The lesser grain borer, Rhyzopertha dominica (F.), is a highly destructive coleopteran pest of stored grains worldwide. Fumigation with phosphine is the preferred control method, however, extensive use of this treatment for almost half a century has led to the development of resistance in insect populations in many parts of the world. Management of resistance requires an understanding of the evolutionary processes involved. In the case of phosphine, the selective agent is applied to all life stages of the insect simultaneously as commodities infested with lesser grain borer are likely to contain eggs, larvae and pupae as well as adults. Thus it is important that the genetic basis of phosphine resistance and delay in development is understood in all life stages of grain pests. Phosphine resistance in the larval and pupal stages of R. dominica is inherited as an incompletely recessive factor, similar to its inheritance in adults. However, in the egg stage it is expressed as a maternally inherited semi-dominant factor that is transient, as it is not observed in later stages of development. In the resistant strain, eggs were less tolerant than adults at the LC50 whilst this was reversed for the susceptible strain. At the LC99, eggs and pupae were more tolerant than adults or larvae of all genotypes. There was also an observed delay in post-fumigation development, measured as percentage of survivors emerged, in all strains that increased with dose.
In order to understand the spread and selection of phosphine resistance, it is very important to explore the ecology of the stored beetles as this influences the dispersal of resistant alleles into susceptible populations or the immigration of susceptible insects to resistant populations. Due to the interaction between phosphine susceptibility and respiration, the effect of phosphine resistance alleles might be expected to negatively affect energy demanding activities such as walking and flying. We used an activity monitoring system to quantify walking of Rhyzopertha dominica and a flight chamber to estimate flight initiation. No significant difference in walking movement was observed between the phosphine strong resistant, weak resistant and susceptible strains of R. dominica whereas females walked significantly more than males regardless of genotype. The walking activity monitor revealed no circadian rhythm and no particular time of peak activity despite reports of peak activity of R. dominica and T. castaneum under field conditions during dawn and dusk. Flight initiation was significantly more at 28°C and 55% RH in first 24 hours after placing beetles in the flight chamber. Starvation and genotype has no significant effect on flight initiation. Our results suggest that known resistance alleles in R. dominica do not affect insect mobility and should therefore not inhibit the dispersal of resistant insects in the field. Recent advances in DNA technology have allowed us to develop a DNA marker specific to a major phosphine resistance allele in this insect. I have screened this marker against field samples from farm storages collected five years apart. An increase in resistance allele frequency on farms that do not use phosphine (i.e. organic growers) was observed over the five year period indicating probable immigration of beetles from neighboring storages either by flight, walking or human-mediated transport of commodities between the farms.
To enhance the international relevance of this work we used genetic complementation analysis to determine whether the same resistance genes are responsible for phosphine resistance between an Indian strain of phosphine resistant R. dominica and a strongly resistant Australian reference strain (QRD569). The resistance loci rph1 and rph2 were both shared between the Indian and Australian strains. Gene sequencing of the rph2 locus, the dihydrolipoamide dehydrogenase gene, confirmed that there was a polymorphism that changed an amino acid within the active site of the protein that is not shared with the QRD569 strain. This shows that the genes for phosphine resistance are likely to be highly conserved worldwide, although there may be allelic differences.