterer, Florida Atlantic University, Jupiter, personal communication, 2003). In human habitations, nutrition may be gained from fats (such as peanut butter) and other oily materials found in homes (Fernald, 1947; Brooks and Nickerson, 2000). W. auropunctata has a venomous sting that gives it a greater ability to subdue vertebrate and large invertebrate prey (Holway et al., 2002).

Foraging

Foraging is less affected by wind, rain, direct sunlight and time of day/night than in populations of Solenopsis geminata and Paratrechina longicornis (Meier, 1994). Workers are highly aggressive to other ant species and in some locations they are able to exclude other ant species completely and dominate an area (Jourdan, 1997; Harris et al., 2005). In their native range they do not defend territories, but recruit to and defend food resources close to their nests (Torres, 1984; Harris et al., 2005).

Environmental

• Biological control

Ants can be detected by conducting surveillance programmes in high risk sites with favourable ant habitats. Surveillance should not occur during or after rain when the ground surface is still wet. Inspection is unsuitable in excessively windy days when ant activity is minimal.

The following is adopted from Vanderwoude et al. (2009):

Three survey methods can be used to detect W. auropunctata: vial baiting, chop stick baiting and visual surveys.

Vial Baiting

Vial baiting with an attractant is systematic and most suitable for surveys of industrial sites and nurseries. Little fire ant vial baits contain peanut butter which can be made by smearing a line of peanut butter (half the size of a pea) on the inner side of each bait vial (60 cc plastic containers with lids). Baits should be freshly made as ants are are not as interested in old and dried-up baits. The survey is carried out by placing bait vials in a grid pattern, with a minimum of one bait container per 10 x 10 m grid. Bait containers should be placed in suitable ant habitats and should be collected within 60-90 minutes after placement in the field.

Chop Stick Bating

Chop stick baiting is useful when targeting habitat trees as well as individual potted plants. One end of the chopstick needs to be painted (both sides) so that these can be easily relocated in the field. Morning or overcast days are the best times to bait. If surveying at midday on hot, low humidity days, baits should be placed in shady spots.

Dip the unpainted end of a chopstick in peanut butter to get a light coating extending about halfway up the stick. Place the chopsticks with peanut butter on the ground every 5-10 m. Only place in suitable habitats such as bases of trees/shrubs and in shady spots. Leave chopsticks with peanut butter for at least 45 minutes, not more than 2 hours before collecting. While collecting check for presence/absence of W. auropunctata.

Visual Survey

Visual surveys are appropriate when targeting discrete locations within a large site or for covering large areas quickly. It is very efficient in high density areas. Causton et al. (2005) used hot-dogs (ca 5 mm thick, made of beef) on the lower ends of 30 cm wire flags that were placed on the ground at 5 m intervals along a transect to detect W. auropunctata. These baits were checked after 45 minutes.

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.

Prevention, quarantine and rapid response are the best management strategies for preventing the establishment of invasive ants. To be successful, they require active surveying, early detection and subsequent rapid treatment procedures, often along with quarantine. This type of management approach remains the most practical strategy for dealing with invasive ants (Krushelnycky et al., 2005). The potential of invasive ants to reach high densities is greater in ecosystems which have been utilized or modified by humans. For example, W. auropunctata is a greater problem in forests and habitats in its native range in South America that have been over-exploited by humans (Armbrecht and Ulloa-Chacón, 2003).

Chemical Control

Eradication programmes are expected to be more successful on small islands or in isolated areas where distributions are less than a few dozen hectares. It has been successfully eradicated from Santa Fe and is thought to have been eradicated from other small islands such as Marchena (Causton et al., 2005). An intensive programme was initiated to eradicate W. auropunctata from ca 21 hectares on Marchena Island, Galapagos. Hydramethylnon with soybean oil, 0.88% active ingredient, was applied three times in the treatment area in Marchena Island at three month intervals during the eradication programme (Causton et al., 2005). This product was developed for Solenopsis fire ants and was the most attractive to W. auropunctata.

W. auropunctata primarily nests and forages in the leaf litter and is behaviourally dominant when competing for small sized baits presented in a dispersed fashion (McGlynn, 1999). W. auropunctata prefers carbohydrate-rich resources such as honey, water or honeydew to protein-rich resources (McGlynn, 1999). W. auropunctata tend to be attracted to lipid baits. Tuna oil and peanut butter were used in the Galapagos archipelago on Santa Fe Island and proved highly attractive to ants, but also to birds, lizards and rats (Abedrabbo, 1994; Stanley, 2004). Researchers used peanut butter painted monitoring sticks on Marchena Island in the Galapagos to detect ant presence (Causton et al., 2005). Peanut butter bait is also used in quarantine in Hawaii to treat plants before shipping (Krushelnycky et al., 2005). Hot dogs on wire flags were used during the eradication programme on Marchena Island in 2001 and were attractive to W. auropunctata; but a large proportion were eaten by lizards and crabs (Causton et al., 2005).

The Hawaiian Department of Agriculture primarily used Amdro® granular ant bait to target W. auropunctata. It is effective when it can be evenly broadcast throughout a population and when excessive humidity or rainfall does not disrupt application operations. Thick vegetation may make it difficult to achieve thorough coverage, which is necessary due to the short foraging distance of the species. In addition, frequent rainfall in some regions has hampered the effectiveness of such applications. Even more problematic are infestations in fruit orchards, where no satisfactory eradication technique is available (Krushelnycky et al., 2005).

Hydramethylnon in soybean oil on corn grit matrix is very effective at controlling S. invicta and W. auropunctata and suggested for use against the latter by Stanley (2004). Methoprene baits (0.4%) used in a field experiment on Santa Cruz Island (Galapagos) in 1989-90 were highly attractive, but population reductions had only reached 50-75% after 3 months (Ulloa-Chacón and Cherix, 1994).

Insect Growth Regulator (IGR) baits developed for S. invicta control (active ingredients: methoprene and pyriproxyfen) appear to be the most effective IGR ant baits available. They have a lipid attractant and are attractive to W. auropunctata. Fenoxycarb (1%) has proved to be repellent to W. auropunctata in laboratory and field tests (Williams and Whelan, 1992; Stanley, 2004).

On Maui Island, Hawaii, three different baits were used during the W. auropunctata eradication programme in 2010 (Vanderwoude et al., 2010). Pyriproxyfen (0.5%) was applied to crop areas and hydramethylnon (0.74%) was applied to turf and ornamental areas every month throughout the year. An experimental indoxacarb (0.18%) bait was applied to all vegetation >1.8 m tall (Vanderwoude et al., 2010). Three-dimensional treatment and repeated treatment is required for successful eradication of this species.

Most if not all ant eradications have employed the use of baits and toxicants, many of which are developed for agriculture or urban settings. However, indiscriminate pesticide use in natural areas and fragile island ecosystems is not advocated. Non-target impacts must be weighed up carefully against the benefits of ant eradication.

29/04/15 Updated by: Disna Gunawardana, Plant Health and Environment Laboratory, Ministry for Primary Industries, New Zealand

31/10/09 Compiled by: Dr James K Wetterer, Florida Atlantic University, Jupiter, USA and IUCN/SSC Invasive Species Specialist Group (ISSG)