Fire Ant

The red imported fire ant’s sting is built around venom that is about 95 percent solenopsins, water-insoluble alkaloids, with the rest toxic proteins. When a mound is disturbed, hundreds of workers pour out and climb onto the intruder. Each ant grips the skin with its mandibles, curls its abdomen under its body, and drives in its stinger. A single ant can withdraw the stinger, pivot its head, and sting several more times, often leaving a circular pattern of sting sites. Stinging typically begins 10 to 20 seconds after the ants climb on, and many ants seem to sting at once. The result is burning pain and, within hours to a day, sterile white pustules that can last several days. Roughly 14 million people in the United States are stung each year.

When floodwater drives red imported fire ants from their nest, workers link together into a living raft. They grip one another’s legs and jaws, and a raft can form in under two minutes. Workers make up the bottom layer, where fine body hairs trap air that keeps them from being fully submerged. The queen and the larvae are carried to the center, where they stay dry on top of the ant mass. Rafts have been recorded floating for 12 days, and some accounts describe survival of up to several weeks, though the colony must reach dry land to restart. This cooperative behavior lets a colony survive floods and, once ashore, build a new mound.

Red imported fire ants are omnivores with a flexible diet. Insects are their preferred food, but they also eat other arthropods, seeds, carrion, small vertebrates, and sugary liquids such as plant sap and honeydew. In South America, competition, predators, parasites, and diseases help keep their numbers in check. Many of those controls are absent in the United States, which is thought to help them spread, and populations reach about five times the density per acre of their native range. Fire ants have excluded many native ant species through competition and predation. In roadside sites in Florida, the native fire ant Solenopsis geminata was present at 83 percent of sites without red imported fire ants, but only 7 percent of sites where they had arrived.

Red imported fire ant colonies come in two forms. Monogyne colonies have a single queen, and their workers are territorial, attacking intruders from neighboring nests. Polygyne colonies have multiple queens, and their workers move freely between mounds without attacking neighbors. This difference changes how crowded the land becomes. Polygyne areas in the United States can have 100 to 1,000 mounds per acre (250 to 2,500 per hectare) and about 40 million ants per acre, compared with 20 to 80 mounds and about 7 million ants per acre for single-queen colonies, the form typical of South America. Multiple-queen colonies are rare in the ant’s native range. Polygyne queens are lighter and lay fewer eggs each, and these colonies produce fewer winged reproductives.

Fire ants coordinate largely through chemical signals. When a forager finds food too large to carry, it lays a trail on the way home with a pheromone called Z,E-alpha-farnesene, made in a gland attached to the stinger, and recruited nestmates follow that trail to the food. An alarm pheromone, a pyrazine compound, makes workers move rapidly and erratically, although this system is described as poorly developed in workers. When a mound is disturbed, hundreds of workers can pour out, run up grasses and other vertical surfaces, and sting an intruder together. Scientists are still unsure what makes them sting in unison. Workers also defend the nest by moving the queen away from danger. Together, these behaviors make a colony an effective defender of its territory.

New colonies begin with nuptial flights. On warm, sunny days after rain, winged males and virgin queens take to the air and mate several feet above the ground, and mating can last up to 30 minutes. Males die soon afterward. A mated queen sheds her wings and builds a small brood cell 1 to 2 inches (2.5 to 5 cm) below the surface, where she lays 10 to 15 eggs and feeds the first larvae herself. Several queens may share a cell, but normally only one survives. Flights occur six to eight times between spring and fall, with up to 4,500 winged reproductives in each. Flights of mated females help the species spread naturally by roughly 8 km (5 mi) a year.