Digging behavior of Solenopsis invicta workers when exposed to contact insecticides.

ABSTRACT Contact between ants and insecticides is a prerequisite for contact insecticides to be effective in the control of red imported fire ants, Solenopsis invicta Buren. Typically, passive contact occurs in the insecticide application process, but ants also may actively contact insecticides by digging in treated soil or walking on a treated soil surface. Laboratory experiments were conducted to determine whether fire ant workers would dig sand treated with contact insecticides in two different scenarios: (1) no-choice bioassays where insecticide-treated sand was the only available digging substrate, and (2) two-choice bioassays where nontoxicant sand was also available for digging. Eight insecticides that are currently registered in the United States for imported fire ant control were tested. They include acephate, bifenthrin, carbaryl, cyfluthrin, deltamethrin, gamma-cyhalothrin, permethrin, and pyrethrin. Workers dug the treated sand for every insecticide tested, even at concentrations up to 10 times of the lowest lethal concentration (LLC) which caused 100% mortality in a toxicity bioassay. However, generally, insecticides significantly reduced the digging effort, even at a concentration that did not cause any significant mortality in the toxicity bioassay.