[Research Update] Mystery Solved

Live Oak Pest Control Technical Director Jeff Lee faced the worst Argentine ant infestation he had ever seen in a North Florida hospital being treated by his company. To make the situation even more challenging, the hospital sat on a lake that had several species of fish, turtles and birds. The city and homeowners shared the lakefront with the hospital. There was even a public recreation trail around the lake.

"This property was particularly challenging," Lee said. "It wasn’t even summertime yet and hospital maintenance was getting six to eight calls per month to control the Argentine ant problem in the building. The infestation extended beyond the hospital property throughout an 11-block area. Ants were infesting the trees in the hospital parking lot. The previous pest control company that had the job already tried several different baits. Nothing had worked."

In response to the problem, Lee implemented a new control program. He corrected sanitation problems around Dumpsters® and cut tree limbs and shrubbery that were too close to the structure. He then treated around the hospital windows, door entries, and trees up to 10 feet above the foundation with Talstar® F insecticide/miticide (bifenthrin, formerly known as Talstar® Lawn & Tree Flowable insecticide/miticide) to prevent ants from accessing structures and tending aphids. He treated the hospital lawn with Talstar® PL Granular insecticide. He also treated the shrubbery with Orthene® (acephate) to control the aphids supplying the Argentine ants with honeydew.

Complaints about ants in the hospital decreased by more than 50 percent after Lee’s treatment, and stayed low throughout the summer. "I think we did a respectable job, considering that Argentines are a neighborhood problem around here. We had to limit our treatment to hospital property," Lee said. Post-treatment evaluation indicated that the Argentines were staying in the trees above the Talstar insecticide spray on the trunks. Superficially, this may have been an indication of a repellent treatment. However, university research points to a different phenomenon.

THE QUESTION. Investigators at the Louisiana State University (LSU) Ag Center, Department of Entomology’s Ant Laboratory wondered whether red imported fire ants (RIFA) and Argentine ants would readily cross barriers of Talstar F insecticide flowable and Talstar PL granular insecticides to reach a food source in the laboratory, so they conducted a study. They reported that neither RIFA nor Argentine ants avoided Talstar-treated soil. When ants were not given a choice (non-choice tests), the majority of "crosses" were successful and the numbers of RIFA and Argentine ants that successfully crossed into soil treated with Talstar F insecticide flowable were statistically similar to the numbers that crossed into untreated soil up to 90 minutes after testing began. At 24 hours in non-choice tests with Talstar flowable, no RIFA and very few Argentine ants crossed through soil treated with Talstar flowable.

When ants were given a choice (choice tests) between either untreated soil or soil treated with Talstar PL, tests resulted in findings similar to that of the non-choice tests. RIFA crossed through treated and untreated soils at statistically similar frequencies through 30 minutes of testing. Crossing frequencies through treated and untreated soil were similar for Argentine ants for up to 60 minutes.

Ants exposed to Talstar F died within 30 minutes, while those exposed to Talstar PL died within 15 minutes.

WHAT IT MEANS. These results indicated that within the first hour of testing neither ant species was repelled by Talstar soil treatments. However, neither species crossed into soil treated with the Talstar flowable 24 hours after testing began. RIFA stopped crossing into soil treated with the Talstar granular at 60 minutes and Argentines stopped at 90 minutes. Does this mean that Talstar exhibited delayed repellency? According to Dr. Michael Rust and graduate student Andrew Soeprono, both of the University of California – Riverside Urban Entomology laboratory, probably not.

Rust and Soeprono took their Talstar flowable investigation to the field. They placed treated and untreated pieces of plywood panels on the ground under orange trees in a grove known to be heavily infested with Argentine ants. Dead cockroaches were placed within the plywood boundaries to tempt ants across the treatments. A digital camera captured all the action at various times after panels were placed on the ground. Rust and Soeprono brought the panels back to the laboratory for storage, and then brought them out again for testing every two weeks over an eight-week period. They counted numbers of ants within treated and untreated areas utilizing the digital images.

"Talstar was highly toxic, suppressed recruitment, and was not repellent to new ant workers," Rust said. The two researchers found Argentine ant recruitment on treated and untreated panels similar for the first count. More significant was the fact that ant numbers on treated panels remained consistent over a six-week period. This indicated that Talstar did not repel ants. Rust and Soeprono, like investigators at the LSU Ag Center, found that Argentine ants died within 15 minutes of exposure.

The fact that Talstar was not repellent to ants was somewhat unexpected, since Talstar and other pyrethroids are known to repel termites. So why weren’t ants repelled by Talstar? Kim Watson of FMC Corporation thinks that it has to do with the unique properties of Talstar and the way different insects sense the environment. "We know that termites are repelled by pyrethroids. But other insects demonstrate different levels of repellency to different pyrethroids. For instance, we know that mosquitoes are repelled very strongly by some pyrethroids but demonstrate very little repellency to Talstar. Ants are in a group of insects that are not repelled by Talstar."

This is good news for professionals trying to control perimeter ants. Dr. Philip Koehler, Professor of Urban Entomology at the University of Florida, warns against the use of repellent chemicals to control ants. According to Koehler, "some of the chemicals currently registered for ant control are so repellent that large numbers of ants get trapped in the house because they won’t cross the treated area. Cutting a group of ants off from their colony may allow them to bud and continue what they started." A treatment that ants won’t avoid, especially one that suppresses recruitment, may be what the industry ordered.

The authors are product development associate for FMC Corporation, Philadelphia, Pa., and assistant professor of entomology at Louisiana State University Agricultural Center, Baton Rouge, La., respectively.