A haplodiploid mite adjusts fecundity and sex ratio in response to density changes during the reproductive period

Abstract

Population density is one of the main socio-environmental factors that have critical impacts on reproduction of animals. Consequently, they need to adjust their reproductive strategies in response to changes of local population density. In this study we used a haplodiploid spider mite, Tetranychus ludeni Zacher (Acari: Tetranychidae), to test how population density dynamics during the reproductive period altered female reproductive performance. We demonstrate that females produced fewer eggs with a significantly higher female-biased sex ratio in dense populations. Reducing fecundity and increasing daughter production in a dense environment could be an advantageous strategy to minimise the intensity of local food competition. However, females also reduced their fecundity after arrival in a new site of larger area from a dense population, which may be associated with higher web production costs because females need to produce more webs to cover the larger area. There was no trade-off between egg number and size, and egg size had little impact on reproductive fitness. Therefore, T. ludeni females could adapt to the shift of population density during their reproductive period by manipulating the fecundity and offspring sex ratio but not the egg size.

Keywords: Fecundity; Resource competition; Sex ratio; Social environment; Spider mite.

Fig. 1

Diagram of experimental design to test the effect of population density shift on reproduction in Tetranychus ludeni: a from high to low/high density (Treatment 1) and b from low to low/high density (Treatment 2). n number of leaf discs (replicates)

Fig. 2

Effects of female population shift between high (black dots) and low (white dots) densities on the mean (± SE) number of eggs laid (a) and egg size (b) in Tetranychus ludeni. Means within a panel with the same letters are not significantly different (Tukey-Kramer test: P > 0.05). Female population density shift occurred on the 4th day of oviposition period

Fig. 3

Relationships between egg size and number of eggs laid (a) and between egg size and immature survival rate (b) in Tetranychus ludeni

Fig. 4

Effects of female population shift between high (black dots) and low (white dots) densities on the mean (± SE) sex ratio (proportion of females) in Tetranychus ludeni. Means with the same letters are not significantly different (Tukey-Kramer test: P > 0.05). Female population density shift occurred on the 4th day of oviposition period

Fig. 5

Relationships between sex ratio (proportion of female offspring) and egg size in Tetranychus ludeni