GROWTH-BLOCKING PEPTIDE OR POLYDNAVIRUS EFFECTS ON THE LAST INSTAR LARVAE OF SOME INSECT SPECIES

The parasitoid wasp Apanteles kariyai perturbs the endocrinological process of the host that controls normal metamorphosis from larva to pupa. The effect is particularly evident when parasitization occurs during the late stages of larval development of the host Pseudaletia separata. This developmental disturbance allows the parasitoid wasp to complete its larval growth and emerge while the host is still in a larval stage, otherwise the parasitoid larvae would be trapped in the sclerotized pupal cuticle. A peptidergic factor, growth-blocking peptide (GBP), has recently been purified from the parasitized larval hemolymph. Injection of GBP into unparasitized last instar larvae of the armyworm clearly retards larval growth and, consequently, delays the onset of pupation of the larvae. Suppression of larval development also occurs when unparasitized last instar larvae are injected with a mixture of symbiotic polydnavirus and venom fluid from the reproductive tract of the Apanteles female wasps. The effects of injecting GBP or a mixture of polydnavirus and venom fluid into the last instar larvae was investigated in three species of Lepidoptera, armyworm (Pseudaletia separata), common cutworm (Spodoptera litura) and silkworm (Bombyx mori), in order to compare the species-specificity of both factors. Injection of GBP causes suppression of larval development in all tested insects. In contrast, developmental suppression by the injection of polydnavirus plus venom fluid occurred only in the armyworm larvae. Common cutworm larvae injected with virus plus venom were slightly suppressed but the larval development of the silkworm was not affected by virus-venom injection. Furthermore, GBP in the armyworm larval plasma was induced by virus-venom injection, but the detectable amount of GBP was not observed in the plasma of the common cutworm and silkworm larvae by the injection.