Synergism of tebufenozide in resistant and susceptible strains of obliquebanded leafroller (Lepidoptera : Tortricidae) and resistance to new insecticides

Cross-resistance of the obliquebanded leafroiler, Choristoneura rosaceana ( Harris), to tebufenozide was reported from laboratory studies before it had been used in commercial orchards in New York State. Bioassays with oblique banded leafroller larvae from tebufenozide and organophosphate susceptible and resistant colonies were conducted with chlorfenapyr, emamectin benzoate , fenoxycarb, fipronil, spinosad, and tebufenozide to determine if cross-re sistance was present before these new insecticides were introduced into com mercial orchards. Resistance ratios ranged from 1.1 to 3.2 for all insectic ides except tebufenozide (12.8). Significant differences between susceptibl e and resistant colonies were found with emamectin benzoate, fenoxycarb, an d fipronil. The effect of the metabolic synergists piperonyl butoxide (PBO) and diethyl maleate (DEM) on tebufenozide toxicity was examined to determi ne mechanisms for obliquebanded leafroller resistance to tebufenozide and p otential mechanisms for other new insecticides. At a concentration of 20 pp m, PBO and DEM significantly synergized the toxicity of tebufenozide in res istant and susceptible colonies (three- to fourfold). Obliquebanded leafrol lers may be resistant to new insecticides with distinct modes of action eve n if these compounds have not been previously used in commercial orchards.