Similar host instar preferences by three sympatric parasitoids of Chielomenes sexmaculata (Coleoptera: Coccinellidae): potential host niche overlapping.

Parasitoids employ diverse oviposition strategies to enhance offspring survival and maximise fitness gains from hosts. Ladybird parasitoids, significant natural enemies of ladybirds, have the potential to disrupt biocontrol efforts, yet their biology and ecology remain poorly understood. This study investigated the host-parasitoid interaction among three sympatric larval endoparasitoids of Cheilomenes sexmaculata (Coleoptera: Coccinellidae): Homalotylus hemipterinus (Hymenoptera: Encyrtidae), Nothoserphus mirabilis (Hymenoptera: Proctotrupidae) and Oomyzus scaposus (Hymenoptera: Eulophidae). Our objective was to understand host instar preferences from five perspectives related to host profitability, handling difficulty or parasitism decision-making, and to examine the occupation rates of each parasitoid in different host instars. Host profitability was determined by development time, adult offspring dry mass, sex ratio, brood size, parasitism success rate and host handling time. Host handling difficulty was evaluated through host defensive behaviour and handling time. Parasitism decision-making was evaluated through acceptance rate and preference score that considered the first reaction of female wasp to the host. Results showed that each parasitoid responded differently to the host from various perspectives. However, the first two suitable hosts of these parasitoids overlap on the third instar host, with first to third instar hosts being ideal for H. hemipterinus, and third to fourth instar hosts being ideal for N. mirabilis and O. scaposus. In the field, the occupation rate of each parasitoid in third instar host was influenced by the population of N. mirabilis, implying its superior competitiveness. This study reveals the host instar preferences of ladybird parasitoids and highlights the potential for interspecific competition.