Establishing baseline susceptibility and discriminating doses of sulfuryl fluoride for resistance monitoring in key stored grain insect pests

Sulfuryl fluoride (SF) has been widely applied for the disinfestation of stored commodities for over 2 decades; however, information on baseline susceptibility in field insect populations is not well established. In the current study, detailed dose mortality response curves were established for 18 Rhyzopertha dominica (Fabricius) (Coleoptera: Bostrichidae) and 12 Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) populations that were collected from various grain storage facilities across Australia. Laboratory reference strains of both species were also used for comparison. Adult beetles (2 wks posteclosion) were exposed to a series of SF concentrations (0.05 to 2.5 mg l-1) over 48 h (2.4 to 120 g hm-3). Mortality responses for each population/strain of R. dominica and C. ferrugineus were recorded and analyzed. Key lethal estimates such as LC50, LC95 and LC99 were calculated for each population/strain and compared to assess inherent variation in susceptibility to SF. Comparison of lethal estimates along with other analyzed outputs identified 2 discriminating doses (DDs): 0.9 mg L-1 (43 g hm-3) for R. dominica and 2.0 mg L-1 (96 g hm-3) for C. ferrugineus over 48 h, which can be effectively used to discriminate SF-resistant individuals from their susceptible counterparts. Comparison of these results with the previously established DD for Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) (1.75 mg L-1 or 84 g hm-3) confirms that toxicity responses to SF are species-specific, highlighting the need for continued monitoring of resistance to SF in the field. This information has significant implications for the grain industry that is currently using SF as a phosphine-resistance breaker.