Nutrient-dependent patterns in the sex ratio of Ovomermis sinensis (Nematoda: Mermithidae), a biological control agent of Helicoverpa armigera (Lepidoptera: Noctuidae)

Ovomermis sinensis (Nematoda: Mermithidae) is an entomophilic nematode and a potential biocontrol agent of lepidopteran pests, including Helicoverpa armigera. The sex ratio of a species can be used to regulate the size of the reproductive population. Parasitic load, parasitic period, host instar, and body size were examined to identify factors affecting the O. sinensis sex ratio. We tested the hypothesis that the O. sinensis sex ratio is correlated with host nutrient supply and the nutrients absorbed by the nematodes. The results show that the proportion of male O. sinensis increased with parasitic load but decreased with host instars and body size. Moreover, the parasitic period of males was significantly shorter than that of females. However, all the factors (host and nematodes) affecting the sex ratio were significantly modified by restricting the host diet, which increased the proportion of males. In turn, juveniles that absorbed fewer nutrients tended to develop into males. Taken together, our findings suggest that factors impacting the O. sinensis sex ratio are related to host nutrient status and provide parameters for mass rearing and a release strategy for this natural enemy.