Efficacy of phosphine (PH3) under hypoxia: pilot and industry-scale trials for controlling strongly resistant stored product pests in rice.

BACKGROUND Phosphine (PH3) is a slow-acting fumigant, therefore control of resistant insect pests requires high concentrations of 540-1080 ppm (0.75-1.5 g m-3) over long exposure periods of up to 21 days. This is difficult to achieve under many practical storage conditions; hence, there is a need to enhance the toxicity of PH3, so that resistant insects can be eradicated in a shorter fumigation time. Co-fumigation with PH3, along with other atmospheric gases (carbon dioxide (CO2) and nitrogen (N2)), enhances PH3 toxicity in laboratory-scale experiments; however, this information has not been translated into practical pest management protocols for adoption by the industry. METHODOLOGY The effectiveness of co-fumigation with PH3 plus two of the atmospheric gases (N2 and CO2) was evaluated in fumigation chambers and commercial-scale rice storage silos. Strongly PH3-resistant insects of mixed life stages of key grain insect pests, Rhyzopertha dominica, Tribolium castaneum, Sitophilus oryzae, and Cryptolestes ferrugineus were placed in cages inside the mini-fumigation chambers or silos containing freshly milled rice grains. Fumigations were performed with currently registered application rates of PH3 (0.75-1.5 g m-3) in combination with selected concentrations of CO2 (5-30%) and N2 (90-98%)/low oxygen (O2, 5-10%) over a short exposure period of 4 days. Test insect cages were retrieved after venting, and the effectiveness of the hypoxic fumigation was evaluated in terms of per cent mortalities of adults and progeny. RESULTS AND CONCLUSION Both the pilot scale and field fumigation research confirmed that the triplet mixture, PH3 0.75-1.0 g m-3 + high N2 (90-95%) + CO2 (5-20%) over 4 days was very effective against three strongly PH3-resistant species, R. dominica, T. castaneum, and S. oryzae and caused complete adult and progeny mortality. However, higher concentrations of this triplet mixture, PH3 1.5 g m-3 + high N2 (90-95%) + CO2 (30%) over 4 days was required to achieve complete mortality of adults and progeny of strongly resistant C. ferrugineus. At both the low and high co-fumigation dose rates, the required minimum effective PH3 concentrations (540-1080 ppm) were maintained over the 4-day exposure period, confirming the reliability of these rates for industrial applications. © 2025 Society of Chemical Industry.