Ovipositional responses of two species of spider mites (Acari: Tetranychidae) to sublethal concentrations of permethrin and methyl parathion on corn

Reproductive stimulation of pests by sublethal dosages of insecticides fits into the general pharmacological hypothesis of hormoligosis. This phenomenon has been suggested as one of the factors causing insecticide-induced spider mite outbreaks. The ovipositional response of twospotted spider mite, Tetranychus urticae Koch, and Banks grass mite, Oligonychus pratensis (Banks), was examined using leaf discs and whole corn, Zea mays (L.) plants. Spider mites were exposed to various sublethal concentrations (LC05, LC10, LC25, and LC50) of permethrin and methyl parathion on corn. Studies using leaf discs were conducted by exposing the mites either directly to the insecticides or to their residues. The whole plant study dealt with only residual exposure. The cumulative number of eggs laid by Banks grass mites, when exposed to insecticide concentrations at LC05 and LC10 levels, was significantly lower than the untreated control under all treatment conditions. In contrast, twospotted spider mites did not show a reduction in oviposition at LC05 and LC10 levels. At higher concentrations (LC25 and LC50), both insecticides caused a significant reduction in the cumulative number of eggs laid by both species of spider mites. Spider mites directly exposed to insecticides laid significantly fewer eggs than mites that were exposed to the insecticide residues. The results of these studies indicate that hormoligosis is not a contributor to mite outbreaks that occur following applications of permethrin or methyl parathion at the rates tested.