Relative tolerance and expression of resistance to phosphine in life stages of the rusty grain beetle, Cryptolestes ferrugineus

Abstract

Cryptolestes ferrugineus is a serious cosmopolitan pest of stored products. Frequent and indiscriminate usage of phosphine has caused the development of high levels of resistance to this fumigant. As there are few alternatives, it is imperative that resistance to phosphine is managed. Effective management requires knowledge of key factors driving the rate of selection. One of the most important factors is the response of each resistance genotype to phosphine, especially heterozygotes. Moreover, it is important to understand the expression of resistance in all life stages as all stages are subjected to selection during fumigation. We determined the relative tolerance and resistance levels to phosphine in all life stages of homozygous parental strains (susceptible and resistant) and their F1 progeny (heterozygous) and estimated relative dominance of resistance within life stages over 48 h. In susceptible insects, relative tolerance was highest in eggs followed by pupae, then adults which had about the same tolerance as larvae. In homozygous resistant insects, the order of tolerance was adult = egg > pupae > larvae and in heterozygotes larvae > eggs > pupae > adults. All life stages expressed resistance with resistance ratios highest in adults > pupae > larvae > eggs. At LC50, resistance was incompletely recessive in eggs, pupae and adults and incompletely dominant in larvae. Eggs and adults were also incompletely recessive at LC95, but larvae were completely dominant and pupae were incompletely dominant. Our data showed that a proportion of heterozygotes in all life stages, the major carriers of resistance in the field, will survive at very high concentrations, particularly in the egg stage, forming a nucleus for reinfestation or dispersal of resistance.