Pest control by the introduction of a conditional lethal trait on multipleloci: Potential, limitations, and optimal strategies

Advances in genetics have made it feasible to genetically engineer insect s trains carrying a conditional lethal trait on multiple loci. We model the r elease into a target pest population of insects carrying a dominant and ful ly penetrant conditional lethal trait on 1-20 loci. Delaying the lethality for several generations after release allows the trait to become widely spr ead in the target population before being activated. To determine effective ness and optimal strategies for such releases, we vary release size, number of generations until the conditional lethality, nonconditional fitness cos t resulting from gene insertions, and fitness reduction associated with lab oratory rearing. We show that conditional lethal releases are potentially o rders of magnitude more effective than sterile male releases of equal size, and that far smaller release sizes may be required for this approach than necessary with sterile males. For example, a release of male insects carryi ng a conditional lethal allele that is activated in the F-4 generation on 1 0 loci reduces the target population to 10(-4) of no-release size if there are initially two released males for every wild male. We show how the effec tiveness of conditional lethal releases decreases as the nonconditional fit ness reduction (i.e., fitness reduction before the trait becomes lethal) as sociated with the conditional lethal genes increases. For example, if there is a 5% nonconditional fitness cost per conditional lethal allele, then a 2:1 (released male:wild male) release with conditional lethal alleles that are activated in the F-4 generation reduces the population to 2-5% ( depend ing on the degree of density dependence) of the no-release size. If there i s a per-allele reduction in fitness, then as the number of loci is increase d there is a trade-off between the fraction of offspring carrying at least one conditional lethal allele and the fitness of the released insects. We c alculate the optimal number of loci on which to insert the conditional leth al gene given various conditions. In addition, we show how laboratory-reari ng fitness costs, density-dependence, and all-male versus male-female relea ses affect the efficiency of conditional lethal releases.