Differential susceptibility to abamectin and two bioactive avermectin analogs in abamectin-resistant and -susceptible strains of Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae)

As resistance to currently used insecticides increases in the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), abamectin and its 4″-epi and 8,9-oxide analogs may serve as likely replacements if proven effective. We previously selected an abamectin-resistant strain of CPB (AB-F) that is suitable for the determination of cross-resistance to these two bioactive avermectin analogs. Using bioassay and logit analysis, the present work shows that, on average and following normalization by weight, the larval stages of the insecticide-susceptible SS strain are equally sensitive to the toxic action of abamectin and its 4″-epi and 8,9-oxide analogs, indicating that all three compounds retain high bioactivity towards the susceptible larval stages. Abamectin and the two analogs also are similar in toxicity to the larval stages of the AB-F strain. However, the AB-F larval stages are significantly less sensitive on average to these insecticides than the SS larval stages, indicating resistance to abamectin and cross-resistance to both the two analogs tested. Likewise, adults of the AB-F strain are significantly less sensitive to the toxic action of abamectin and the two analogs compared to SS adults, also indicative of resistance and cross-resistance. Abamectin is significantly more toxic, however, to both SS and AB-F adults, respectively, than either of the two analogs. The reduction in sensitivity was particularly evident in SS adults to both the 4″-epi and 8,9-oxide analogs. Additionally, adults of the SS strain are significantly less sensitive to the toxic action of abamectin and the two analogs when compared to SS larval stages. There is no significant differences, however, in the toxicity of these three insecticides, respectively, between larval and adults stages of the AB-F strain. This phenomenon results in lower resistance ratio (RR) values calculated for the two avermectin analogs compared to those calculated for abamectin regardless of the insect stage examined but is particularly evident and significant in the adult stage. This relative decrease in resistance levels is primarily associated with SS adults that are less sensitive to the toxic action of these insecticides. The decrease in abamectin toxicity is apparently due to significantly increased levels of P450 associated with SS adults versus forth instars and to similar levels in adults of the SS and AB-F strains. Because abamectin resistance in CPB is due in large part to enhanced oxidative metabolism of abamectin, it is likely that the SS adults are more tolerant to abamectin due to an enhanced level of oxidative detoxification. Finally, neither abamectin nor the two analogs are structurally protected at the specific molecular locations, C₃″, C₂₄, and C₂₆ carbons, which leads to enhanced oxidative metabolism, resistance and cross-resistance. Structurally-protected avermectin analogs at these vulnerable intramolecular sites are likely to be more effective insecticides in suppressing the development of oxidative detoxification-based resistance to abamectin, as would the use of oxidative synergists.