Behaviour and development of the parasitoid Bracon brevicornis - an enemy of the European corn borer Ostrinia nubilalis

In recent years, in Germany, the ectoparasitic wasp Bracon brevicornis (Hym. Braconidae) was quite often detected parasitising Ostrinia nubilalis larvae in infested maize crops. For this reason a video film (duration 14 1/2 min.) was produced that documents the behaviour and development of the braconid parasitoid. Infested maize stalks were partially cut open to reveal the host larvae (L4-L5) and were then presented under a stereomicroscope to mated B. brevicornis females. As a typical response, the Ostrinia larvae first tried to protect themselves by spinning a web within the cut stalk that was then partially covered by frass and faeces. The wasp, standing nearby, continuously observed this mending process with great care, waiting for the right moment for the first sting to paralyse the mighty larva by injecting a venom. Several attempts with the partially protruded ovipositor preceded the first very quick successful sting, upon which the host larva responded violently. Several minutes elapsed, continuously observed by the parasitoid, until the affected larva gradually calmed down. Then it was stung again. The paralysed Ostrinia larva was then removed from the stalk to document host feeding and oviposition. First host feeding stings evoked again a strong defensive response. Finally, after the larva had calmed down completely, host feeding was initiated by forceful deep ovipositor stings. The emerging haemolymph was immediately imbibed. Host feeding was followed by protruding the ovipositor at almost full length along the body of the host. The egg was then seen to flow out from a lateral slit of the ovipositor. Oviposition of two successive eggs could be documented. Subsequent sequences show that many (up to 30) eggs are deposited at different portions of the paralysed, still pulsating host. The development of the parasitoid from egg deposition until adult emergence was recorded at 27-28°C. Emphasis was placed on the following features: Embryonic development, hatch of the L1 larvae that immediately start feeding, food ingestion at high magnification, the rapid growth of the larvae until they are fully developed after about three days. These larvae start to spin a dense silken pupation cocoon. Final sequences show the praepupa and pupa within the cocoon and how, five days after the onset of pupation, an adult male and female emerged from the cocoon.