Comparison of two pirimiphos-methyl formulations against major stored-product insect species

• Capsule suspension of pirimiphos-methyl was equally effective with emulsifiable concentrate.
• Rhyzopertha dominica was the least susceptible to both formulations.
• Pirimiphos-methyl was very effective against Sitophilus spp. and Liposcelis entomophila.
• There was no consistent effect of temperature and relative humidity on insect mortality.

The insecticidal effect of two pirimiphos-methyl formulations [emulsifiable concentrate (EC) and capsule suspension (CS)] was evaluated in laboratory bioassays against adults of Sitophilus granarius, S. oryzae, Rhyzopertha dominica, Tribolium confusum, Oryzaephilus surinamensis, Prostephanus truncatus and Liposcelis entomophila. Pirimiphos-methyl was applied at five dose rates, 0.5, 1, 2, 3 and 4 ppm, and, depending on the natural mortality level, insect mortality was assessed after 1, 2, 7, 14 and 21 d of exposure. After the final mortality count, all individuals were removed and progeny production was evaluated after an additional period of 65 d. In another series of laboratory bioassays, the insecticidal efficacy of these two pirimiphos-methyl formulations was evaluated against S. granarius, T. confusum and R. dominica at three temperatures (20, 25 and 30 °C) and two relative humidity (r.h.) levels (55 and 75%). Pirimiphos-methyl was applied at two dose rates, 2 and 4 ppm, and mortality of beetles was measured after 7, 14 and 21 d of exposure. In all cases, Sitophilus species were highly susceptible to both pirimiphos-methyl formulations, since complete (100%) mortality was noted with doses ≥1 ppm after 7 d of exposure. In contrast, R. dominica and P. truncatus were the least susceptible to pirimiphos-methyl, as mortality did not exceed 82 and 57%, respectively, for any of the dose rates or formulations tested. Progeny production was in most cases suppressed by pirimiphos-methyl, with the exception of R. dominica. Moreover, in most of the combinations tested, there was no significant and consistent effect of temperature and r.h. on S. granarius, T. confusum and R. dominica mortality.