The mechanism of the emergence of Cotesia kariyai (Hymenoptera: Braconidae) larvae from the host

At maturity, the endoparasitoid larvae of several subfamilies of the Braconidae have to emerge from inside of the host to pupate. Although the hosts hormonal milieu and the timing of larval parasitoid emergence have been studied, no report has yet focused on the physiological state of the host in connection with the emergence behavior of endoparasitoids. We investigated the mechanism of larval emergence behavior in a gregarious endoparasitoid, Cotesia kariyai. The parasitoid larvae inserted their mandibles into the host cuticle and perforated the integument by moving their head-capsule backwards and forwards. The emerging parasitoid larva must have a physical support (an “anchor”) with the terminal appendages in order to exert the necessary pressure to cut the host integument. Morphological observations revealed that each parasitoid larva was enveloped in a capsule just before emerging from their host. Eight and nine day-old parasitoid larvae secreted material around their bodies to form these capsules. This material consisted of acid-glycoproteins which coated the exuvium of the 2 instar larvae. The haemolymph volume of the parasitised host also decreased in later stages and was dramatically reduced immediatly prior to parasitoid emergence. This final reduction of the host haemolymph volume is the result of absorption by parasitoid larvae. This mechanism allows the parasitoid larvae to create an anchor more easily. The parasitoid larvae could also adhere to each other with the glycoprotein. In addition, these capsules prevent the leaking of host haemolymph through the emergence hole; these holes on the host integument were plugged by the capsules after parasitoid emergence. Although the pressure acquired by the anchor was lost once the head of the parasitoid larvae emerges from the host integument, the parasitoid larvae crawls out of the host cavity using backward pointing spines which enable the parasitoid to grip the capsule and move forward via peristaltic contractions.