Issue 38, July 2009
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Controlling Imported Fire Ants

Imported fire ants, native to South America, first appeared in Mobile, Ala., in the 1930s and have since advanced throughout the southern United States. These pests have a powerful sting, and their venom gives rise to the painful burning sensation for which the ants were named. Imported fire ants are extremely irritating to large animals, and they can be fatal for young livestock.

Fortunately, Agricultural Research Service (ARS) scientists are collaborating to combat this painful pest.

Much of this work has occurred at the ARS Biological Control of Pests Research Unit in Stoneville, Miss., the ARS Center for Medical, Agricultural and Veterinary Entomology (CMAVE) in Gainesville, Fla., and the ARS South American Biological Control Laboratory (SABCL) in Hurlingham, Argentina.

The ARS scientists at Stoneville have evaluated baits for controlling imported fire ants and have identified biological control strategies to reduce the ants’ numbers. They've also conducted surveys to compare the population sizes of native ants and imported fire ants in different regions and under different management strategies.

In one study, entomologist Douglas Streett is developing commercial biopesticides using insect-killing fungi. He is currently evaluating two registered fungal products to assess their effectiveness in controlling imported fire ants. This work can be used to develop novel fire ant controls and improve existing tools.

In related work, entomologist Jian Chen developed fire ant baits with improved water resistance. Many fire ant baits use soybean oil to attract the ants, and defatted corn grit to carry the oil and a delayed toxicant. Unfortunately, defatted corn grit lacks water resistance, which reduces the efficacy of many fire ant baits.

Chen developed novel methods for improving the ability of fire ant baits to withstand water damage. These methods not only improve the sturdiness of the baits, but also increase their ability to attract ants. Chen is now working with pest control companies to develop commercial baits employing these technologies.

At CMAVE, scientists led by chemist Bob Vander Meer are also working to improve fire ant targets using pheromone attractants specific to fire ants.

"Most baits affect both fire ants and non-target ant species, which are important predators of new fire ant queens," Vander Meer says. "Specifically targeting fire ants will decrease the likelihood of new infestations because native ants will be more likely to survive and attack newly mated fire ant queens."

Collaborative Control of Red Imported Fire Ants

Fungi aren't the only biological control agents ARS researchers are pitting against red imported fire ants. ARS researchers are also collaborating to assess potential biological control agents like phorid flies, microsporidia diseases and viruses.

Two pathogenic microsporidia, Kneallhazia solenopsae and Vairimorpha invictae, are associated with population declines of red imported fire ants in Argentina, according to CMAVE entomologist David Oi. And phorid flies parasitize fire ants with fatal results. Now, CMAVE and SABCL scientists are investigating the possibility of using the phorid fly as a vector for infecting the fire ant population with these pathogens.

Although preliminary data showed that V. invictae didn't successfully transmit to phorid flies effectively, CMAVE scientists were able to infect phorid flies with K. solenospae without harming the flies. The scientists observed that K. solenopsae reduced the size of the fire ant colony and the number of reproducing ants.

Understanding how fire ants behave in their native environment is crucial to developing effective control strategies. ARS scientists at CMAVE and SABCL have collaborated for more than 20 years to research the biology of red imported fire ants and assess potential biological controls.

Led by director Juan Briano, the SABCL staff has conducted more than 340 field trips in Argentina, Uruguay, Brazil, Paraguay, Bolivia and Chile for exploration and monitoring of fire ants and their natural enemies.

Despite the widespread effects of this pest, little is known about its competitive nature or how it interacts with other ants in its homeland. SABCL scientists have studied interactions between the red imported fire ant, Solenopsis invicta, and other aboveground foraging ants in two habitats in northeastern Argentina.

Prior to the study, it was thought that the fire ants—now established throughout the Americas—were not dominant in their native land, but this study showed them to be numerically and behaviorally dominant, winning 78 percent of their encounters with other ants.

For more information about ARS research on fire ants, contact Dan Strickman, Leader of ARS National Program #104: Veterinary, Medical and Urban Entomology.