When fire ants were introduced into Alabama in the early 1900s, almost all their natural enemies were left behind in South America. As a result, fire ant densities in Alabama are much higher than they are in South America. But researchers with the AAES are finding new ways to control this pest, including the use of fly species that behead fire ants.

Two species of imported fire ant occur in Alabama. The red imported fire ant, Solenopsis invicta, is the more aggressive of the two species, and is located primarily in the southern portion of the state. The black imported fire ant, Solenopsis richteri, is now only found in a small population located in Northwest Alabama and Northeast Mississippi. Like native ants, the black imported fire ant has also lost the battle for territory to the red imported fire ant. A hybrid of the two species populates the northern part of Alabama.

One group of natural enemies showing promise in the battle against fire ants are phorid flies in the genus Pseudacteon. These flies have the unusual habit of decapitating fire ant workers. The adult fly deposits her egg inside the body of the fire ant. The developing maggot then migrates to the head of the ant. Just before the fly pupates, it causes the ant’s head to fall off. The fly pupa remains in the ant head until the adult fly emerges to mate and look for fire ant workers to attack.

Just before pupating, the phorid maggot pushes off the ant's head and uses the head capsule as a pupal case.

Phorid flies cannot decapitate enough fire ants to effectively reduce populations. However, fire ant workers are very aware of the presence of the flies. An attack by a single fly is enough to inhibit foraging by the workers. When the workers become aware of the flies, they run for cover or assume a C-shaped defensive posture that has only been reported when the ants are under attack by phorids (see photo at top of page). This reduction in foraging by the fire ant workers should allow native ants a better opportunity to compete with the imported fire ants.

The first attempt at establishing a population of the phorid fly, P. tricuspis, was made in a hybrid fire ant population in Talladega County in 1998. This initial release was a failure. To date, no P. tricuspis have been located in Talladega County.

In spring of 1999, a second attempt at establishing P. tricuspis was conducted in Macon County, where the local fire ant is the red imported fire ant. More than 2,000 flies were released during a two-week period near Notasulga, Alabama. A control site that had habitat features similar to the release site was established approximately six miles to the west of the release site. No flies were released at the control site. Observations were made at the release site during the summer, but a resident population of the flies was not observed until mid September.

The flies were sighted again in the spring of 2000, but numbers were low. By September, not only had fly numbers increased, the flies had dispersed more than five miles from the release site. Surveys for the flies in fall 2001 found them more than 11 miles from the release site. The original control site is now populated with the flies and estimates suggest that the flies have spread across more than 380 square miles in the Macon, Tallapoosa, and Lee county areas.

Ovipositor of P. curvatus.

Ovipositor of P. tricuspis. Phorid flies are host specific and only attack fire ants of a certain size range. The ovipositor used to inject the egg into the ant is designed to fit the correct size ant.

Due to the success with the Macon County release, another phorid species was released into the hybrid fire ant population in Talladega County. The phorid fly, P. curvatus, was released in the spring of 2000. As in Macon County, a control site with similar habitat features was located approximately six miles from the release site. In September, a resident population of the flies had become established. By late-summer of this year, the population had spread approximately a mile from the release site. This site is the first successful release of P. curvatus in North America.

In Houston County during September 2000 and in Lowndes County during May 2001, P. tricuspis was released again in red imported fire ant populations. Control sites in these counties were established approximately 20 miles from the release site due to the rapid spread of the flies from the Macon County site. There is a thriving population of phorid flies in Lowndes County, but the population in Houston County is still small.

Fire ant populations are sampled at each release site and at each control site two times a year. Mounds are counted at each site and located using GPS technology. Mound size is also measured. Numbers of foraging ants in the area are measured by two methods. One method of sampling is placing baits in a grid. Fire ant foragers usually recruit to a new food source in an area within 10 minutes. Baits are allowed to remain in the field for 30 minutes. Random foragers are also sampled using pitfall traps. Pitfall traps are placed in the same grid used for the baits and are allowed to remain in the field for three days. These traps collect ants and other arthropods when they fall into the traps filled with environmentally safe antifreeze. Data from the sites are currently being analyzed.

Anytime a new species is introduced, there are concerns about unintended consequences. Phorid flies have only been observed attacking ants, and they have never been reported to attack any other insect. In fact, the phorid flies that attack fire ants appear to be specific to fire ants. Their ovipositors, which are used for egg-laying, are so specialized that eggs are only deposited in ants within a specific size range. The adult flies only live from one to three days in the field.

This is a joint study with the AAES and the USDA-ARS in Gainesville, Florida, in cooperation with the Alabama Cooperative Extension System and the Tuskegee Extension System. All photos by Sanford Porter, USDA..

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