Biological pest control

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Predatory Polistes wasp looking for bollworms or other caterpillars on a cotton plant

Biological control of pests and diseases is a method of controlling pests (including weeds and diseases) in agriculture that relies on natural predation, parasitism or other natural mechanism, rather than introduced chemicals.

1 Overview
2 Examples of predators
3 Parasitic wasps
4 Plants to regulate insect pests
5 Directly introducing biological controls
6 Economics of biological pest control
7 Negative results of biological pest control
8 References
9 See also
10 External links and references

[edit] Overview

Diagram illustrating the natural enemies of cabbage pests

Biological pest control works to create a system where the insects referred to as pests and the pathogens that cause diseases are kept at manageable levels utilizing the natural processes, particularly predation, within an ecosystem.

[edit] Examples of predators

Ladybird larva eating wooly apple aphids

Lacewings are available from biocontrol dealers.

Ladybugs, and in particular their larvae which are active between May and July in the northern hemisphere, 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 various colored markings, whilst its larvae are initially small and spidery, growing up to 17 mm long. The larvae 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.

Hoverflies. 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 (1 mm), pale yellow white and laid singly near greenfly colonies. Larvae are 8-17 mm 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.

Dragonflies are important predators of mosquitoes, both in the water, where the dragonfly nyads eat mosquito 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 larger fauna such as frogs, toads, hedgehogs, slow-worms and birds. Cats and rat terriers kill field mice, rats, june bugs, and birds. Dogs chase away many types of pest animals. Dachshunds are bred specifically to fit inside tunnels underground to kill gophers and rabbits.

[edit] 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: (5-10 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.

[edit] 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, hedgerow, or beetle bank where beneficial insects can live and reproduce. 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.

The following are plants often used in vegetable gardens to deter insects ^[1]

Basil — Repels flies and mosquitoes.
Catnip — Deters flea beetle.
Garlic — Deters Japanese beetle.
Horseradish — Deters potato bugs.
Marigold — The workhorse of pest deterrents. Discourages Mexian bean beetles, nematodes and others.
Mint — Deters white cabbage moth, ants.
Nasturium — Deters aphids, squash bugs and striped pumpkin beetles.
Pot Marigold — Deters asparagus beetles, tomato worm, and general garden pests.
Peppermint — Repels the white cabbage butterfly.
Rosemary — Deters cabbage moth, bean beetles and carrot fly.
Sage — Deters cabbage moth and carrot fly.
Southern Wood — Deters cabbage moth.
Summer Savory — Deters bean beetles.
Tansy — Deters flying insects, Japanese beetles, striped cucumber beetles, squash bugs and ants.
Thyme — Deters cabbage worm.
Wormwood — Deters animals from garden.

[edit] Directly introducing biological controls

Diagram illustrating the life cycles of Greenhouse whitefly and its parasitoid wasp 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 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.

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 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 (Hymenopterae: Mymaridae) has been introduced to control the glassy-winged sharpshooter Homalodisca vitripennis (Hemipterae: Cicadellidae) in French Polynesia and has successfully controled ~95% of the pest density

[edit] 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 13:1.^{[citation needed]}

[edit] Negative results of biological pest control

In some cases, biological pest control can have negative results, even having more damaging effects than positive. An example is when the mongoose was introduced to Hawaii in order to control the rat population. In overall, the mongoose ate Endemic birds of Hawaii, especially their eggs, more often than it ate rats.

[edit] References

Hoddle M.S., Grandgirard J., Petit J., Roderick G.K., Davies N., 2006. Glassy-winged sharpshooter Ko'ed - First round - in French Polynesia. Biocontrol News and Information 27(3), 47N-62N
U.S. Congress, Office of Technology Assessment 1995. Biologically based technologies for pest control, OTA-ENV-636, Washington, DC. http://www.wws.princeton.edu/ota/ns20/year_f.html