Persistence and residual toxicity of two pirimiphos-methyl formulations on wheat against three stored-product pests

The persistence and residual efficacy of two pirimiphos-methyl formulations [emulsifiable concentrate (EC) and capsule suspension (CS)] on wheat were evaluated in laboratory bioassays against adults of Sitophilus granarius, Rhyzopertha dominica and Tribolium confusum. Pirimiphos-methyl was applied at two doses, 2 and 4 mg kg−1, on wheat and a bioassay was initiated at the day of application and every 4 weeks for 7 consecutive months (8 bioassays). In each bioassay, insect mortality was assessed after 7, 14 and 21 d of exposure, whereas after the final mortality count, all individuals were removed and progeny production was evaluated after an additional period of 65 d. Moreover, samples of the treated wheat were taken at the beginning of each bioassay to study the fate of pirimiphos-methyl during the storage period of 7 months by measuring insecticide residues by high performance liquid chromatography (HPLC). Both pirimiphos-methyl formulations provided long-term protection against S. granarius adults. In most cases, complete (100%) mortality was noted, while mortality exceeded 95% after 21 d of exposure even 7 months after application for both doses and formulations. Tribolium confusum mortality did not reach 100% for any of the combinations tested, while a gradual decline of mortality levels was recorded over the 7-month period of storage. Rhyzopertha dominica was the least susceptible species to pirimiphos-methyl with very low mortality levels already after 1 month of storage. The residue analysis results show that pirimiphos-methyl is persistent under the conditions tested, as more than 50% of the initial concentration was still present on the wheat grains after the 7-month period of storage, for both doses and formulations. From the residue analysis results CS was shown to be slightly more persistent than EC, however this was not expressed as a significantly higher insecticidal efficacy of CS over time compared to EC.