Title: Exotic Intruders

Author: Joan Druett

Publication details: Heinemann, 1983, Auckland

Digital publication kindly authorised by: Joan Druett

Part of: New Zealand Texts Collection

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Exotic Intruders

Insect assistant number one—Rhyssa

page 179

Insect assistant number one—Rhyssa

After the damage that the accidentally-introduced Sirex wasp caused in pine forests became obvious, the New Zealand Government consulted with the Imperial Institute of Entomology in Britain, and then received, in 1929, a shipment of 1 300 larvae of the European ichneumon wasp Rhyssa persuasoria. These had been collected from infested tree-trunks in English woods. Further importations took place, and the wasp was also bred in the Cawthron Institute in Nelson.

Rhyssa is an attractive little insect, with the female easily distinguished by the very long ovipositor that trails behind her. The body is black, with white markings. The female Rhyssa is attracted to trees that have the fungus carried by Sirex growing inside the trunk, enticed by the smell of fungus. She drills around the trunk in a random pattern until, at last, she strikes a Sirex larva. She then stings the grub, and paralyses it so that it is motionless but living prey for her young to feed on. An egg passes down her ovipositor and lies on the skin of the Sirex grub. The egg hatches in a day or so, and the Rhyssa grub feeds luxuriously on its captive meat, in leisurely fashion, and then lies in the Sirex tunnel and eventually pupates. The adult, when it hatches, bites its way out of the wood and flies away, continuing its honeyed existence by feeding on nectar and the sweet dew excreted by aphids and scale insects.

Rhyssa drilling into a radiata pine tree to sting, paralyse and lay an egg on a Sirex larva. The ovipositor is about 40 mm long, while the body is 35 mm long.

Rhyssa drilling into a radiata pine tree to sting, paralyse and lay an egg on a Sirex larva. The ovipositor is about 40 mm long, while the body is 35 mm long.