Tony Derwent, from Alexandra, asks :-

We found six of these insects in our garage at Alexandra. What are they?

John Marris, an entomologist in charge of the Lincoln University Entomology Research Collection, responded.

The photo is of a male sirex wasp, or Sirex noctilio in its scientific form. The female is similar in appearance but has a steel-blue abdomen tipped with a long ovipositor – a specialised egg laying device - looking like a giant stinger. Sirex wasps are European in origin but were accidentally introduced into New Zealand in the early 1900s. These insects belong to an unusual group of wasps characterised by their broad abdomen, in contrast to the narrow waists of most wasp species and their ant and bee relatives. The great majority of wasps are predatory or parasitic, whereas sirex wasps and their wide-bodied kin are herbivores.

Sirex wasps are often found around pine plantations but are most frequently encountered when brought into houses with firewood. Despite their large size, up to 36 mm in length, and frightening waspish appearance, these insects are quite harmless to humans and do not sting.

Sirex larvae feed mainly on the wood of pine trees. They are a minor forestry pest due to larval feeding damage, which can also introduce harmful bacteria. Adult wasps, once mature, exit the trees from December through to April, leaving tell-tale circular emergence holes around 3 to 6 mm in diameter.

Sirex damage in New Zealand forests has been minimised by improved forestry management practices and through the successful introduction of a suite of parasitic wasps that attack and kill sirex larvae.

These were introduced to New Zealand as biological control agents to combat the threat posed to forestry by sirex wasps. Most remarkable among these is the giant sirex wasp parasite (Megarhyssa nortoni), which is able to detect a sirex larva merrily munching inside a tree. Having located her prey, the female parasite then proceeds to drill down through pine wood with her needle-like ovipositor, which can be up to 75 mm in length. An egg is then passed down the egg tube with sufficient precision to lay it on an unsuspecting sirex larva. The emergent parasite larva then devours the sirex larva, killing it in the process, and later developing into an adult wasp to carry on the process.