Parasitic wasp

The use of the parasitic wasp for aphid or pest control is nothing new. Victorian greenhouses used such natural methods and in fact, their use was widespread throughout the world until the invention of DDT in the early 1940s. From then on through to the 70s chemical means were sought to eradicate aphids, with varying degrees of success.

Since the 1970s, however, we have recognised the dangers to human health of consuming fruit and vegetables which have been treated with pesticides, and the move nowadays is towards natural means of aphid control.

To this end, parasitic wasp farms have been set up, and you can buy their eggs to introduce a colony into your greenhouse to bring an aphid attack under control. Parasitic wasps do survive in the wild, but in most of the cooler temperate areas of the world, they are killed off by frost.

They can live year round in a heated greenhouse however.

Parasitic wasps' own survival involves a co-dependent relationship with the aphid. There are over 60,000 different species of parasitic wasp, and each one seems to target a specific type of aphid. If your greenhouse suffers from whitefly, greenfly and black fly, you may have to consider introducing three distinct species of parasitic wasp to counteract your problem. The parasitic wasp will not kill all your aphids, but they will keep their numbers under control.

They do this by injecting an egg into the abdomen of the aphid. Depending on the species, an adult female parasitic wasp can inject eggs into up to 15 aphids each day of their average 8 day life cycle.

Within two days, the aphid usually dies as the larva grows inside it, sustaining itself by eating the aphid from the inside out. You can spot an oviposited aphid as its body will be swollen accompanied by a color change - greenflies turn brown, whiteflies turn black. After 8 -10 days the pupa mutates into an adult and leaves the aphid body (known as a mummy) by eating a hole in the abdomen to escape.

It all sounds pretty gruesome, but this is how nature designed them.

The parasitic wasp has another trick up its sleeve to ensure continuation of the species. The female can choose the sex of her offspring. Unfertilised eggs are always female, and fertilized eggs are male. Depending on how many aphids there are, she can manipulate their numbers by having more, or less, female offspring. The males in some species of parasitic wasp assist in the control of aphids by feeding off them. In others, the males have very little function except to occasionally mate with a female, and this usually only happens towards the end of a season when the females are considering hibernating. An impregnated female can hibernate for months and the fertilised egg inside her remains in a state of suspended animation until the temperature rises.

The parasitic wasp does not function in temperatures under 57F. Their optimum temperature for activity and breeding is around 70F. Aphids, on other hand, continue breeding until the temperature falls below 40F. This gap ensures the continuation of their species.