Temperature and sex allocation in a spider mite.

Although temperature is the most important environmental factor regulating arthropod development and reproduction, its influence on sex allocation in haplodiploid arthropods remains largely unexplored. We investigated under laboratory conditions how maternal age and temperature mediate offspring sex ratio of the spider mite Tetranychus mcdanieli (Acari: Tetranychidae). Over nine temperature regimes, female-biased sex ratios were consistently observed, varying from 57 to 87% among progeny produced over lifetime. Spider mite sex ratio was affected by maternal age: more male progeny were produced at both the beginning and the end of the female lifespan, yielding a dome shaped curvilinear relationship. This pattern of variation with age probably results from constraints on using sperm at young ages and sperm depletion or viability at older ages. We found a significant curvilinear relationship between temperature and sex ratio. The proportion of female offspring was lowest at intermediate temperatures and highest at extreme temperatures. We suggest that increased female-biased sex ratio at extreme temperatures is an evolutionary response of spider mites to deteriorating habitats as, in the Tetranychidae, females have better capacities than males to disperse and survive under harsh conditions.