Demographic effects on the use of genetic options for the control of mosquitofish, Gambusia holbrooki.

This study tests the sensitivity of genetically based pest control options based on sex ratio distortion to intra- and intersexual aggressive interactions that affect male and female survival and fitness. Data on these interactions and their impacts were gathered for the mosquitofish Gambusia holbrooki (Poeciliidae), a promiscuous species with a strongly male-biased operational sex ratio and well-documented male harassment of females. The experimental design consisted of an orthogonal combination of two population densities and three sex ratios, ranging from strongly male-biased to strongly female-biased, and long-term observations of laboratory populations. Contrary to expectations, the number of males in a population had little evident effect on population demographics. Rather, the density of adult females determined population fecundity (as a result of a stock-recruitment relationship involving females, but not males), constrained male densities (apparently as a result of cannibalism or intersexual aggression), and regulated itself (most likely through effects of intrasexual aggression on female recruitment). The principal effect of males was to constrain their own densities via effects of male-male aggression on adult male mortality rates. Through use of a realistically parameterized genetic/demographic model, we show that of three different genetic options applied to control G. holbrooki, one based on recombinant sex ratio distortion (release of Female Lethal carriers) is marginally more efficient than a sterile male release program, and both outperform an option based on chromosomal sex ratio distortion (Trojan W). Nonlinear dependence of reproductive rate on female density reduces the efficacy of all three approaches. The major effect of intra- and intersexual aggression is mediated through females, whose interactions reduce female numbers and increase the efficacy of a control program based on sex ratio. Socially mediated male mortality has a small impact on control programs due to operational sex ratios that are heavily male-biased. The sensitivity of sex ratio-based control options to social factors will depend on the mating system of the targeted pest, but evidence of widespread density-dependent population regulation suggests that, for most species, the effects of elevated adult mortality (due to intra- and intersexual aggression) on control programs are likely to be slight.