Steinernematid Nematode Drenches for Control of Fire Ants, Solenopsis invicta, in Florida

Abstract

Recent developments in low-cost, large-scale production of entomogenous nematodes of the genera Steinernema and Heterorhabditis (Bedding 1984) have facilitated largescale field testing against various soil-infesting insect pests (Kaya 1985, Poinar 1986), with good potential for commercial control in some applications. Early research by Poole (1976) and Quattlebaum (1980) showed promise that entomogenous nematodes have potential for controlling fire ants, Solenopsis spp. Furthemore, these nematodes have the ability to invade adult insects, and thus potential to kill fire ant queens. Agents which only kill worker ants do not effectively eliminate colonies, although they may temporarily reduce their size. A variety of entomopathogens, including viruses, bacteria, fungi, protozoa, and other species of nematodes have been evaluated for fire ant control. These organisms, with the possible exception of a fungus (Beauveria sp.) isolated from fire ants in Brazil (Stimac et al. 1987), produced little or no mortality in laboratory and/or field tests. Jouvenaz (in press) recently reviewed these evaluations and discussed the special problems attending the development of biological formicides. We selected a strain of Steinernemafeltiae produced by Biosys Inc. (Palo Alto, CA) for field evaluation after screening several strains of Steinernema and Heterorhabditis spp. in the laboratory. In the laboratory tests, groups of ants composed of queens (from a polygynous colony), about 25-30 workers, and a small quantity of brood were placed in 16 x 125 mm glass culture tubes containing 25 mm of slightly moistened sandy soil. The tubes were capped with plastic tops which allowed gas exchange and were maintained at 290 C. After two or three days of acclimation, one ml of deionized water without nematodes or containing 5,000 or 50,000 nematodes was added to each tube, and survival of queens was scored daily for four days. Ten replicates (a total of 50 queens) were used for each nematode concentration and for the control. After 96 h, the strain we selected had produced queen mortalities of 40% and 58% in 5,000 and 50,000 nematode dosaged, respectively. Mortality of workers and brood, although not quantitatively determined, was estimated to be at least 80%. Mortality of queens, workers or brood was not observed in the control group. In separate tests we observed that colonies of ants vacated soil treated with high concentrations of nematodes. The first of the two field evaluations reported here was conducted in October and November, 1987, at Site No. 1 in southeast Gainesville, Alachua Co., Florida. This site was a utiities right-of-way measuring about 365 x 23-30 m, with sandy soil and surrounded by trees. The site had been mowed regularly prior to the test, but was not mowed during the test period. The second test was conducted during June and July, 1988, at Site No. 2, 4 mi north of LaCrosse, Alachua Co., Florida. This site was an approximately rectangular plot 1.4 ha in size on a sod farm and was also bordered by trees. On both plots, the grass varied from 10-30 cm high, and was not mowed during the test period.