Parasitic wasps are by far the largest group of wasps. They do not sting, but the females use their needle-like ovipositor (egg-laying tube) to place eggs on or inside another insect, usually its egg, larva or pupa.
Eating the host
When the wasp larva hatches from the egg, it feeds on its host – at first on non-essential body tissues so as not to kill it too quickly. Eventually it consumes the host. The fully fed wasp larva then forms a pupa, and eventually an adult parasitic wasp emerges.
Choice of host
Parasitic wasps use a wide range of hosts. Some attack only eggs, others only larvae, others pupae. Only rarely do they attack adult insects. Some species lay a single egg per host, others several. Some eggs are laid on the outside of the host’s body, others are injected inside it.
Each parasitic species usually attacks just one or a small group of related species. This makes them useful for controlling insect pests – many parasitic wasp species have been brought into New Zealand for this purpose. For example, the parasitic wasp Apanteles glomeratus helps control white butterfly, a pest to horticulture.
The tiny fig wasp lives and feeds inside growing figs. As it enters a fig to lay its eggs, it also fertilises the plant with pollen carried on its body. Each species of fig has its own species of pollinating wasp. New Zealand has two Australian wasp species, which pollinate the Moreton Bay and Port Jackson fig trees.
Native parasitic wasps
Most of New Zealand’s native wasps are parasitic, and new species are constantly being discovered. Recently a whole new family (Maamingidae) was found in New Zealand; this family is found nowhere else.
Most are small – the size they can reach is limited by the size of their host. The smallest, with a body length under 1 millimetre, develop inside small hosts like insects’ eggs, or scale insects. The largest, 4 centimetres long, is Certonotus fractinervis, which attacks the fleshy wood-boring larvae of the native elephant weevil (Rhyncodes ursus). The female drills its long ovipositor into dead beech tree trunks to find a larva of the weevil.
Place in the ecosystem
Within their natural ecosystems, native parasitic wasps act as natural enemies that control populations of native insects. Many live in leaf litter. Some have reduced or absent wings, so they cannot fly, and look rather like ants. One of these, Betyla fulva, is a natural parasite of the native glow-worm. It attacks glow-worm pupae in the bush and in damp gullies, but apparently not those living in caves.
Take one egg …
A single egg of one tiny species of parasitic wasp divides into 1,000–2,000 embryos inside a single green looper caterpillar. Each embryo then becomes a wasp larva. The larvae wait until the caterpillar is fully grown before they eat it out. The caterpillar dies while starting to spin its silken cocoon.
Some associations between parasitic wasps and their hosts are ancient. For example, several species of a primitive parasitic wasp (Archaeoteleia) parasitise the eggs of the cave wētā. The presence of closely related but different Archaeoteleia species in Chile indicates a common ancestry going back at least to the Cretaceous period. At this time, up until 85 million years ago, New Zealand and Chile were joined in the supercontinent of Gondwana. Cave wētā are also found in other lands that were once part of Gondwana, and it seems probable that their eggs have been parasitised by Archaeoteleia for at least the last 85 million years.