BIOLOGY AND MORPHOLOGY OF THE PARASITOID CHELONUS-INANITUS (BRACONIDAE, HYMENOPTERA) AND EFFECTS ON THE DEVELOPMENT OF ITS HOST SPODOPTERA-LITTORALIS (NOCTUIDAE, LEPIDOPTERA)

The solitary egg-larval parasitoids of the genus Chelonus have the uni que effect of inducing in their lepidopterous hosts the precocious ons et of metamorphosis and developmental arrest in the precocious prepupa . As a prerequisite to identifying the physiological and biochemical m echanisms underlying these developmental effects, we analyzed the morp hology and various growth parameters in the course of development of C helonus inanitus and studied the effect of this parasitoid on growth a nd development of its host Spodoptera littoralis. We show the first sc anning electron microscopic study of the internal stages of an endopar asitic wasp from the embryo to the third instar. By including observat ions from microscopic and histological analyses we describe the differ ent stages in the development of the anal vesicle and the formation of new segments and show that the anal vesicle has surface structures wh ich were never described before. We also demonstrate that the head and mouthparts change entirely with each instar. Measurement of the lengt h, width and head width at daily intervals reveals that the parasitoid s grow extremely rapidly from day 6 (final phase of first instar) unti l day 12. According to our findings, developmental stages of endoparas itic larvae can be unequivocally defined on the basis of the parasitoi d's length, the morphology of the head and the morphology of the abdom en or the anal vesicle, respectively. C. inanitus lay their eggs into eggs of S. littoralis, but the major effect on host development is see n much later when the parasitized larvae begin metamorphosis in the fi fth instar while nonparasitized larvae do so in the sixth stadium. Our measurements of weight and head capsule width nevertheless show that the average weight of parasitized larvae is less than that of nonparas itized larvae already in the first instar and that the head capsule wi dth is significantly smaller in parasitized larvae from the third stad ium onwards. This demonstrates that subtle effects become manifest ver y shortly after parasitization. Furthermore, parasitization by C. inan itus inhibits testes development in S. littoralis. A comparison of par asitoid development in relation to host development under various cond itions reveals that the parasitoid's growth rate varies with the age o f the host egg at parasitization and that the parasitoid molts into th e second instar always after the host has molted into its precocious l ast instar. Thus C. inanitus, on one hand, regulates its host by induc ing precocious onset of metamorphosis and developmental arrest in the precocious prepupa. On the other hand, it adapts to host development b y varying its growth rate and synchronizing its molt into the second i nstar with host development.