Fire Ant

The red imported fire ant, Solenopsis invicta, is a small but highly organized ant in the family Formicidae, native to South America and now established across much of the southern United States and other warm regions worldwide following accidental introduction. It is one of the best-known invasive ant species due to its aggressive colony defense and distinctive mound-building habit.

Fire ants live in large, highly social colonies with a caste system of queens, workers, and reproductive alates (winged ants), and a single colony can contain tens of thousands of individuals. Colonies construct visible, dome-shaped earthen mounds, particularly noticeable in open, sunny turf after rain.

Ecologically, fire ants are opportunistic omnivores and aggressive competitors, often displacing native ant species in areas where they become established, which has made them a notable subject of ecological study as an invasive species.

• Body: reddish-brown to copper-colored head and thorax with a darker abdomen; workers show noticeable size variation (polymorphic) within a single colony.
• Size: workers range roughly 2–6 mm.
• Waist: two-segmented petiole connecting thorax to abdomen, a general ant characteristic.
• Mounds: builds loose, dome-shaped dirt mounds without a visible central entrance hole, often 20–45 cm across.
• Lookalikes: confused with other reddish ant species, but the combination of a fluffy, crater-less mound and rapid, coordinated swarming response to disturbance is characteristic.

Fire ants favor open, sunny areas with well-drained soil—lawns, pastures, roadsides, and agricultural fields—where they build their characteristic mound nests. In its introduced range across the southern United States it is common from the Carolinas west to Texas and California, and it also occurs in parts of the Caribbean, Australia, and Asia.

Colonies are active year-round in warm climates, with foraging activity increasing in spring and fall when temperatures are moderate, and mounds becoming more prominent after rainfall as ants move soil upward.

Fire ants are aggressive, highly coordinated foragers that feed on a wide range of food including insects, other invertebrates, seeds, and sugary substances, making them opportunistic omnivores. Workers communicate using pheromone trails to recruit nestmates rapidly to a food source or to a perceived threat.

When a mound is disturbed, large numbers of workers emerge quickly and can sting in defense of the colony, a coordinated response that is one of the species' most notable behavioral traits. Within the colony, fire ants also engage in cooperative brood care and can relocate an entire mound in response to flooding or disturbance.

Fire ants undergo complete metamorphosis: egg, larva, pupa, and adult. A queen lays eggs continuously within the colony, and larvae are fed and tended by worker ants before pupating.

Colonies can be either single-queen (monogyne) or multiple-queen (polygyne), affecting colony size and mound density. Winged reproductive males and females periodically leave the nest in mating flights, after which fertilized queens shed their wings and attempt to found new colonies, which can grow to full size within one to two years under favorable conditions.