Matthew Aubuchon, Gary Mullen, and Michelle Perdue

For Alabama cattle producers, the red imported fire ant (Solenopsis invicta Buren) is a widespread and often troublesome pest in pastures. Control of this insect is desirable, but conventional methods can be expensive and labor-intensive. Most even require that cows be removed from the pastures during and after treatment.

The only commercially available ant bait registered for use in grazed pastures is Extinguish®. The product, registered in 1998, contains the insect growth regulator S-methoprene. It may be applied without removing cattle during treatment.

In research over the past year, AAES entomologists have been looking at the development of management plans that employ efficient application of environmentally safe insecticides to reduce treatment costs and labor for landowners. For this particular study, the research objective was to evaluate the effectiveness of Extinguish® in both whole-plot applications at a rate of one pound of bait per acre and perimeter applications at one-fourth pound of bait per acre to determine which method and application volume offers significant fire ant reduction.

The tests were conducted on privately owned grazed pastures in Chambers County. Nine one-acre plots were divided into three whole-plot treatments, three perimeter treatments, and three untreated controls. Using a backpack Global Positioning System (GPS) unit, researchers counted and mapped all fire ant mounds. They then applied Extinguish® with a Toro® bait spreader that provided a 10-foot treatment swath. Twenty swaths were run inside the whole-plot treatments, delivering one pound of bait per acre. One swath was run along the borders of the perimeter plots, delivering one-fourth pound of bait per acre.

The differences in fire ant control were impressive. In the plots receiving whole-plot applications of Extinguish®, the average number of mounds decreased by 31% at eight weeks following treatment and 85% at 16 weeks following treatment (see figure). The average number of mounds within plots receiving perimeter applications, meanwhile, dropped by only 9% at eight weeks and 23% at 16 weeks. However, the average number of ant mounds in the perimeter plots was similar to the average mound count in untreated controls.

Average number of fire ant mounds per treatment plot.

Recording amount of fire ant activity for individual mounds.

Whole-plot applications of Extinguish® at one pound of bait per acre proved effective for gradual control of fire ants in grazed pastures, with highest mortality measured four months after treatment. Perimeter applications of Extinguish® using the reduced application rate of one-fourth pound of bait per acre did not provide effective control of fire ants within the treatment plots. While perimeter applications may be effective on a smaller scale, they may not be practical for pasture situations.

Future trials with perimeter treatments using wider application swaths and alternative volumes of bait should be conducted to determine more suitable application patterns to achieve better management of fire ants with methoprene ant baits in grazed pastures.