An assessment of methacrifos as a grain protectant and its efficacy against oryzaephilus surinamensis L., Tribolium castaneum herbst, Sitophilus granarius L., Sitophilus oryzae L., Acarus siro L., Glycyphagus destructor schrank and Tyrophagus longior gervais

Abstract A trial was carried out to investigate the efficacy of methacrifos against susceptible and multi‐resistant strains of four species of insect and three species of mite. Methacrifos was applied to two separate 20‐tonne loads of English wheat which were then stored under ambient conditions for 24 weeks. The efficacy of methacrifos was assessed using established bioassay techniques which showed complete control for 24 weeks of all susceptible insects except S. oryzae, where there were survivors at week 24. Methacrifos also gave 100% mortality of resistant O. surinamensis for 24 weeks. There was >95% mortality for 16 weeks of resistant strains of T. castaneum and of resistant S. granarius and S. oryzae for 20 weeks. After S. granarius and S. oryzae were removed, the grain was incubated for 10 weeks. Subsequent F1 emergence was considerably less from the treatments than from the controls. Methacrifos produced 100% mortality for T. longior for 24 weeks, but control of G. destructor fluctuated and was <100% at week 24. There were survivors of A. siro from week 4.