Population studies of spider mites (Acari: Tetranychidae), and their predators in corn following insecticide applications directed at the European corn borer, Ostrinia nubilalis (Hubner)

Field studies were conducted during 1992, 1993, and 1994 in corn fields in northeastern and western Nebraska to characterize the population dynamics of phytophagous spider mites and their predators following applications of methyl parathion and permethrin. Insecticide applications were timed for control of first and second generation European corn borer, Ostrinia nubilalis (Hubner), larvae. Densities of twospotted spider mites, Tetranychus urticae (Koch), significantly increased only in those plots receiving an application of permethrin for control of second generation European corn borer larvae. Banks grass mite, Oligonychus pratensis (Banks), densities did not significantly increase compared to control plots following any of the insecticide treatments. Mite predator numbers were generally low throughout the study and no significant changes in densities were observed between pre-treatment and post-treatment sampling. Field temperatures were sub-optimal for spider mite reproduction throughout the study period. These suboptimal temperatures might have altered some or all of the causative factors of spider mite increases. Laboratory studies were conducted to determine if dispersal and reproductive behavior of spider mites are affected when exposed to direct and residual sub-lethal concentrations of methyl parathion and permethrin. Results indicated increased dispersal of both species of spider mites at all concentrations. Mites showed greater dispersal when exposed to insecticides directly compared with residual exposure. There were significant differences in mite ovipositional rates between lower and higher insecticide doses. Reduced oviposition was observed at higher concentrations of both insecticides. Our studies show that neither destruction of mite predators nor pesticide-induced reproductive stimulation are important factors causing mite increases. However, insecticide-induced dispersal of spider mites might play a role in the spider mite outbreaks. Future research should focus on the effect of the insecticides on the fungal pathogens (Neozygites sp.) which can be an important mortality factor for spider mites under field conditions