A co-fumigation strategy utilizing reduced rates of phosphine (PH3) and sulfuryl fluoride (SF) to control strongly resistant rusty grain beetle, Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae)

BACKGROUND Managing resistance to phosphine (PH3) in rusty grain beetle, Cryptolestes ferrugineus, is challenging, as strongly resistant insects of this species require very high concentrations over lengthy exposure periods (>10 days). Recently, approaches that enhance the efficacy of PH3 have gained momentum to control this pest, especially co-fumigations. In this study, efficacy of co-fumigating PH3 with another commercially available fumigant, sulfuryl fluoride (SF), has been evaluated against adults and eggs of two PH3-resistant strains of C. ferrugineus. Concentrations of the mixture, representing lower than current application rates of both fumigants, were tested towards its field use. RESULTS Co-fumigation of PH3 with SF was achieved in two patterns: over a continuous exposure period of 168 h simultaneously and sequentially over two periods of 78 h, in which insects were exposed to SF first followed by PH3 with 12 h aeration in-between. Results of simultaneous fumigations identified two effective co-fumigation rates, SF 185 + PH3 168 g hm−3 and SF 370 + PH3 84 g hm−3 that yielded complete control of adults and eggs. These two rates also were equally effective when they were applied sequentially and produced consistent results. Irrespective of application methods, concentrations of both PH3 and SF failed individually in achieving complete mortality of either adults or eggs or both. CONCLUSION Our results confirmed that a co-fumigation strategy involving half the current standard rate of PH3 (84 g hm−3) with one-fourth of the current maximal registered rate of SF (370 g hm−3) can provide effective control of strongly PH3-resistant C. ferrugineus.