An assessment of chlorpyrifos-methyl,etrimfos, fenitrothion and pirimiphos-methyl as grain protectants

Trials were carried out over two consecutive years to compare the efficacy of chlorpyrifos-methyl, etrimfos, fenitrothion and pirimiphos-methyl against susceptible and organophosphorus-resistant strains of Tribolium castaneum Herbst, Sitophilus granarius L. and Oryzaephilus surinamensis L., gamma-HCH-resistant strains of Acarus siro L. and Glycyphagus destructor Schrank, and a susceptible strain of Tyrophagus longior Ger-vais. Pirimiphos-methyl was not evaluated against the three mite species as data on the efficacy of this material had previously been published. Etrimfos and pirimiphos-methyl were also tested against a susceptible and an organophosphorus-resistant strain of Sitophilus oryzae L. Each pesticide was applied to two separate 20-tonne batches of English wheat and stored under ambient conditions for 36 weeks. The efficacy of the pesticides was assessed regularly over the storage period using established bioassay techniques. However, chlorpyrifos-methyl and fenitrothion were assessed against susceptible and resistant strains of S. granarius for 32 weeks only. Each of the four pesticides produced >95% mortality of the susceptible strain of all insect species tested for the entire duration of the trial with the exception of fenitrothion, when survival of T. castaneum increased after 32 weeks. Only fenitrothion failed to give 100% mortality of the resistant strain of S. granarius throughout the trial, but even so, the mortality was always >95%. Chlorpyrifos-methyl, etrimfos and pirimiphos-methyl gave much better control of the resistant T. castaneum than fenitrothion, and etrimfos gave slightly better control of resistant O. surinamensis than pirimiphos-methyl which was in turn better than chlorpyrifos-methyl at 36 weeks. Both etrimfos and pirimiphos-methyl gave 100% mortality of the susceptible and resistant strains of S. oryzae. None of the pesticides maintained 100% kill of A. siro throughout the trial and fenitrothion failed to achieve this level of mortality after only 4 weeks. Etrimfos produced 100% mortality of the other two mites species tested throughout the trial, whereas both chlorpyrifos-methyl and fenitrothion achieved this level of mortality for 32 weeks.