Every gardener's nightmare : GARDENING

Anna Pavord visits Wye College, where they breed common pests - and the useful insects that prey on them

There is rather a ghastly fascination, as I discovered recently, in peering through a magnifying glass at a purposeful kind of ladybird which is hoovering up mealybugs and cramming them into its mouth with all the delight of a five-year-old at a b irthday tea.

The brown-and-black ladybird, all of 4mm long, is the latest in a series of predators which you can let loose in your garden or greenhouse to attack some of the most troublesome garden pests. Professional growers, forced by supermarkets such as Safeway into producing crops that have been grown without chemical pesticides, have been using this kind of biological control for some time. Gardeners are also learning how to get bugs to fight on their side.

Take whitefly. This is a menace in greenhouses and conservatories. It can ruin tomatoes, cucumbers and aubergines as well as ornamental plants such as geranium and fuchsia. The whitefly's young offspring suck in sap at one end and excrete a sticky syrup called honeydew at the other. The honeydew in turn attracts sooty mould which stops the leaves doing their stuff, converting light into energy. Only a day or two after maturing, the adult whitefly starts to lay a frightening number of eggs. Having hatched, the larva wanders about a bit until it finds a good feeding station. When it has plugged into a leaf vein, it stays at the same trough until it pupates and itself becomes a fly. Then the whole ghastly cycle starts again, the plant meanwhile looking distinctly sick.

The whitefly's most effective enemy is a minute wasp, called encarsia, which commercial growers have used for many years to protect their crops. Encarsia's method of attack is quite grisly. If you are squeamish, skip this bit. The adult wasp lays its eggs inside the larva of the whitefly and, like revivified mummies in a horror film, the young eat their way out from inside, emerging after about three weeks as fully fledged new wasps.

To operate productively, the wasp has to have comfortable living conditions. It will not work while night temperatures are below 10C/50F and is most energetic in day temperatures of 18C/64F and above. If the temperature is too low, the wasp does not breed as fast as the whitefly and so cannot keep its end up in the battle.

Encarsia is a well-behaved guest. If you leave the doors and windows of your greenhouse or conservatory open, it will not suddenly try to bolt. While there are whitefly to attack, it will remain at its post.

The biggest problem, says John Dale, a research director at Biological Crop Protection in Kent, lies in persuading gardeners to get cracking on pests before rather than after they have exploded to plague proportions. Encarsia works best if it is introduced three times at two weekly intervals as temperatures warm up in spring. Hatching at regular intervals, the wasps are brilliantly poised to take on the whitefly where it hurts most, right in the middle of their reproductive cycle. They are powerless against clouds of adult whitefly on the wing.

If this is the stage you are at, you need to reduce the levels of adult whitefly by some other means, before unleashing the wasps. Sticky traps hung above affected plants are effective. So is a portable vacuum cleaner. You just shake the plant gently to get the whitefly on the wing and then suck them up with the nozzle. Solutions of soft soap or washing-up liquid also work.

The scenes inside Biological Crop Protection's glasshouses at Wye College are the stuff of gardeners' nightmares. Here are row upon row of beautifully grown tobacco plants and French beans, sacrificial victims raised only to be deliberately infested withpests. BCP sells a whole range of helpful bugs under its Defenders trademark and breeds them in its glasshouses at Wye by letting them loose on pests.

The tobacco plants are raised as fodder for whitefly and one plant can be unwilling host to more than 50,000 eggs. When the eggs have developed, tobacco leaves, covered with the tell-tale black husks of whitefly larva already taken over by developing encarsia, are hung between the tobacco plants like washing on a line. The young wasps hatch, move on to the infected tobacco plants next to them and convert the whitefly larva into yet more baby wasps. It is all completely back to front in terms of what gardeners are usually doing, which is what makes it so odd to see.

The French beans are there to feed red spider mite and BCP's glasshouse shows in a hideously graphic way what damage these pests can do. The beans are laid out in blocks. The first block is full of newly sown beans, the next of juicy young plants. At thethird stage, the plants are deliberately infected with red spider and at the fourth you can see only too clearly the classic signs of infestation: leaves mottling, bleaching and eventually dropping as the mites suck away at sap on the underside of the foliage. At the fifth stage the predator is introduced and when it has fattened itself up happily on red spider mite, it is harvested, the sixth and final stage. By this time, the beans have had it. They are textbook examples of What Not To Do.

The spider mite predator is another mite called phytoseiulus which unlike the waspish encarsia, will feed on its prey at any stage, juvenile or adult. This makes it a very effective ally and you can use it inside or out, provided the temperature is at least 10C/50F. It is more effective at higher temperatures as it breeds faster.

Commercial growers have used this mite very effectively to protect strawberries growing outside, but gardeners tend to find red spider mite most problematic under glass, especially in conservatories where conditions are hot and dry. "Damping down", keeping the air moist by watering or misting, may persuade them to go elsewhere. If it doesn't, you can now call up the phytoseiulus to help.

All the above predators are available direct from Defenders, PO Box 131, Wye, Ashford, Kent TN25 5TQ. Telephone orders to 0233 813121 and the bugs will be dispatched by first-class post on Thursday to arrive in time for the weekend. The mealybug predator(the ladybird) is for indoor use only and needs a minimum temperature of 20C/68F. Price £9.95 for 10. The whitefly parasite operates (indoors only) at a minimum air temperature of 18C/64F. Use it at the first signs of adult whitefly. Three doses, delivered at two weekly intervals cost £14.95. The red spider mite predator which you can use inside or out, requires a minimum temperature of 10C/50F. Use it at the first signs of leaf mottling. Three doses, delivered together, cost £14.95, a single dose £5.95.

There will be a Bug Workshop at Hadlow College, Kent on 22 March (1-30-4.30pm). Cost £25. Information from Louise Labuschagne, 11 Pyrus Close, Chatham, Kent ME5 9QF (0634 200978).

Wye College in Kent has arranged a biocontrol workshop for Saturday 1 April. It includes a visit to BCP's glasshouses, buffet lunch and samples of biological controls. Cost £40. Further information from Mike Copland or Jon Varley at Wye (0233 812401 ext 369)