Toxicological and biochemical studies on spinosad and synergism with piperonyl butoxide in susceptible and resistant strains of Tribolium castaneum.

Evolution of resistance in the red flour beetle Tribolium castaneum against several insecticides threatens sound storage of stored grain products. Dose-response bioassays were conducted on area-wise collection of beetle populations from major wheat growing cities of Pakistan, while biochemical techniques were used to evaluate basic mechanisms underlying resistance. All the collected field strains were susceptible to spinosad. High resistance to deltamethrin resulted in cross resistance (380 folds) against spinosad just up to 8 generations. Spinosad and deltamethrin resistance is primarily associated with detoxification by enhanced levels of several enzymes. Enzymatic assays showed that catalase, amylase and acetylcholinesterase activities, but not phosphatases (alkaline and acidic), were positively correlated with resistance to deltamethrin and spinosad. Piperonyl butoxide (PBO) resulted in significant synergism with spinosad as LC50 value was greatly reduced i.e. from 231375.2 to 305.4ppm when used in 1:4 against R-MDA strain. Knowledge of the mechanisms involved in spinosad and deltamethrin cross resistance is a key to devising new resistance management strategies aimed at restoring the efficacy of spinosad-based programmes.