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. ferrugineuswas fumigated with PH₃and 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 PH₃over 168 h achieved complete control against resistantC. ferrugineuseggs and adults, whereas each of the tested doses failed individually. Our study confirms that SF and PH₃enhance the efficacy of each other when used in combination, which holds great potential for managing resistant C. ferrugineus.