Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
Apparently native from southern USA to Guianas, western Amazonia, coastal Peru. Populations in the Antilles, Galapagos, and possibly southeastern USA are probably introduced (Trager 1991). Costa Rica: throughout the country at elevations where ants occur.
Size large; color red brown; polymorphic, varying continuously from small minor workers to large major workers with allometrically enlarged heads; even on smallest workers eyes with 10 or more ommatidia; petiolar node thin, scale-like, shorter than peduncle.
Solenopsis geminata is the tropical fire ant. Fire ants are a group of related species, the Solenopsis geminata group, that has its center of diversity in southern South America. Solenopsis geminata is the only member of the group that occurs in Costa Rica, although it occurs in a "red form" that is more abundant in open areas and a "black form" that prefers forested areas. The environmental or genetic determinants of these forms are unknown.
Solenopsis geminata is most abundant in open sunny areas. It is common in agricultural areas and around human settlements. In the lowlands it is found not only in the open but may also penetrate into forest understory, albeit at lower density. At higher elevations it is restricted to open areas and does not extend into closed-canopy forest. There is anecdotal evidence that S. geminata occurrence in forest understory, even in mature forest habitats, is increasing, perhaps due to effects of fragmentation. Increased abundance in forest understory could be due to a greatly increased source population in the surrounding pasture areas, or to microclimate change that favors fire ant establishment in the forest understory.
Solenopsis geminata colonies are large, with tens to hundreds of thousands of workers. Nests are in the soil, usually in the form of a large exposed soil mound. Galleries extend out into the surrounding soil, surfacing at foraging zones at a distance from the nest. Most foraging is at the soil surface, but I have seen fire ants foraging several meters up on tree trunks or treefalls when there are abundant epiphytes and epiphytic soil. Workers form galleries extending from the ground up through the epiphytic soil.
Workers are generalized scavengers and they recruit rapidly to resources. Oil and protein sources, such as tuna baits, are particularly attractive. When large resources are discovered, workers often rapidly cover them with soil. I once observed S. geminata workers tending petiolar extrafloral nectaries at each leaf of a long Passiflora vine. The vine looped from the vegetation down to the ground for part of its length, and wherever a leaf petiole was touching the ground the ants had built a soil pavilion covering it.
Workers have powerful stings and are the bane of children running barefoot in the grass. If you mistakenly stand on a nest, workers will slowly cover your feet and lower legs and then all sting at once. Farmers generally despise them.
Individual colonies have large nuptial flights, with abundant males and alate queens issuing from nests. Workers swarm over the nest surface and surrounding vegetation, and they appear to be driving the males and alate queens from the nest. Nuptial flights do not seem highly syncronized among colonies and they may occur at any time of year.
Trager, J. C. 1991. A revision of the fire ants, Solenopsis geminata group (Hymenoptera: Formicidae: Myrmicinae). Journal of the New York Entomological Society 99:141-198.
John T. Longino, The Evergreen State College, Olympia WA 98505 USA.firstname.lastname@example.org
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