Parasitic insects
Most insect parasitoids are wasps or flies. Parasitiods comprise a diverse range of insects that lay their eggs on or in the body of an insect host, which is then used as a food for developing larvae. 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. Four of the most important groups are:
Ichneumonid wasps: (5-10 mm). Prey mainly on caterpillars of butterflies and moths.
Braconid wasps: Tiny wasps (up to 5 mm) 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 (<3 mm). Parasitize eggs/larvae of greenfly, whitefly, cabbage caterpillars, scale insects and strawberry tortrix moth.
Tachinid flies: Parasitize a wide range of insects including caterpillars, adult and larval beetles, true bugs, and others.
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.
Encarsia formosa
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 chalcid 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.
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. Bt israelensis is effective against mosquito larvae and some midges.
A biological pest 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.
The parasitoid Gonatocerus ashmeadi (Hymenoptera: Mymaridae) has been introduced to control the glassy-winged sharpshooter Homalodisca vitripennis (Hemipterae: Cicadellidae) in French Polynesia and has successfully controlled ~95% of the pest density