In a healthy natural woodland where there is little direct human intervention, pest and disease organisms will always be present, but, unless there is a drastic environmental change, will normally kept in a state of equilibrium where they are not able to get out of hand to a level which is detrimental to the overall woodland community. This is the model for which the sensitive organic gardener will strive, but at the same time it is wise to recognise that there will often be some degree of intervention needed to manipulate the natural checks and balances to the gardener's favour, particularly if growing plants for food or ornament which would not normally occur in natural situations. Therefore biological control is about developing a range of techniques that use living organisms to maintain the beneficial equilibria in garden landscapes without causing adverse effects to humans or the wider environment. Of course, introducing exotic insects into any ecosystem entails taking risks of moving further from the intended equilibrium through unexpected interactions.

An important part of the biological gardening approach is to become familiar with the various life forms that inhabit the garden, predators as well as pests, and also their life cycles, patterns of feeding and the habitats that they prefer.

Insects have devised many ways to eat other insects. Predators will directly attack and devour their prey, whilst parasitoids will deposit an egg within another insects body, from which a larva will emerge which will devour the host's innards.

Examples of predators:

Ladybugs, and in particular their larvae which are active between May and July, are voracious predators of aphids such as greenfly and blackfly, and will also consume mites, scale insects and small caterpillars. The ladybug is a very familiar beetle with red and black markings, whilst its larvae are initially small and spidery, growing up to 17mm long. It has a tapering segmented grey/black body with orange/yellow markings nettles in the garden and by leaving hollow stems and some plant debris over-winter so that they can hibernate over winter.

Hoverfly are another very welcome garden predator. Resembling slightly darker bees or wasps, they have characteristic hovering, darting flight patterns. There are over 100 species of hoverfly whose larvae principally feed upon greenfly, one larva devouring up to fifty a day, or 1000 in its lifetime. They also eat fruit tree spider mites and small caterpillars. Adults feed on nectar and pollen, which they require for egg production. Eggs are minute (1mm), pale yellow white and laid singly near greenfly colonies. Larvae are 8-17mm long, disguised to resemble bird droppings, they are legless and have no distinct head. Semi-transparent in a range of colours from green, white, brown and black.

Hoverflies can be encouraged by growing attractant flowers such as the poached egg plant (Limnanthes douglasii), marigolds or phacelia throughout the growing season.

Dragonflys are important predators of mosquitoes, both in the water, where the dragonfly nyads eat mosquito larva|larvae, and in the air, where adult dragonflies capture and eat adult mosquitoes. Community wide mosquito control programs that spray adult mosquitoes also kill dragonflies, thus removing an important biocontrol agent, and can actually increase mosquito populations in the long term.

Other useful garden predators include lacewings, Anthrocorid bugs, rove and ground beetles, aphid midge, centipedes, predatory mites, as well as megafauna such as frogs, toads, hedgehogs, slow-worms and birds.

Parasitic wasps

A diverse range of wasps lay their eggs on or in the body of an insect host, which is then used as a food for developing wasps. Parasitic wasps take much longer than predators to consume their victims, for if the larvae were to eat too fast they would run out of food before they became adults. Such parasites are very useful in the organic garden, for they are very efficient hunters, always at work searching for pest invaders. As adults they require high energy fuel as they fly from place to place, and feed upon nectar, pollen and sap, therefore planting plenty of flowering plants, particularly buckwheat, umbellifers and composites will encourage their presence.

Three of the most important groups are;

• Ichneumon flies: (5-10mm). Prey mainly on caterpillars of butterflies and moths.
• Braconid wasps: Tiny wasps (up to 5mm) attack caterpillars and a wide range of other insects including greenfly. A common parasite of the cabbage white caterpillar- seen as clusters of sulphur yellow cocoons bursting from collapsed caterpillar skin.
• Chalcid wasps: Among the smallest of insects (<3mm). Parasitize eggs/larvae of greenfly, whitefly, cabbage caterpillars, scale insects and strawberry tortrix moth.

Plants to regulate insect pests

Choosing a diverse range of plants for the garden can help to regulate pests in a variety of ways, including;

• Masking the crop plants from pests, depending on the proximity of the companion or intercrop.
• Producing olfactory inhibitors, odors that confuse and deter pests.

Acting as trap plants by providing an alluring food that entices pests away from crops.

• Serving as nursery plants, providing breeding grounds for beneficial insects.
• Providing an alternative habitat, usually in a form of a shelterbelt or hedgerow where beneficial insects can live and reproduce during the 'off' season. Nectar rich plants that bloom for long periods are especially good, as many beneficials are nectivorous during the adult stage, but parasitic or predatory as larvae. A good example of this is the soldier beetle which is frequently found on flowers as an adult, but whose larvae eat aphids, caterpillars, grasshopper eggs, and other beetles.

Directly introducing biological controls

Most of the biological controls listed above depend on providing incentives in order to 'naturally' attract beneficial insects to the garden. However there are occasions when biological controls can be directly introduced. Common biocontrol agents include parasitoids, predators, pathogens or weed feeders. This is particularly appropriate in situations such as the greenhouse, a largely artificial environment, and are usually purchased by mail order.

Some biocontrol agents that can be introduced include;

• Encarsia formosa. This is a small predatory chaclid wasp which is parasitical on whitefly, a sap-feeding insect which can cause wilting and black sooty moulds. It is most effective when dealing with low level infestations, giving protection over a long period of time. The wasp lays its eggs in young whitefly 'scales', turning them black as the parasite larvae pupates. It should be introduced as soon as possible after the first adult whitefly are seen. Should be used in conjunction with insecticidal soap.
  

Above; diagram illustrating the life cycles of Greenhouse whitefly and its parasitoid wasp Encarsia formosa. Wasp inserts its egg into a whitefly larva that has reached its third developmental stage, egg hatches and the young wasp larva eats the whitefly nymph from within. Unparasitized whitefly gives rise to second generation adults

• Red spider mite, another pest found in the greenhouse, can be controlled with the predatory mite Phytoseilus persimilis. This is slightly larger than its prey and has an orange body. It develops from egg to adult twice as fast as the red spider mite and once established quickly overcomes infestation.

• A fairly recent development in the control of slugs is the introduction of 'Nemaslug', a microscopic nematode (Phasmarhabditis hermaphrodita) which will seek out and Parasitize slugs, reproducing inside them and killing them. The nematode is applied by watering onto moist soil, and gives protection for up to six weeks in optimum conditions, though is mainly effective with small and young slugs under the soil surface.

• A bacterial biological control which can be introduced in order to control butterfly caterpillars is Bacillus thuringiensis. This available in sachets of dried spores which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees. The bacterial disease will kill the caterpillars, but leave other insects unharmed. There are strains of Bt that are effective against other insect larvae. Bacillus thuringiensis israelensis|Bt israelensis is effective against mosquito larvae and some midges.

• A biological control being developed for use in the treatment of plant disease is the fungus Trichoderma viride. This has been used against Dutch Elm disease, and to treat the spread of fungal and bacterial growth on tree wounds. It may also have potential as a means of combating silver leaf disease.

• Economics of biological pest control

  Biological control proves to be very successful economically, and even when the method has been less successful, it still produces a benefit-to-cost ratio of 11:1. One study has estimated that a successful biocontrol program returns £32 in benefits for each £1 invested in developing and implementing the program, i.e., a 32:1 benefit-to-cost ratio. The same study had shown that an average chemical pesticide program only returned profits in the ratio of 2.5:1.