Effects of population size and sex ratio on reproduction, sex-specific survival, and population growth in spider mite Tetranychus urticae Koch.

Population size and sex ratio are fundamental components of the social environment. Tetranychus urticae Koch (Acari: Tetranychidae) is a globally significant invasive pest, infesting a wide range of economically important agricultural crops. Understanding how population size and sex ratio affect individual fitness and population growth of T. urticae provides crucial insight into population dynamics and management. Here, we tested the influence of population size and sex ratio on reproduction, longevity, and life table parameters in T. urticae. We set up a factorial design with three population sizes and three sex ratios yielding nine combination treatments. Results showed that females in larger populations had greater fecundity and longer lives; however, sex ratio had no significant effect on female fecundity and longevity. In addition, females in larger population sizes had a higher net reproductive rate, a higher intrinsic rate of natural increase, and a shorter population doubling time, suggesting that control strategies should focus on proactive treatments to keep population sizes small to suppress population growth. Offspring survival rate was positively affected by parental population size but negatively affected by offspring population density. In contrast to females, males in larger and male-biased populations had significantly shorter longevity. The findings from this study advance our understanding of how population size and sex ratio shape spider mite life history traits and population dynamics, offering insights for population management.