Fumigant toxicity of allyl isothiocyanate against phosphine-resistant populations of five major stored-grain insect pests

Given the development of resistance in stored-grain insect pests to phosphine (PH3), the grain industry is seeking alternative methods for effective pest and resistance management. We evaluated the efficacy of allyl isothiocyanate (AITC), a potential alternative fumigant against adults of phosphine-susceptible (PH3-S) and resistant strains (PH3-R) of five major grain insect pests, including Sitophilus oryzae (Linnaeus), Tribolium castaneum (Herbst), Rhyzopertha dominica (Fabricius), Oryzaephilus surinamensis (Linnaeus), and Cryptolestes ferrugineus (Stephens). Adult dose-mortality response curves were established for each species, and the mortality endpoints of post-fumigated adult S. oryzae and T. castaneum were compared. The effect of commodities on the efficacy of AITC was briefly investigated from the perspective of adult insect mortality. The PH3-R strain of S. oryzae was the most tolerant and required the highest dose, LC50: 1.75 µL a.i. L-1, whereas the PH3-R strain of C. ferrugineus was the most susceptible to AITC, requiring the lowest LC50: 0.59 µL a.i. L-1. Comparisons of LC99.9 across the species and strains confirmed that AITC at 2.59 µL a.i. L-1 was adequate in achieving complete control of adults across all five insect species tested, irrespective of their resistance status to phosphine. These results suggest that phosphine-resistant insects fail to confer cross-resistance to AITC. Post-exposure endpoint mortality studies revealed a steady increase in mortality in S. oryzae (from 18% at 24 h to 100% at 168 h). In contrast, no such changes were recorded with T. castaneum, suggesting the existence of species-specific differences in responding to AITC. The presence of insect-infested commodities, such as rolled oats and cracked sorghum, reduced the efficacy of AITC, indicating that this fumigant could be sorptive.