Co-fumigation with phosphine and sulfuryl fluoride: Potential for managing strongly phosphine-resistant rusty grain beetle, Cryptolestes ferrugineus (Stephens)

Populations of rusty grain beetle, Cryptolestes ferrugineus, have developed a very high level of resistance (1300×) to the fumigant phosphine (PH3) in Australia. Resistant insects triggered control failures, threatening the country’s annual grain market worth AU$8 billion. Although PH3 protocols were amended to manage this new resistance, fumigation requires lengthy exposure periods which has practical difficulties. While there is no suitable replacement for PH3, the current study explores potential approaches to enhance the efficacy of this fumigant. One possibility is co-fumigation of PH3 with another complementary fumigant, sulfuryl fluoride (SO2F2 or SF), with the dual goals: enhanced efficacy and minimise use of both fumigants. A cohort of mixed age eggs and adults of PH3-resistant C. ferrugineus was fumigated with PH3 and SF individually, as well as in combination inside desiccators at 25°C and 60%RH for 168 h. Two doses below the maximal registered rates for SF (8.9 mg L- 1, equivalent to 1500 g hm-3) and PH3 (1.0 mg L- 1) were tested. Co-fumigation was performed simultaneously for 168 h. Our results revealed that, the mixture of 1.1 mg L- 1 or 2.2 mg L- 1 of SF and 0.5 mg L- 1 of PH3 over 168 h achieved complete control against resistant C. ferrugineus eggs and adults, whereas each of the tested doses failed individually. Our study confirms that SF and PH3 enhance the efficacy of each other when used in combination, which holds great potential for managing resistant C. ferrugineus.