Baseline monitoring of codling moth (Lepidoptera : Tortricidae) larval response to benzoylhydrazine insecticides

A diet-incorporation larval bioassay was developed to measure the response of codling moth, Cydia pomonella (L.), to the benzoylhydrazine insecticides tebufenozide and methoxyfenozide. The bioassay tested neonates and third, fourth, and fifth instars from a laboratory colony and neonates and fourth instars from a pooled population collected from five certified-organic appl e orchards. Bioassays were scored after 6 and 14 d. No differences between the laboratory and field population were found for either insecticide. Sign ificant differences were found in the response of third and fifth instars b etween the 6 and 14 d bioassays, primarily due to a high proportion of mori bund larvae in the shorter assay. Larval age had a significant effect in bi oassays and was more pronounced in 6- versus 14-d tests. Fifth instars were significantly less susceptible to both insecticides than other stages, whi le responses of third and fourth instars were similar. The response of neon ates was significantly different from third and fourth instars to tebufenoz ide but not with methoxyfenozide in the 14-d test. Field bioassays excluded the use of fifth instars and were scored after 14 d. LC(50)s estimated for 18 field-collected populations varied five- and ninefold for tebufenozide and methoxyfenozide, respectively. The responses of all but six field-colle cted populations were significantly different from the laboratory strain. F ive of these six populations were collected from orchards with no history o f organophosphate insecticide use. The LC50 for methoxyfenozide of one fiel d-collected population reared in the laboratory for three generations decli ned fourfold, but was still significantly different from the laboratory pop ulation. These data suggest that transforming current codling moth manageme nt programs in Washington from a reliance on organophosphate insecticides t o benzoylhydrazines may be difficult.