Polydnavirus and venom of the egg-larval parasitoid Chelonus inanitus (Braconidae) induce developmental arrest in the prepupa of its host Spodoptera littoralis (Noctuidae)

The egg-larval parasitoid Chelonus inanitus induces in its host Spodoptera littoralis precocious onset of metamorphosis in the penultimate stadium and developmental arrest in the precocious prepupa. At oviposition the wasp injects, along with the parasitoid egg, venom and calyx fluid that contains polydnaviruses. By using indirect and direct approaches we have investigated the role of polydnaviruses and venom in influencing host development. When parasitoid development was prevented by either X-ray irradiation of wasp females or heat-treatment of parasitized eggs, hosts developed normally up to the prepupal stage but then became developmentally arrested. Injection of calyx fluid and venom into eggs also led to developmental arrest in the prepupa, while injection of anti-polydnavirus antibodies into eggs before parasitization prevented successful parasitoid development and rescued hosts so that they pupated normally. These results show that polydnavirus and venom induce developmental arrest in the prepupa but do not induce precocious onset of metamorphosis. Venom alone was ineffective in causing developmental arrest, and calyx fluid alone was effective only at high doses; but together with 0.0048 wasp equivalents of venom, as little as 0.0005 wasp equivalents of calyx fluid per egg was sufficient to induce developmental arrest. SDS-gel electrophoretic analyses of purified polydnavirus and venom showed that both consist of a great number of polypeptides of various sizes, and Western blots revealed that the anti-polydnavirus antibodies crossreacted with several venom proteins.