Application of spinosad increases the susceptibility of insecticide-resistant Alphitobius diaperinus (Coleoptera: Tenebrionidae) to pyrethroids.

The effect of spinosad exposure on the susceptibility of pyrethroid- and organophosphate-resistant populations of lesser mealworm, Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae), to insecticides was investigated in broiler house farm and laboratory studies. A field pyrethroid- and organophosphate-resistant population showed a 3.6-fold increase in susceptibility to gamma-cyhalothrin following spinosad treatment. Overall, cyfluthrin- and fenitrothion-resistant field populations were more susceptible to these insecticides following spinosad treatments, but populations that were not resistant showed no change in susceptibility following spinosad treatment. In a related study, three broiler farm beetle populations with very similar levels of cyfluthrin and gamma-cyhalothrin resistance and similar susceptibilities to spinosad were used to investigate temporal effects of spinosad field treatments on the susceptibility to pyrethroids. Farm insecticide regimes applied at the start of each flock differed: the control broiler house received no insecticide applications, another house was systematically treated with cyfluthrin at the start of each study flock, and the third house was systematically treated with spinosad at the start of five flocks. Afterwards, treatments reverted to cyfluthrin on all farms. At the end of flocks, beetles were tested with cyfluthrin, gamma-cyhalothrin, and spinosad. The control and cyfluthrin house beetles did not change susceptibility to pyrethroids over the period of the study. In the spinosad house, spinosad had no effect on spinosad susceptibility but dramatically increased cyfluthrin and gamma-cyhalothrin susceptibilities. These new susceptibilities were maintained while spinosad applications continued, but pyrethroid susceptibility declined once spinosad applications ceased. This study provides evidence of a synergistic interaction between spinosad and pyrethroids in pyrethroid-resistant beetles. This evidence has significant implications for management of insecticide-resistant populations through an integrated spinosad-pyrethroid strategy that aims to minimize insecticide use while enhancing control.