Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Ichneumonidae
Subfamily: Rhyssinae
Genus: Megarhyssa
Species: M. nortoni
Binomial name: Megarhyssa nortoni
Common name: Giant sirex wasp parasite, Giant ichneumonid, Western giant ichneumon
Megarhyssa nortoni is large, predatory, ichneumon wasp that was introduced to New Zealand from North America in 1962 and 1964 as a parasite of the larval woodwasp, Sirex noctilio.
Megarhyssa nortoni is now established in many forest areas.
It is a black/reddish brown colour and has distinguishing round yellow spots down the side of the metasoma (abdomen). Its legs are mostly yellow. Its wings are transparent, and the body is elongated with a length 2-3cm. The female is notable for her ovipositor that is twice as long as its body (5 cc to 7.6cc in length). There is a similar native wasp Certonotus fractinervis but it has distinctive yellow spots on top of its metasoma.
The female uses her long ovipositor to deposit eggs on Sirex grubs deep in timber of coniferous trees. She can smell the wood-eating fungus that is utilised by the Sirex larvae to predigest wood pulp. She also uses her antennae to detect vibrations made by the Sarix larvae. When a host is located she curls her ovipositor over her abdomen and inserts the tip of her ovipositor at a right angle into the bark and cuts into the tree until it reaches the sirex’s larval tunnel. The female then deposits a very slender egg through its ovipositor into the tunnel on or near the Sirex larva. When the egg hatches it eats the live Sirex larval host from the inside causing the Sarix larva's eventual death. The wasp’s larva pupates inside its host and emerges the following summer as an adult.
The female does not sting and is harmless to humans. The male wasp is not as colourful and has no ovipositor. The adults feed on nectar, honeydew and water.
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