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Line drawing of a S. invicta worker. Modified from K-State Research and Extension, Kansas State Univversity.

  

Range expansion of S. invicta in the southeastern United States over the 80-year period, 1918-1998. Graphic courtesy USDA/APHIS.

Species Name: 
Solenopsis invicta Buren 1972

Common Name(s):
Red Imported Fire Ant (RIFA), Fire Ant, Red Ant

Species Description:
Macroscopic identification of the mounds of red imported fire ants (RIFA, Solenopsis invicta) is typically made by lay residents of Florida while in the midst of a hasty retreat once the mounds are accidentally disturbed and large numbers of irritated stinging red ants emerge to attack the source of the disturbance.

S, invicta soil mounds tend to be 46 cm or less in diameter (Cohen 1992). Red fire ant stings usually result in the formation of a white pustule within 24 hours.

Positive identification of individual RIFA is aided by confirmation of a red to brown body color, a two-segmented body pedicel ("waist"), a four-toothed mandible, a sting at the tip of the gaster (terminal abdominal segment), and 10-segmented antennae ending in two-segmented clubs (Hedges 1997).

Potentially Misidentified Species:
Florida is home to two different species of fire ant. In addition to the ubiquitous non-native red imported fire ant (Solenopsis invicta), Florida is also home to the less frequently encountered native tropical fire ant, S. geminata. Drees (1997) notes that S. geminata mounds can be distinguished by the presence of workers with disproportionately large square heads that are lacking in S. invicta worker ants.

A number of other fire ants of genus Solenopsis occur in the United States, including the black imported fire ant, Solenopsis richteri. Introduced to the U.S. in 1918, Solenopsis richteri co-occurs with S. invicta within a portion of its non-native distribution in the U.S. (e.g., Mississippi and western Georgia) and is capable of hybridizing with S. invicta (ARS 2003, Collins and Scheffrahn 2005).

There are more than 280 species of Solenopsis worldwide.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
S. invecta is a southern South American species native to Brazil. It was likely introduced to the United States in the 1920s and 1930s and to parts of Australia in the 1970s. The species has also become established in New Zealand, Puerto Rico, and the Virgin islands. S. invecta now infests more than 320 million acres in 12 states in the southeastern U.S. and Puerto Rico. The species has also recently been reported from California and New Mexico (Holway et al. 2002, ARS 2003).

IRL Distribution:
S. invicta occurs throughout the entire state of Florida, including all six counties within the IRL watershed.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Solenopsis invicta is a small ant, with workers averaging 3-6 mm in length.

Abundance:
Vinson & Sorenson (1986) report that a mature S. invicta colony can have nearly one-quarter million workers while typical colonies have about one-third as many. These authors also estimate that in the southern U.S. where S. invicta occurs, nearly 100,000 new queens per year can be produced in every acre of infested land.

Reproduction:
Winged reproductive males called alates are produced once a colony is a year old. As many as 4,500 alates join winged reproductive female alates (the new queens) to take part in a mating flight during which the females are fertilized in the air. The males die shortly thereafter and the fertilized females establish new colonies. Six and eight mating flights in the spring and fall months are typical for mature colonies(Vinson and Sorenson 1986, Collins and Scheffrahn, 2005).

Newly mated queens will often cluster together in sheltered areas after mating and may cooperate to establish a new colony. Vinson and Sorenson (1986) indicate that multiple-queen S. invicta colonies do occur in some cases, while in other cases all but one queen will die off as the colony matures (Collins and Scheffrahn, 2005).

Embryology:
After mating, a female alate will lose its wings and locate a suitable spot for a new colony. After burrowing into the soil and sealing herself into a self-excavated chamber, the young queen will lay an initial clutch of usually 10-15 eggs. These will hatch into the first cohort of sterile female workers in 8-10 days, during which time the queen will have produced on the order of 100 more eggs (Holldobler and Wilson 1990, Collins and Scheffrahn 2005).

As with most insects, S. invicta early life history includes both a larval and a pupal stage. Larvae are the immature, wormlike first feeding forms that emerge from the eggs and pupae are subsequent quiescent stage prior to metamorphosis. In S. invicta each of these stages persists for approximately 1-2 weeks, and the initial cohort of worker ants becomes mature 2-4 weeks after hatching. Until this first clutch grows to maturity, the young queen will refrain from laying more eggs. The queen provides nutrition to the first batch of developing young in a number of ways, including the production of non-developmental trophic eggs, regurgitation of energy-rich oil, salivary secretions, and via breakdown of the now-unneeded wing muscles. All of these sources of nutrition are used to provide nutrients to larvae and pupae of this initial clutch (Vinson & Sorenson 1986, Collins and Scheffrahn 2005). Because the young queen can provide only limited nutrition these initial workers are notably smaller when they emerge than are successive cohorts. On emerging, however, this group of workers sets to work foraging for food for the queen and the next generation of larvae.

Temperature:
Low temperatures appear to be key to limiting northward expansion of the range of Solenopsis invicta in the United States. Ecological models predict that the species will fail to become established in regions where the soil persists at near-freezing temperatures for more than a couple of weeks (ARS 2003).