Parasitoid wasp life cycle

Introduced wasps cause all sort of problems for people and native species, but did you know we have native wasps in New Zealand?

What is a native wasp?

Our native wasps are very different to the common idea of what a wasp is. First, they don’t live socially in nests like the invasive yellowjackets and paper wasps that plague our bush and cities. Second, they don’t sting people and cause dangerous allergic reactions. Third, we have about 2,500 species of native wasps compared to five or so species of introduced social wasps.

Parasitoid wasps

Our native wasps are classified as parasitoid wasps. A parasitoid is an organism that feeds on a living host for part of its development, but unlike a parasite, a parasitoid eventually kills its host. Some species of parasitoid wasp develop inside the eggs of other invertebrates, or more rarely, others develop inside an adult host. Most parasitoid wasp larvae develop on the larvae of other insects, especially moths, flies and beetles. Each species of wasp usually develops on a small number of species, but some are restricted to a single host species, and others are generalists, developing in a wide variety of hosts.

The parasitoid life cycle

The parasitoid wasp life cycle begins with the female wasp searching for a host with her antennae. Once she finds a suitable host, she uses her ovipositor, the stinger-like appendage on her abdomen, to lay her egg. Some wasps even use their ovipositors to drill through bark or plant tissue to reach their hosts living inside a tree or plant.

Some parasitoid wasps lay their eggs on the outside of the host, while others stab their ovipositor into the host to inject an egg inside it. The wasp egg is squeezed into a long thin shape as it passes down the ovipositor, but it soon snaps back into shape once inside the host. Special adaptations, including a mixture of viruses injected by the mother, allow the wasp egg to resist the host’s immune system.

Once the wasp egg hatches, a larva emerges and begins to feed on the host. A wasp larva living inside a host will sometimes feed selectively to ensure the host lives for as long as possible, providing a warm cosy place for the wasp larva to develop. Eventually, this feeding kills the host, and the wasp larva forms a pupa. After a while, an adult wasp emerges from the pupa and flies off to find a mate and begin the process all over again.

Helpful wasps

Because parasitoid wasps kill their hosts, they can be useful for biocontrol – the practice of using a beneficial species to manage the impact of a pest. Scientists can travel to the native range of an introduced pest to search for natural enemies there. Once they’ve found a natural enemy, they need to carefully test how it might respond to native species back in New Zealand. This is called host-range testing, and it is a legal requirement before a new species can be released as a biocontrol agent in New Zealand.

Several parasitoid wasps have been successfully introduced to New Zealand to control pests. One of these is the giant sirex wasp parasite Megarhyssa nortoni. This huge wasp was introduced in the 1960s from North America by the New Zealand Forest Service. As its name suggests, it attacks the larvae of sirex wood wasps, which cause damage to pine plantations by burrowing into trees and infecting them with fungi. This species can now be found in Auckland, Waikato, Bay of Plenty, Hawke’s Bay and Nelson regions, where it provides good control of wood wasps. An Irish parasitoid wasp has also been important in controlling the clover root weevil on dairy farms.