Predators succeed where chemicals fail

Growers of cucumbers and tomatoes in hothouses now routinely use tiny predators to control pests as an alternative to pesticides.

Parasitic wasps are released which consume whitefly larvae from within, and a mite-eating spider mite is used to control herbivorous pest species.

In fact, growers are now deliberately introducing pest species into their hothouses. The predator is brought in a certain number of days afterwards and it then keeps a close check on the number of its prey throughout the growing season, holding it down to a level which does negligible damage to the greenhouse crop.

This is more effective than waiting until the pest arrives of its own accord and only then introducing the predator. By then it could be too late, with the prey population explosion doing severe damage.

In the Netherlands, which is one of the world's leading countries for greenhouse cultivation, growers rely almost entirely on using pests' natural enemies to control them. One reason for this is that the whitefly and spider mites have evolved resistance to pesticides.

The world outside the glass is much harder to control and predators are rarely used against pest species in Britain.

The Forestry Commission, however, has used Rhizophagus grandis, a predatory beetle, to attack the great spruce bark beetle.

The latter is a European invader which arrived in 1982, threatening severe damage to the commission's millions of Sitka spruces. The former, another European species, which eats only the bark beetle, was deliberately introduced in affected areas a few years later. It now seems to have established itself in Britain and is holding back the bark beetle.

Myxomatosis was a devastatingly effective method of reducing rabbit numbers but it was uncontrolled and transient. The disease originated in South America, is caused by a virus and transmitted by fleas.

In 1952 it was deliberately introduced to an estate in France. A year later it reached Kent and spread rapidly through Britain's rabbits. But their numbers have now reached pest proportions once more - and they are resistant to the myxomatosis virus.