Combining pest control and resistance management: Synergy of engineered insects with Bt crops

Transgenic crops producing insecticidal toxins are widely used to control insect pests. Their benefits would be lost if resistance to the toxins became widespread in pest populations. The most widely used resistance management method is the high-dose/refuge strategy. This requires toxin-free host plants as refuges near the insecticidal crops, and the use of toxin doses intended to be sufficiently high to kill insects heterozygous for a resistant allele, thereby rendering resistance functionally recessive. We have previously shown by mathematical modeling that mass-release of harmless susceptible (toxin-sensitive) insects engineered with repressible female-specific lethality using RIDL^® (Release of Insects carrying a Dominant Lethal) technology could substantially delay or reverse the spread of resistance and reduce refuge sizes. We have now explored this proposal in depth, studying a wide range of scenarios, considering impacts on population dynamics as well as evolution of allele frequencies, comparing with releases of natural fertile susceptible insects, and examining the effect of seasonality. We investigate the outcome for pest control for which the plant-incorporated toxins are not necessarily at a high dose. We demonstrate that a RIDL-based approach could form an effective component of a resistance management strategy in a wide range of genetic and ecological circumstances. There are significant threshold effects for several variables, so a comfortable margin of error would be advisable in setting release ratios and refuge sizes, especially as the frequency and properties of resistant alleles are difficult to measure accurately in the field.