Biological Control of Insect Pests

2001-09-01

Although chemical pesticides are cheap and often very effective, they are causing a number of problems. For farmers, the most serious are pest resistance, and health problems when chemicals are applied, particularly in the enclosed space of greenhouses. For consumers, the two main problems are pesticide residues on food, and the damage done to the environment.

Because of these concerns, there is a great deal of interest in biological pest control, based on living organisms. These include predators, parasites and entomopathogens (i.e. disease-causing microbes and fungi which infect insects). Sometimes biological control measures are used alone. More often, they are combined with chemical applications and cultural methods in systems of integrated pest management.

Biological Control in Greenhouses

Greenhouses are very vulnerable to invasions by insect pests, which enter though open vents and doorways. Once pests are inside, they find a protected habitat which is favorable for the rapid buildup of pest populations.

Greenhouses as a closed and controlled environment are well suited to biological control, particularly of pests which attack the non-food parts of the plant (e.g. the roots, stem and leaves of fruit vegetables). A little insect damage to these parts does not matter, since the consumer will never see them.

Biological control is also an advantage when fruit vegetables are grown. These need pollinating insects such as bees, and biological control agents are more selective than chemicals.

Good sanitation is a simple and effective form of biological control. Growers should remove weeds growing around the greenhouse, since these may harbor pests. They must also be careful to use clean planting materials.

WHY Do Greenhouse Growers Still Favor Chemicals?

There are many successful programs using biological control in greenhouses. However, chemicals are still used to control pests in 95% of the world's greenhouses. Why is this? There are several reasons.

• - Chemical pesticides are a multimillion dollar business. The multinational companies which produce them have a great deal of influence. At both at a government level and a farm level, most of the information about pest control is coming from chemical companies.
• - Growers are accustomed to the way chemicals work. They see pest control as a "magic bullet" which destroys pests suddenly and immediately. Biological control tends to be slow and gradual. It takes time for natural enemies to increase in numbers and have an effect on pest populations.
• - While this is happening, the pests continue to attack crops. Most Asian consumers like to buy perfect fruit and vegetables without any insect damage. If biological control is to succeed, both farmers and consumers will have to expect vegetables and fruit to have some slight signs of pest damage.
• - Costs and returns - biological control needs to be economic. At the moment, many commercially produced biocontrol agents are more expensive than chemicals.
• - Pioneer farmers who are the first to begin practicing biological control may find their farms becoming a "wildlife refuge". Pests may come streaming onto their farms when neighboring farmers apply chemicals. This is a poor incentive to begin biological control!!
• - The application of biological control methods needs more expertise than the use of chemicals . Growers need to understand the population dynamics of the pest, and the relationship between the pest and the control agent. They often need to monitor the levels of pests, which is technically quite difficult, but necessary if the right species of natural enemy is to be applied at the right time. Biological control agents should be applied early, before population levels build up.

Some Major Greenhouse Pests

Two-Spotted Spider Mite (TSSM) (Tetranychus Urticae)

The two-spotted spider mite is a common cause of crop damage in greenhouses. It is difficult to control with chemicals. Most species have become resistant, and the mite hides underneath leaves where sprays and powders cannot reach. There is also the problem of toxic chemical residues, especially with crops such as strawberry which are eaten complete with the skin.

A species of predatory mite, Phytoseius persimilis, is being mass reared in Korea as a biological control of two-spotted spider mite in strawberry. Two releases per month at a rate of only three mites per square meter proved to be an effective control.

The control effect can be enhanced by spraying selected miticides such as abamectin, to reduce the density of spider mites before the release of the predatory mites.

P. persimilis is also used outdoors. One paper presented at the seminar described its use to control spider mites in grape orchards in Japan. It was not applied directly to the vines, since it cannot live on tall, woody plants like grape. Instead, it was applied to ground cover where the spider mites spend the winter. Grape orchards treated in this way had only 10% of the spider mites found in untreated orchards.

Whitefly

The greenhouse whitefly is an introduced pest in Asia. It is a major pest which damages a wide range of crops. The tiny wasp Encarsia formosa is a parasitoid of the whitefly. It lays its eggs on the whitefly eggs, which serve as food for the young wasp larvae after they hatch.

This wasp has been used as a biological control agent in Europe for some years. It is also proving an effective control of greenhouse whitefly in Korea. However, greenhouses must be heated so that winter temperatures do not fall below 10oC.

Selected strains of a fungal pathogen, Verticillum lecanii, are also being used as a means of controlling whitefly. This funguis are easy to mass produce, the conidia being sprayed in a suspension. The same fungus is being tested as a control of cotton aphid (Aphis gossypii), an important pest of greenhouse vegetable crops.

Thrips

Thrips are tiny insects less than a tenth of an inch long. Like mites, they damage the plant by sucking the juices. Predatory mites are sometimes introduced into greenhouses to control thrips. Other predators are Anthocorid mites such as Orius.

Which predatory species is used depends partly on the thrips species, and partly on the greenhouse environment. Different species of predatory mites prefer different temperatures and humidity levels. For example, the predatory mite Phytoseius persimilis, used to control two-spotted spider mites in Korea (see above), can only survive if the relative humidity is 60% or more.

Nematodes

Plant-parasitic nematodes are a common greenhouse pest. Root-knot (Meloidogne) nematode species cause serious damage in Asian countries, attacking cucurbit crops such as melon and cucumber. The nematodes injure the roots of plants, reducing their take-up of water and nutrients, and creating wounds which are entry points for fungi and bacteria.

Current research in Korea is studying the control of nematodes by a bacterium, Pasteuria bacteriuma, which is parasitic on root-knot nematodes. Infected nematodes remain alive, but they do not reproduce, so numbers remain small.

Pasteuria is resistant to chemicals, and to fluctuations in the environment. However, we do not yet know how to mass produce Pasteuria for distribution to farmers. At the moment, it can be made to reproduce only inside a living nematode.

Biological Control in Open Fields

Biological control is much more difficult to apply in the open field. Control agents which are effective in the laboratory or in greenhouses often "disappear" when they are used in the field, their effect drowned among that of hundreds of other species. However, biological control in field crops is receiving a lot of attention. In most countries, these are the bulk of crops being grown by farmers.

Perennial outdoor crops have featured in successful biological control programs. The predatory mite Amblyseius womersleyi has been used to control spider mites in Korean apple orchards.

The same species has been used in tea fields in Japan to control spider mites and other pests. Strains of A. womersleyi were found which were resistant to synthetic pyrethroids and organophosphates.

Unfortunately, this resistance was not stable. It did not persist unless the mites were sprayed repeatedly. However, release of resistant strains of A. womersleyi combined with adequate applications of pyrethroids, gave good control of spider mites, yellow tea thrips and tea green leafhopper.

Combining Biological Control and Pesticides

Farmers need to control a whole range of pests, not just one, and crop diseases as well. Often the most effective control programs are based on a combination of biological and chemical control.

For this to be possible, scientists must select strains and species of natural enemies which are resistant to chemicals. They must also identify those chemicals which are compatible with natural enemies. For example, the fungal pathogen V. lecanii, used to control whitefly and cotton aphid (see above), is highly resistant to the fungicides dimethomorph and procymidone. Other common fungicides, such as benomyl and propineb, were highly toxic.

Conclusion

The key to biological pest control is effective combinations of different control methods in an IPM program. This often involves the use of chemical pesticides that are compatible with natural enemies.

On the demand side, consumers must be educated to accept agricultural products which are slightly damaged by pests. Participants in the FFTC seminar recommended that researchers should select the pests and agricultural systems which are amenable to biological control, and where effective results can be expected which growers can use.

Index of Images

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  Figure 1 Insect Pests Can Enter Greenhouses through Doors and Other Openings

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  Figure 2 Vents Left for Ventilation Allow the Entry of Greenhouse Pests

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  Figure 3 Larvae of Black Vine Weevil Infected with the Nematode Heterohabditis (Healthy White Larva on Right)

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  Figure 4 Two Entomopathogenic Nematodes