Bigger animals tend to live longer than the small ones across species, but whether body size also has a robust relationship with survival within species remains to be determined. Here, the association between body size and fitness traits was examined through two food treatments (starvation and fed ad libitum) for both virgin and mated spider mites, Tetranychus urticae. The longevity of spider mites differed significantly across treatments, with feeding ad libitum increasing the survival of both males and females, mating decreasing male survival when starved and female survival when fed ad libitum. The body size of females but not of males increased with food. For each treatment, no clear correlations between body size and longevity were found. However, female fecundity was shown to have a positive relationship with body size. These results suggested that within species, for spider mites, there is no clear association between body size and longevity, but the fecundity increased with the body size, although this association is weak.

Some factors affecting the development, survival and prey consumption of Neoseiulus cucumeris (Acari: Phytoseiidae) feeding on Tetranychus urticae eggs (Acari: Tetranychidae)

Intermittent fasting (IF) is receiving increasing attention as an alternative to continuous restriction of calories because of its benefits in aging-related disease prevention and lifespan extension. However, whether both sexes with sexual dimorphism have similar response to IF have rarely been assayed. In this study, we determined how different durations of IF influence lifespan and whether males and females differed in their responses to IF. We also tested whether there is a trade-off between lifespan and lifetime reproduction in females under IF. We used spider mite Tetranychus urticae, with female-biased sexual size dimorphism (SSD), as our model species to investigate the survival and lifespan difference of both sexes at different durations of IF regimes, and explore the association between longevity and fecundity in females within and across treatments. The lifespan of females increased before intermediate level of IF and then decreased afterwards, but males showed a decreasing trend in lifespan when subjected to IF. Within each treatment, female longevity was positively associated with their fecundity. However, the females fed ad libitum had a higher lifetime fecundity with a shorter lifespan, whereas mites fed 50% IF outlived ad libitum fed ones with lower fecundity because of the later onset of reproduction and lower daily fecundity, showing clear survival and reproduction trade-off when variation of resource availability enhanced across treatments. We showed sex-specific response to IF in lifespan, indicating that sexes with SSD have different optimal level of IF. These findings showed trade-off between survival and reproduction between treatments but not within treatments, suggesting that variation in resource availability is the necessary precondition for life history trade-off, and IF extends lifespan of females at the cost of reproductive success.

Establishment and maintenance of the natural enemy are critical for successful biological control of pests on plants without alternative food for predators. Great efforts have been devoted to exploring suitable supplementary food for predators. In this study, we evaluated the feasibility of using Tyrophagus curvipenis (Fain and Fauvel) (Acari: Acaridae) as a supplementary food source for Neoseiulus cucumeris (Oudemans) (Acari: Phytoseiidae) by investigating the survival and development of N. cucumeris at different prey densities with the influence of conspecifics. Furthermore, we investigated predation rates and their body size at adult emergence. The results showed that N. cucumeris developed from egg to adult in approximately six days. Survival rates of immature predators increased significantly with the given prey density. No significant difference in body size was found between the survived adults fed at different prey density, but the females were always larger than males. To conclude, T. curvipenis can be an excellent alternative food source for the biological control agent N. cucumeris.

Sexual interaction is an important activity that determines the reproductive schedule of organisms and can ultimately influence the fitness traits of both sexes. Although the influence of sexual interaction on the fitness of females has been extensively determined, little is known about the effects on males, which often have different mating strategies and optimal mating regimes from those of females. To understand how mating regimes (timing and frequency) modulate the fitness in both sexes, we used spider mite (Tetranychus urticae) to investigate the influence of delayed mating and repeated mating on the fitness of male and female. For females, the unmated and the delayed mating females outlived those mated immediately after adult emergence. The repeated mating shortened the lifespan of females that mated at 1-day-old, but not that mated 7-day-old. However, no significant variation in lifespan was observed for males across different mating regimes. We found although delayed mating significantly reduced the daily reproductive rate of the females, there was no significant difference in lifetime reproduction of females across treatments because the delayed mating females increased their reproductive lifespan as a compensation. Our study highlighted that the time and frequency of sexual interaction showed a sex-specific consequence on male and female spider mites, indicating that sexual interaction incurs a higher cost to females which have a much lower optimal mating frequency than males.