Modelling pulsed releases for sterile insect techniques: fitness costs of sterile and transgenic males and the effects on mosquito dynamics

Abstract

Summary1. The development of transgenic technologies, coupled with sterile insect techniques (SIT), is being explored in relation to new approaches for the biological control of insect pests. Recent studies have shown that there are often fitness costs associated with transgenic insect strains, but the impact of these costs on their potential use in pest control is poorly understood. 2. In this paper, we explore the impact of an insect fitness cost on two control strategies (classical SIT and transgenic late-acting bisex lethality) using a stage-structured mathematical model, which is parameterized for the mosquito Aedes aegypti. Counter to the majority of studies, we use realistic pulsed release strategies and incorporate a fitness cost, which is manifested as a reduction in male mating competitiveness. 3. For both models we show that the level of control of a pest mosquito population is highly sensitive to the rate at which the transgenic or sterile males are released. Population control is more effective when smaller numbers of sterile/transgenic males are released more frequently than larger and less frequent releases. 4. If the wild-type mosquito population exhibits cycles of peaks and troughs in abundance, as is the case for many insect species, then high frequency releases of transgenic males not only reduce mosquito abundance, but they may dampen future pest outbreaks, whereas the use of SIT alone may have an adverse effect, causing an increase in mosquito abundance. Additionally, the timing of sterile/transgenic male release during the mosquito population cycle is critical in reducing pest outbreak levels. 5. In all cases, the reduced fitness of the sterile/transgenic males causes reductions in control, thus requiring more frequent or greater magnitude releases. 6. Synthesis and applications. The sterile insect technique is considered to be a valuable non-chemical tool for pest management. With the potential application of recent genetic developments to enhance the technique, it is becoming increasingly important to consider the wider ecological implications of this biological control strategy. Predicting the most efficient release strategies will be important in combating pest and vector insects as well as for limiting potential broader ecological effects. Although the focus of our models are based on the mosquito, A. aegypti, which can spread yellow fever, dengue fever and Chikungunya disease, our modelling approach and results can be applied more broadly to other species.

New revised simple models for interactive wild and sterile mosquito populations and their dynamics

“…To make the models as simple as possible, we still assume homogeneous populations such that no stage distinction is concerned. We focus the density dependence on the larvae development process and assume the death rates for wild and sterile mosquitoes are density independent [19,20,22]. We first give general modelling descriptions in Section 2.…”

Optimal barrier zones for stopping the invasion of Aedes aegypti mosquitoes via transgenic or sterile insect techniques

“…The temporal model of the wild-type mosquito population we consider here is derived from the framework of Dye (1984) which has been utilized in numerous previous studies (Barclay 2001;Phuc et al 2007;Yakob et al 2008;White et al 2010). We define the density of wild-type female mosquitoes to be N (t), and that of sterile/transgenic male mosquitoes to be S(t), at time t. We assume: (i) mosquito population growth proceeds via a stage-structured process and density-dependent mortality acts on a pre-adult developmental stage; (ii) the sex ratio of the wild-type mosquitoes is 1:1; (iii) wild-type male mosquitoes mate in proportion to their relative abundance (Knipling 1955;Phuc et al 2007), at a rate given by N (t)/(N (t) + cS(t)) where 0 < c ≤ 1 represents the reduced mating competitive ability of sterile male or transgenic male mosquitoes so that c = 1 implies that the control competes equally with the wild-type female mosquito, whereas small c, in contrast, entails the biological control is extensively out-competed in reproducing with females; (iv) the female wild-type and male sterile/transgenic mosquitoes have a per capita death rate µ.…”

“…However, these models fail to take into account the stage-structured life-history of the insects, which can have significant effects on their dynamics and hence the accuracy of the model predictions. This is now being addressed in the literature using delayed differential equations (Phuc et al 2007;White et al 2010).…”

“…In the first field trial conducted in the Caymen Islands (Harris et al 2011), males (strain OX513A) were released in a 10-hectare (ha) area at an average release rate of 465 males/ha/week for four weeks, resulting in a significant reduction in wild-type mosquitoes (approximately 80%), thus demonstrating the feasibility of the technology. Whilst the SIT strategy carries certain fitness costs for the sterilised mosquitoes, there seems to be little costs carried by the transgenic mosquitoes (see White et al (2010) and references therein). Thus, there is potential for RIDL to be an effective control for A.…”

“…In particular, late acting genes, which induce death after the density-dependent larval stage, have a significant advantage over SIT strategies because of an additional reduction in pest abundance as a result of larval competition (Atkinson et al 2007;Phuc et al 2007;White et al 2010).…”