re, the U.S. Congress appropriated $2.4 million in 1957 for control and eradication efforts. As part of an overall plan quarantine was imposed to retard or prevent the artificial dissemination of these now notorious pests. On May 6th 1958, regulations governing the movement of nursery stock, grass sod and some other items were instituted through the Federal Quarantine 301.81. By that time, however, imported fire ants had moved into 8 southern states. This spread, although slowed considerably by federal regulations and climatic conditions, continues even today. In recent years, isolated infestations of imported fire ants have been found as far west as New Mexico, Arizona California and as far north as Kansas and Maryland. In the case of California it is thought that the spread of these ants into this state was due to movement of bee hives as most initial infestations were found in around orchards and other situations where bees were moved into these areas for crop pollination. Based on USDA prediction these ants have a potential of even expanding the spread throughout much of the United States.
Current and potential distribution of imported red fire ant. Image courtesy of USDA.
Adult fire ants (Genus Solenopisis) are characterized by the presence of a ten-segmented antennae, two-segmented antennal club and two-segmented waste that joins the thorax to the abdomen which is typically darker than the rest of the body. Species identification is somewhat more difficult due to hybridization between the 2 "native" species as well as between the 2 imported species.
Diagram of diagnostic characteristics of red imported fire ant.
A mature colony of fire ants typically consists of 4 main stages: egg, larva, pupa, and adults. Since these are true social insect there is a caste system with different shaped and sized adults that carry out different function within the colony. These adult forms include the polymorphic workers, winged males, winged females and one or more reproductive queens. The egg, larval, and pupal stages (Figure 3) occur within the underground nest and are only seen when nests are disturbed or when they are being carried to a different location by workers (Figure 4). The eggs are small but can be seen with the unaided eye. They hatch into the grub-like legless larvae that are fed by the workers. Ant larvae in general are legless, carrot shaped with a well-developed unpigmented head capsule and are typically hooked at the head end. The larvae will grow and molt (shed their exoskeleton) 3 times prior to molting into the pupae, which are similar in appearance to the adults except that their legs and antennae are held tightly against the body. As with the larvae the pupae lack pigmentation, especially in the early instars. As they mature they begin to turn darker. As discussed in honeybees, the function of the polymorphic workers is determined primarily by age (and to a much lesser degree by size). Younger workers typically care for and feed the brood while middle-aged workers maintain and protect the colony. The oldest workers forage for food.
Figure 3. Egg, larval and pupal stage of fire ants. Image courtesy of USDA Archives.
Figure 4. Worker fire ants carrying larva for relocation. Image courtesy of USDA, Calif. Dept Agriculture.
The alates, or winged reproductives (Figure 5), are most abundant in the late spring and early summer, but can be found at any time of the year. The males are decidedly smaller than the females, glossy black and have a smaller head. Although both winged males and females can be found in the same colony, as a general rule one form will be dominant. It is possible that this functions to increase the chances of one sex mating with another from a different colony. Most colonies in a given area typically swarm on nuptial flights at the same time. This benefits the species and insures mixing of the gene pool. It is well-documented that inbreeding is generally detrimental to a given species.
Figure 5. A winged reproductive of the red imported fire ant. Image courtesy of USDA Archives.
Mating flights