How Residual Fertility Impacts the Efficiency of Crop Pest Control by the Sterile Insect Technique.

The sterile insect technique (SIT) is a biological control technique based on mass-rearing, radiation-based sterilization that can induce fitness costs, and releases of the pest species targeted for population control. Sterile matings, between females and sterilized males, can reduce the overall population growth rate and cause a fall in population density. However, a proportion of irradiated males may escape sterilization, resulting in what is called residual fertility. Our aim in this study was to examine the impact of residual fertility on pest control employing a modeling approach. We modeled pest population dynamics with three generic differential equations representing sterilized males, wild males and wild females. We explored the impact of residual fertility, in the presence or absence of fitness costs, on potential pest control outcomes using a scenario with male sterilization as our standard of reference. We carried out a detailed mathematical analysis of the model's dynamics by calculating model equilibria and the latter's stability. Bifurcation analyses were performed with parameters for the Mediterranean fruit fly Ceratitis capitata. We showed that, when residual fertility is below a threshold value, wild populations can be eradicated by flooding the landscape with irradiated males. This threshold is higher when residual fertility is associated with fitness costs. Too high a level of residual fertility makes SIT less effective and hinders population eradication. That said, substantial decreases in population density can be achieved even when residual fertility is much larger than the above threshold.