Sex-biased dispersal in Tetranychus urticae mediated by population density but independent of sex ratio

Dispersal is a multiple-stage process which can be affected by intrinsic phenotypes and extrinsic environmental conditions. Understanding how animals adjust dispersal strategies in response to these factors is essential for predicting population dynamics and providing knowledge for population management. Tetranychus urticae Koch (Acari: Tetranychidae) is a notorious invasive pest of global concern, damaging numerous economically important crops. Here, we examined the effect of sex, population density, and sex ratio on dispersal probability and distance in T. urticae . We set up a factorial design with three densities and three sex ratios generating nine treatments. Our results show that dispersal was generally female-biased, and dispersal probability and distance increased with population density in both sexes, but the increase was significantly stronger in females. In contrast, sex ratio and interaction between sex and sex ratio had no significant effects on dispersal probability and distance. Females that dispersed farther laid eggs over longer distances. Females at female-biased sex ratios and dense populations produced more eggs but resulted in lower offspring survival rates, indicating severe intrasexual competition. We suggest that sexual dimorphism in body size, along with the relatively high benefits and low costs of female dispersal compared to males, may lead to female-biased dispersal in spider mites. Multiple mechanisms including female intrasexual competition and male harassment to females may account for the observed dispersal patterns in relation to population density and sex ratio. Female intrasexual competition and male harassment may drive female dispersal, resulting in wider egg distribution across habitats and potentially facilitating pest population outbreaks and range expansion.