Autographa nigrisigna looper (Lepidoptera: Noctuidae) excludes parasitoid egg using cuticular encystment induced by parasitoid ovarian fluid

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Autographa nigrisigna (Lepidoptera: Noctuidae) loopers form cuticular cysts on the dorsal site of the penultimate segment by parasitization of endoparasitoid Campoletis chlorideae (Hymenoptera: Ichneumonidae) and interrupt successful parasitization by excluding the parasitoid egg and larva in the hemocoel into the cyst. This study elucidated for the first time the difference in the exclusion rates with host age at the time of parasitization. All parasitized host loopers of D1L1 (Day 1, 1st instars) and D0L2 formed cuticular cysts and nearly 70% could exclude parasitoid eggs successfully; however, the number of loopers that could form cuticular cysts decreased with host aging at the time of parasitization. To elucidate the difference between the encapsulation system and cuticular encystment, it was examined whether the substances encapsulated are excluded by cuticular encystment. A small piece of antenna the same size as an egg used as a biotic foreign substance was absolutely encapsulated and was not excluded by encystment. Further, it was clarified whether parasitoid eggs with a different surface were encapsulated or excluded by cuticular encystment with or without ovarian fluid. UV eggs (with an intact surface and development ceased by UV treatment) were hard to encapsulate, but were excluded with encystment. Plasma eggs with a degenerate surface (treated with heating for 5 min at 60°C after 10-min immersion in plasma of Pseudaletia separata) were encapsulated and not excluded by cuticular encystment, although ovarian fluid increased the cyst formation rate. These results show that cuticular encystment functions to exclude parasitoid eggs incapable of encapsulation.