Biology
Several biological factors ensure the success of this species as a notorious pest. These include: multi-queened (polygynous) colonies – hence a high fecundity; large numbers of offspring and rapid recruitment lead to dominance over larger ants at food sources; an ability to exploit a diversity of habitats and food sources; a propensity for forming supercolonies through successful mixing of individuals from separate nests that are linked by foraging trails.
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A “tramp” species |
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Foraging and diet |
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Abundance |
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Climate associations |
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Habitat associations |
A “tramp” species
Argentine ant is commonly referred to as one of the “tramp” ant species (Passera 1993). Tramp species generally have the following common features that have enabled them to be highly successful invaders:
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Strong tendency to move and associate with humans
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Unicoloniality (not aggressive to Argentine ants from different nests)
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Aggressive to other ant species
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Polygyny (colonies with many queens)
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Disperse by budding (queens and a group of workers separate from the colony and walk to a new site).
Foraging and diet
Argentine ants are effective at recruiting foragers to, and monopolising, food resources. Workers are aggressive to other ant species and their numerical dominance leads to them displacing other ants (Holway 1999). Occasionally Argentine ants are displaced by other species (e.g., Solenopsis invicta — Porter et al. 1988). Pheidole megacephala (the big-headed ant) appears to survive in the presence of Argentine ants, but not to coexist – there is a dynamic equilibrium between the two species, with each species holding possession of sites for extended periods (Haskins & Haskins 1988).
Argentine ants have a generalist diet that includes nectar, insects, seeds, carrion, and honeydew secreted by homopterans (Suarez et al. 1998). The diet of workers consists primarily of sugar, while larvae and queens are mainly fed protein (Vega & Rust 2001). Queens are unusual in that they take part in foraging and grooming (Vega & Rust 2001).
Dispersal
Human-assisted dispersal has resulted in the successful establishment of Argentine ants on six continents and many oceanic islands (Suarez et al. 2001). Establishment patterns in many areas follow human transport networks (roadways and towns) (e.g., Holway 1995; Human et al. 1998). Their distribution is often patchy due to dispersal by budding off from existing colonies and jump-dispersal often through human-assisted transportation (e.g., in potted plants, rubbish, freight) (Van Schagen et al. 1993; Suarez et al. 2001). The rate of spread due to budding is relatively slow, averaging about 150 m/yr where climate or habitat is not limiting (reviewed by Suarez et al. 2001), but can range from near zero in areas of climatic extremes up to 800m/yr in highly favourable recently invaded habitat (Holway 1998a; Way et al. 1997). Colonies are able to relocate and survive in response to high levels of disturbance (Passera 1993). Workers pick up larvae and eggs and relocate their nest in response to changes in weather, human activity, and/or food supply. Queens appear to need workers to establish a colony successfully, but a colony with as few as 10 workers can grow quickly in the laboratory (Hee et al. 2000).
Abundance
Argentine ants are usually extremely abundant wherever they have established. Any suitable nesting site will be occupied, and thousands of ants will pour out if the nest is disturbed. Nests are inter-connected via foraging trails, and workers are freely exchanged between them. The activity along foraging trails indicates the high abundance. Markin 1967 (in Vega & Rust 2001) estimated 50 000–600 000 ants ascending single trees daily. There is considerable seasonal variation in the density and distribution patterns. In favourable conditions, new nests containing queens and workers bud off and the infestation expands, whereas in adverse conditions, small nests merge to form fewer larger colonies (Davis & Van Schagen 1993).
Climate associations
Argentine ants mostly occur in 30–36oC latitude belts of both hemispheres (Majer 1993). Isolated populations can establish and thrive outside these latitudes, where climatic conditions are favourable (e.g., California, 37–38.5oC (Ward 1987); Hawaii, 20.5–21oC (Cole et al. 1992); Easter Island, 27oC (Morrison 1997)). In Hawaii, coastal areas appear to be too hot, and Argentine ants are restricted to higher, cooler elevations (Cole et al. 1992). In arid climates, a lack of water probably restricts their distribution (Ward 1987; Kennedy 1998). In the colder states of the US, such as Minnesota and Illinois, Argentine ants cannot survive outside human modified landscapes (Suarez et al. 2001) where human activities and structures create warmer microclimates.
During cooler times of the year foraging will be restricted, as 10oC is the reported lower limit for foraging (Markin 1970). Foragers are most active when the temperature ranges from 10–30oC, and cease foraging when surface temperatures reach 32oC or drop below 15oC (Hedges 1998). Oviposition slows over the winter and development rates of eggs, larvae, and pupae slow. Oviposition does not occur below a daily mean temperature of 18.3oC, and the population reaches a seasonal low in numbers during winter (Vega & Rust 2001). Environments with high rainfall reduce foraging time and may reduce establishment chances (Vega & Rust 2001).
Within a site, distribution can be patchy and related to microclimate. For example, in a citrus orchard in California, 93% of colonies were found in the southwest (hotter) quadrant around orchard trees (Phillips 1986).
Habitat associations
Although frequently associated with human settlement, Argentine ants are not restricted to modified habitats and occur in native vegetation in a number of locations. In Hawaii, they are present in undisturbed montane habitat in dry and mesic areas, and shrubland/grassland sites, but not wet forests (Reimer 1993; Cole et al. 1992). In Southern California, they occur in scrub habitat fragments, and along the edge of a large continuous area of native vegetation (Suarez et al. 1998). However, 100 m into the fragments, L. humile numbers begin to decline. It is unclear if Argentine ants are still invading the fragments or not penetrating further into the scrub fragments because they lacked water (Suarez et al. 1998). In South Africa they have invaded the fynbos (Giliomere 1986) and in Chile the Matorral (Fuentes 1991).
Information is scarce on whether substrate influences the distribution of Argentine ants. In western and southern Portugal they are common in sand and clay loam soils, but largely absent in sandy loam soils associated with metamorphic rocks, despite otherwise favourable conditions (Way et al. 1997).
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