Seth Johnson, Don Henne, Anna Mészáros and Lee Eisenberg

The red imported fire ant invaded the United States from South America more than 75 years ago. It was first discovered in Louisiana in the early 1950s. Because of its aggressive behavior and lack of natural enemies, the red imported fire ant has expanded its range into at least 13 states and Puerto Rico. It is present throughout most of the southeastern United States, where it is considered an economically important pest.

Early efforts to eradicate fire ants with chemical controls were met with limited success, and it was eventually recognized that alternative control tactics were required for effective management. Because the red imported fire ant invaded the United States without its natural enemies, attention has recently focused on the potential for biological control by importing specialist parasitoids in the phorid fly genus, Pseudacteon.

In South America, where fire ant populations are one-fifth of those in the United States, at least 24 species of Pseudacteon attack fire ants. These parasitic flies locate their host by detecting alarm pheromones that worker ants release when the mound is disturbed or when the ants are fighting with other ants or among themselves. These tiny flies, which are less than 1.5 millimeters in length, are known as decapitating flies. They attack fire ants exclusively. These include red and black fire ants and their hybrids. The female phorid fly lays a single egg in the thorax of a worker ant. When the egg hatches, the maggot moves into the head and consumes the head contents. The maggot releases a specific enzyme that eventually causes the decapitation of the ant, and the larva uses the empty head capsule as a pupal case.

Different species of phorids attack host ants at different times of day in different locations and prefer different host fire ant worker sizes. Pseudacteon flies are effective in disrupting foraging activity of fire ants, allowing for more balanced competition among ant species. These decapitating flies will not eradicate fire ants completely, but they could theoretically reduce the red imported fire ant competitive dominance in ant communities and allow native ant species to better compete for resources. If this happens, North American red imported fire ant populations could drop up to 90 percent to levels more similar to those found in South America. In Louisiana, so far, we have released four phorid species: P. tricuspis, P. curvatus, P. litoralis and P. obtusus (Figure 1).

The first successful release of P. tricuspis in the U.S. was by the U.S. Department of Agriculture Agricultural Research Service in Florida in 1997. The first successful release with P. tricuspis by LSU AgCenter scientists was northeast of Covington in 1999. Releases with P. tricuspis were made at seven locations (twice at the same site in Natchitoches Parish) around the state from 1999-2006, with establishment confirmed in St. Tammany, East Feliciana, Natchitoches and Vernon parishes (Figure 2).

The flies for the first five P. tricuspis releases were provided by the USDA-ARS lab oratory in Gainesville, Fla. Flies for the sixth release came from the USDA’s Animal, Plant, Health Inspection Service and the Florida Department of Plant Industry rearing program. Flies for the successful seventh and eight releases came from the LSU AgCenter’s own small-scale rearing program from 2003-2006. The founding flies for the AgCenter colony came from the USDA-APHIS and FL-DPI rearing program. Successful P. tricuspis releases were made by releasing 30-40 flies at each of 10 disturbed mounds daily for two weeks. Mounds were continually disturbed for two hours to keep the worker ants on the surface and available for parasitization.

AgCenter scientists made three successful releases with P. curvatus in Madison, East Feliciana and Natchitoches parishes (Figure 3). A single unsuccessful release of P. litoralis was made in East Feliciana (2006) with flies from the USDA-ARS lab in Gainesville, Fla. P. obtusus was released in Natchitoches (2009) and Iberville (2010) parishes. Establishment has not been confirmed for the 2009 release. The P. curvatus and P. obtusus releases were conducted in cooperation with the USDA-APHIS and FL-DPI rearing program. Colony fragments of worker ants were collected from Louisiana release sites, shipped to Gainesville, Fla., exposed to attacking flies for several days and shipped back for release into the mound where they originated. The colony fragment method was also used for the P. litoralis release, but only the largest workers were shipped to Florida for exposure to attacking flies because of the preference for this worker size class.

Field and laboratory studies were conducted on the ecology of P. tricuspis in Louisiana. AgCenter scientists found the pattern of range expansion by P. tricuspis involved both neighborhood diffusion and a long-distance dispersal pattern, known as stratified or jump dispersal. The annual rates of spread were a few miles in the first two years. The rates increased rapidly in years 3 to 4 and slowed down or leveled off by years 5 to 6 at 9-15 miles per year, with northward spread being about 40 percent greater than the spread in the other directions. Hourly fly abundance gradually increased through the morning to a peak at midday and gradually declined in the evening.

Hourly fly abundance was positively correlated with wind speed and negatively correlated with relative humidity and dew point temperature. Peak seasonal abundance occurred in the late summer and early autumn. Mass-release resighting experiments found a dispersal pattern in the short-term consistent with a simple diffusion model. Within two hours of release 50 percent of the flies dispersed less than 10 yards and 95 percent dispersed less than 30 yards. The distribution of this species at a location over time was largely random. However, high P. tricuspsis counts were associated with areas of polygyne (multiple queens) host colonies, and there were occasional aggregations into discrete patches with distribution gaps between these patches. Thus, population surveys of P. tricuspis should be conducted in several areas at a location to account for patchiness in fly populations and varying densities and social forms of fire ant colonies.

The most fascinating discovery involved the fate of the parasitized fire ant worker. AgCenter scientists found through intensive observation that parasitized workers remained inside the nest during parasitoid larval development and left the nest 8-10 hours before decapitation. Parasitized ants were highly mobile after they left the nest and ultimately entered the thatch layer at the soil surface where they were decapitated, and the fly maggot pupated. The term "zombie" fire ant workers was coined to characterize the behavior while under parasitoid control.

In 2009, AgCenter scientists conducted a survey of phorid fly distribution in Louisiana. All 64 parishes were sampled at a total of 137 sites. At each sampling site, 10 fire ant mounds were selected and sampled by disturbing the mound, crushing ants with a shovel to allow release of alarm pheromone, and collecting the phorid flies that showed up with an aspirator for two to three minutes at each mound (Figure 4). Scientists found P. tricuspis in 46 out of 64 parishes. It was not found in a tier of 13 northern parishes or in five parishes in the south central part of the state including Vermilion, Acadia, Evangeline, Lafayette and Allen. The parishes with the highest counts were Vernon, Natchitoches, Sabine, St. Helena, Livingston, St. John the Baptist and West Baton Rouge (Figure 2).

Scientists found P. curvatus in 57 of the 64 parishes. It was not found in Avoyelles, Evangeline, Allen, Acadia, Vermilion, Assumption and St. James. The parishes with the highest counts were in the north central part of the state – Union, Lincoln, Jackson and Caldwell – and Washington in the southeast (Figure 3). Even though the first P. curvatus release occurred six years after the first P. tricuspis release, P. curvatus was already more widely distributed. This is due to a reported dispersal rate of twice that of P. tricuspis, and spread from Texas and Mississippi releases. Populations in southwest Louisiana probably came from releases in Texas, and the southeast and northeast Louisiana populations were aided by releases in Mississippi. Although dispersal was slowed by the cold this past winter, every parish in the state is expected to have at least one of the fly species by the end of 2010. The successful establishment and spread of P. tricuspis and P. curvatus in Louisiana warrants further research on additional phorid species, pathogens and other natural enemies that can be safely released for biological control against the red imported fire ant in the United States.

A previous study in Florida attempted to measure the effect of phorid fly releases on the red imported fire ant after P. tricuspis was established there. The study showed that P. tricuspis alone exerted no significant impact on fire ant populations. It was concluded that multiple species of phorids would be required, as naturally occurs in South America, before fire ant populations would be reduced.

AgCenter scientists are in the second year of attempting to evaluate the impact of two established species of phorids on fire ant populations in Louisiana. However, the study may be ended early because the control area, Vermilion Parish, may be overrun with one or both phorid fly species in 2010. Regardless of the results of this study, additional species of phorid flies will soon be available for release in Louisiana. Pseudacteon cutellatus is currently in quarantine in Gainesville, Fla., awaiting approval for release, and P. nocens is being reared and released in Austin, Texas. In addition to the established phorid species, P. cutellatus and P. nocens are expected to increasingly reduce red imported fire ant populations in Louisiana and help mitigate their negative effects on humans, livestock and wildlife.

Seth J. Johnson, Professor, Department of Entomology, LSU AgCenter; Donald C. Henne, Assistant Professor, Texas A&M AgriLife Research-Weslaco, Texas; Anna Mészáros and Lee J. Eisenberg, Research Associates, Department of Entomology, LSU AgCenter, Baton Rouge, La.

(This article was published in the fall 2010 issue of Louisiana Agriculture.)