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.