Spinosad is a relatively new insecticide with a unique mode of action that is being evaluated for control of larval mosquitoes. Whereas a number of toxicological studies have measured effects of spinosad on various animals, few have been conducted on the effects of spinosad on nontarget, aquatic insect species. Such studies are important as these species might be found in the same environments as mosquito larvae targeted for control. A neighborhood pond was surveyed to find a representative species of mosquito as well as other common aquatic insects with which to examine susceptibility to spinosad and nontarget effects. The mosquito species chosen was Culex quinquefasciatus and the most common nontarget taxa were immature stages of a mayfly (Caenis sp., Ephemeroptera: Caenidae), a damselfly (Ischnura sp., Odonata: Coenagrionidae), and a dragonfly (Pachydiplax longipennis, Odonata: Libellulidae). Bioassays of mosquitoes from a reference susceptible strain (Sebring-S) and field collections of Cx. quinquefasciatus were used to determine susceptibility to spinosad. In addition, susceptibility was examined in nontarget taxa using spinosad concentrations corresponding to the LC50 of field-collected mosquitoes (0.031 ppm) and the maximum label rate (1.6 ppm) of spinosad (Natular®EC). Susceptibility to spinosad did not differ between Sebring-S and field-collected mosquitoes. However, there was a marked difference in susceptibility among nontarget taxa. Susceptibility was greatest in Caenis sp., followed by Ischnura sp., and then P. longipennis.
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