The joint effect of climate condition and interspecific competition influences the seasonal population dynamics of three ladybird parasitoids

Climate factors and interspecific interactions both play important roles in organism population dynamics and can influence the effectiveness of biocontrol agents in agricultural ecosystems. However, little is known about cases involving fourth trophic level parasitoids. In this study, we conducted a single location field survey to investigate the joint effect of climate factors and interspecific competition on solitary pructotropid Nothoserphus mirabilis, gregarious encyrtid Homalotylus flaminus, and gregarious eulophid Oomyzus scaposus, which are major parasitoids of Cheilomenes sexmaculata (Coleoptera: Coccinellidae) in Taiwan. The results showed that the increasing abundance and frequency of competitors will have a negative impact on the parasitoid population. High precipitation, maximum temperature, and minimum temperature all had a significant negative effect on ladybird, N. mirabilis and O. scaposus, but had an indirect positive effect on H. flaminus due to the decrease of N. mirabilis. As a result, H. flaminus and N. mirabilis exhibited reverse population dynamics with each other, and O. scaposus had a partially reverse trend with the other two parasitoids. Climate factors had a greater impact on ladybird abundance than parasitoids. However, the proportion of ladybirds escaping parasitism reversed with the trend of ladybird abundance, and the parasitism rate reached more than 70% during the peak of ladybird abundance, indicating the potential for a disruptive effect in this aphid-ladybird-parasitoid system. The present study enhance the knowledge about ladybird parasitoids in agricultural ecosystem and have implications for the use of biocontrol agents.