Horizontal transfer of barrier insecticides in Argentine ants (Hymenoptera: Formicidae).

Barrier applications of sprays and granules have been widely used to control Argentine ants, Linepithema humile (Mayr), around structures. Their activity against ants, especially the pyrethroids, has been ascribed to toxicity and repellency. Recent studies have shown that bifenthrin and cyfluthrin produced rapid knockdown of foraging ants preventing them from establishing trails across treated surfaces. Slower-acting insecticides such as fipronil provided delayed mortality and the potential for horizontal transfer. Workers exposed to simulated barriers treated with fipronil, bifenthrin, and cyfluthrin for 1 minute and returned to colony boxes provided 95% kill of the colonies in 19.8, 110.1 and 68.6 days, respectively. In a similar test with workers that had been exposed to barriers and killed by freezing, fipronil, bifenthrin, and cyfluthrin provided 95% kill of colonies in 8.2, 78.6, and 54.9 days, respectively. The mortality of unexposed ants in the colonies was as follows: fipronil > bifenthrin > cyfluthrin = control. Barriers of each insecticide were applied at label rates around structures to corroborate these findings. The barriers provided mixed results. Reduced applications and narrow barriers (0.3 by 0.3 m) of fipronil around the foundation provided about 50% reductions for 8 weeks. Thorough applications of bifenthrin and cyfluthrin provided good control for 2 and 8 weeks respectively. Field studies were consistent with the laboratory observations of insecticide transference. The studies suggest that more directed applications of fipronil and bifenthrin to trails and nest sites might provide much better control that regimented barrier applications.