Co-fumigation with phosphine (PH3) and sulfuryl fluoride (SO2F2) for the management of strongly phosphine-resistant insect pests of stored grain

Introduction: Phosphine (PH3), an effective fumigant used to protect stored grains from insect pests. As resistance to PH3 is widespread, effective fumigation requires lengthy exposure periods (>7 days) which has practical difficulties. There is an urgent need to develop alternative approaches that can increase the efficacy of PH3. One possibility is co-fumigation with PH3 and another fumigant such as sulfuryl-fluoride (SO2F2), which has properties complementary to PH3. Here we present the results of PH3+SO2F2co-fumigation of key stored grain insect pests. Methodology: Adult beetles were fumigated with PH3 and SO2F2individually as well as in combination inside air-tight desiccators at 25°C and 65% RH for 48 hours. Mortality after a recovery period of 14 days was recorded and the data was subjected to probit analysis. Experiments were replicated twice. Results: Comparison of estimated effective doses (LC99.9) of PH3, SO2F2 and PH3+SO2F2 for each of the four PH3-resistant grain insect species, Tribolium castaneum, Cryptolestes ferrugineus, Sitophilus oryzae and Rhyzopertha dominca indicated that co-fumigation with SO2F2 at half of its effective dose (0.22-0.82 mg L-1) reduced the amount of PH3 required to one third (0.33 to 5.8 mg L-1). There was no interaction between resistance to PH3 and sensitivity to SO2F2.The analysis on observed and expected mortality response curves of PH3+SO2F2in all the four species confirmed that the interaction was additive. Conclusion: SO2F2 enhances the toxicity of PH3 in an “additive” manner in PH3-resistant insect species, thus treating PH3 jointly with SO2F2 holds great potentials for pest and resistance management.