The Giant Wood Wasp is an impressive beast. The 'Giant' part of its name refers to its impressively large size and it is popularly known as a 'Wasp' because the adults are banded black and yellow and the female has a long, needle-like projection that looks suspiciously like something that could inflict a painful sting.
Not so, thankfully. The female's long, needle-like projection is not a stinger; it is an ovipositor, a narrow tube, like a drinking straw, through which she lays her eggs. So, despite her large size and slightly fearsome appearance, she is quite harmless. The reason the ovipositor is so long is that she inserts it under loose flakes of tree bark to lay her eggs as close to wood as possible and to protect them from predators and the elements.
The Giant Wood Wasp is not a wasp. It is a sawfly, a member of a group of insects closely related to ants, bees and wasps. Sawflies differ from wasps in that they don't have a narrow wasp-waist and the females' ovipositors are often equipped with a saw edge for cutting or drilling into wood before laying.
Adult sawflies are not well-known as they live for only one week or so. While life expectancy for the adult can be as short as one week, its grub lives for several years depending on the species. The Giant Wood Wasp adult lives for 7-9 days between late May to early October and its grub lives for up to three years.
Females lay several hundred eggs under the bark of dead or unhealthy trees or freshly-felled logs. The eggs hatch into grubs that are creamy white in colour and cylindrical in shape, resembling the familiar Leatherjacket, the grub of the Crane Fly popularly known as Daddy-long-legs.
The Giant Wood Wasp grub bores into the timber eating the wood ahead of it. As it progresses it bores a round tunnel that gets wider as the grub gets fatter. It has a dark spine on its rear end and that can be seen in the image above together with the insect's droppings, known as frass.
Grubs can grow to 30mm long and tunnels can be up to 8mm across and 75cm long resulting in serious damage to commercially-valuable stands of coniferous trees. Exit holes can often be seen on dead trees where the grubs emerged and tunnels are regularly seen by those sawing or splitting timber.