Genetics-based methods for agricultural insect pest management

Abstract

The sterile insect technique is an area-wide pest control method that reduces agricultural pest populations by releasing mass-reared sterile insects, which then compete for mates with wild insects. Contemporary genetics-based technologies use insects that are homozygous for a repressible dominant lethal genetic construct rather than being sterilized by irradiation.Engineered strains of agricultural pest species, including moths such as the diamondback moth Plutella xylostella and fruit flies such as the Mediterranean fruit fly Ceratitis capitata, have been developed with lethality that only operates on females.Transgenic crops expressing insecticidal toxins are widely used; the economic benefits of these crops would be lost if toxin resistance spread through the pest population. The primary resistance management method is a high-dose/refuge strategy, requiring toxin-free crops as refuges near the insecticidal crops, as well as toxin doses sufficiently high to kill wild-type insects and insects heterozygous for a resistance allele.Mass-release of toxin-sensitive engineered males (carrying female-lethal genes), as well as suppressing populations, could substantially delay or reverse the spread of resistance. These transgenic insect technologies could form an effective resistance management strategy.We outline some policy considerations for taking genetic insect control systems through to field implementation.

Keywords: Bt crops; genetic insect control; resistance management; self‐limiting constructs; sterile insect technique.

Figure 1

Genetics. Engineered males carry two copies of a genetic construct ‘L’, which is a repressible dominant lethal. The wild‐type allele ‘w’ represents the absence of the engineered construct. All offspring of released males and wild females inherit one copy of the dominant lethal. In (A), those progeny are not viable. In (B), the engineered lethality is female‐specific; thus, female progeny are nonviable and male progeny survive and can pass on the construct to half of their own offspring.

Figure 2

Population dynamics. Insect population over time, with no release (black) or release of ‘sterile’ males homozygous for a bisex‐lethal construct (grey lines) at a release ratio of 1 (dotted), 1.193 (dash‐dot), 1.2 (dashed) or 10 (solid) released males maintained in the environment to the number of wild males at equilibrium. The critical release ratio lies between 1.193 : 1 and 2 : 1. This example is a deterministic, continuous‐time model of a density‐regulated Aedes aegypti mosquito population, illustrating the simulated number of adult females (these are the disease vectors). Model and parameter values as in reported in Alphey et al. (2011a).

Figure 3

Resistance management. (A) Frequency of the resistant r allele in emerging adults and (B) population size relative to initial size, over time (insect generations) with no insect releases (black lines) and with release of toxin‐susceptible modified males carrying a female‐lethal genetic construct at fixed ratio of 1 : 2 to males emerging in the wild (grey lines). The released insects act in synergy with the Bt crops. This represents a generic pest for which resistance to Bt plants is recessive with partially‐dominant fitness costs, where refuge is 4% of the habitat. Based on a deterministic, discrete‐generation model reported in Alphey (2009).