Lepidopteran larvae can exclude some organelles that have invaded the hemocoel by defense systems such as phagocytosis, encapsulation, and nodule formation. However, Plusiinae loopers have another system that can exclude organelles such as endoparasitoids by forming a cyst on the penultimate segment, i.e., cuticular encystment. In this study, we examined whether cuticular encystment is common in six species of Plusiinae loopers [Autographa nigrisigna Walker, Anadevidia peponis F., Trichoplusia ni (Hübner), Acanthoplusia agnata Staudinger, Erythroplusia rutilifrons Walker, and Trichoplusia intermixta Warren) from the first to the fourth stadia. Loopers were parasitized artificially by the gregarious endoparasitoid Cotesia glomerata L. Different rates of cuticular cyst formation were shown among species. More than one-half of the loopers formed a cuticular cyst in all species except T. intermixta. A. nigrisigna and A. peponis, in particular, formed >80% cysts in the first to third instars. However, almost all parasitized host larvae died before the larval emergence of the parasitoid in spite of the high proportion of cyst formation. Death of the parasitized host larvae seemed to be caused by an overload in the number of eggs laid. We also examined, in A. nigrisigna, the number of parasitoid larvae that this looper can exclude by cuticular encystment. The number of eggs laid in the gregarious endoparasitoid can be regulated artificially. A sudden disturbance of the oviposition sequence by removing the female C. glomerata with a brush caused various numbers of eggs to be laid in the host. When the oviposition time was restricted to 0.4–4.0 s (i.e., the number of eggs laid was <5), survival rate of the loopers clearly increased. We conclude that parasitoid larvae can be expelled by cuticular encystment only if a few eggs are laid in Plusiinae loopers.