Topical toxicity of imidacloprid, fipronil, and seven conventional insecticides to the adult convergent lady beetle (Coleoptera: Coccinellidae)

Abstract

The relative toxicities (comparing LD50 and LT50s) of two synthetic pyrethroids (cypermethrin and fenvalerate), two organophosphorus insecticides (chlorpyrifos, diazinon), three carbamates (propoxur, carbaryl, bendiocarb), a phenylpyrazole representative (fipronil), and a heterocyclic nitromethylene representative (imidacloprid or NTN 33893) were assessed with topical bioassays in the laboratory against the convergent lady beetle, Hippodamia convergens Guerin Meneville. LD50 values decreased (i.e., toxicity increased) with an increased time after application of a specific insecticide. The differences between the LD50 values caused by various insecticides were significant. Among tested insecticides, cypermethrin and bendiocarb were the most toxic; fipronil was the least toxic. H. convergens responded differently to different insecticides within the same class. Beetles exhibited similar responses to both organophosphorothionates chlorpyrifos and diazinon 24 to 72 h after application. Of the carbamates, propoxur was 2.4 and 3.5 times less toxic than carbaryl and bendiocarb, respectively. Of the pyrethroids tested, cypermethrin was significantly more toxic than fenvalerate. At 800 ppm, cypermethrin and bendiocarb were the fastest in killing H. convergens among the tested insecticides. The ranking of insecticides in decreasing order of LT50 values was as follows: fipronil > diazinon > chlorpyrifos > propoxur > carbaryl > fenvalerate and imidacloprid.