Edward Vargo is a professor of entomology at N.C. State. Here he explains why summertime encounters with fire ants are no picnic. Questions and answers have been edited.
Fire ant stings are painful due to certain chemicals called alkaloids that kill cells they come into contact with. Fire ants are unusual in producing this class of compounds, which are normally produced by plants. Well-known plant-produced alkaloids include caffeine and nicotine. Fire ants inject their venom with a sting located on the tip of their abdomen. Although fire ants do bite, the bite is largely painless, and it’s the sting that causes the burning sensation.
Fire ants originally come from South America and were introduced through human trade into Mobile, Ala., in the 1920s before spreading throughout the Eastern and Central United States. Most ants come in either one of two flavors of social structure: colonies with a single queen per nest or colonies with multiple queens per nest. Fire ants are unusual in that they have both social forms, and these two forms occur in North Carolina. The multiple-queen form has much higher mound density, so it supports a much larger population and therefore causes more problems. It is also harder to control since successful control of a colony requires killing all the queens – which in the case of fire ants can number in the hundreds.
Many ranchers have noticed that tick and chigger populations have declined after fire ants invade an area, so they seem to do a good job of getting rid of some other noxious pests. Scientifically they have been a valuable study system for learning more about the role of ants in ecosystems and the biological mechanisms underlying the social behavior we see in ants, bees, some wasps and termites.
Fire ant queens are pretty amazing. Each queen mates with a single male and does so about 100 feet in the air during the so-called nuptial flight. After mating, the queen lands, breaks off her wings and starts a colony on her own; having stored her mate’s sperm in a special organ, she has no more use for the male. In a mature colony, a queen is an egg-laying machine. She can lay her own weight in eggs every day – about 2,000 eggs!