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 F 1 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 LC 50 , resistance was incompletely recessive in eggs, pupae and adults and incompletely dominant in larvae. Eggs and adults were also incompletely recessive at LC 95 , 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.

Effect of Nitrogen on Phosphine-Susceptible and -Resistant Populations of Stored Product Insects

“…Apart from testing parental individuals, progeny production capacity of the populations was tested. In many studies on aerial insecticides, the efficacy trials are focused on using parental adults, which may provide a false impression that the application was successful, given that for many species, adult is the most susceptible life stage [ 31 , 32 ]. In addition, laboratory handling to provide the individuals that are to be exposed is likely to cause an additional stress/damage, especially in the case of eggs, which may give eventually high mortality levels that do not correspond to “real world” applications.…”

Resistance to the Fumigant Phosphine and Its Management in Insect Pests of Stored Products: A Global Perspective

“…Use of adult beetles, rather than beetles at other life stages, was recommended for ease and consistency in light of challenges with isolating immature life stages for some grain insects. Detailed studies are needed to describe variation in tolerance among different life stages of the resistant population (123). Examples of diagnostic doses for phosphine resistance in common storage pests are as little as 20 ppm for R. dominica and up to 50 ppm for S. granarius.…”

“…An important biological factor affecting phosphine efficacy is insect developmental stage. Eggs and pupae of resistant insects tend to be more tolerant to phosphine than larvae or adults (123). In addition, delayed egg hatching after exposure to phosphine has been reported for T. castaneum (95), R. dominica (97), L. bostrychophila (74), and C. ferrugineus (98).…”

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)

“…Two PH 3 ‐resistant reference strains of this pest species were used, and, in each strain, two life stages (adults and eggs) were tested. This is because adults of strongly resistant C. ferrugineus express the highest level of resistance (~×1400) to PH 3 , 41 whereas eggs exhibit very high tolerance to SF (~×30) among the other life stages 38–40 . The results of this study will have direct relevance to industry for future adoption if the co‐fumigation strategy would show its potential in combating PH 3 resistance in C. ferrugineus .…”

“…In Australia, for example, SF has been used as a ‘resistance breaker’ to control strongly resistant populations of rusty grain beetle, Cryptolestes ferrugineus (Stephens), which have developed very high levels of resistance and cannot be controlled with the current label rates of PH 3 22 . Although SF is not highly effective against eggs, particularly at low concentrations, increased ovicidal effect of PH 3 41 could complement the weakness of SF against eggs. Thus, a co‐fumigation of SF and PH 3 may provide an opportunity to eliminate PH 3 ‐resistant insects.…”