There are several kinds of biological control. Classical biological control introduces exotic enemies to control exotic pests. The cost to the farmer is generally low. The same is true of conservation techniques, such as the preservation of natural enemies or their enhancement. The third approach is based on augmentation. It involves the mass rearing and release of a large number of natural enemies. In augmentation, it is the insects actually released which are expected to achieve most of the control effect. (This can be compared to classical control, in which the offspring of future generations are seen as the control agents). Augmentation is usually used in greenhouses.
Many of the most successful biological pest control programs have been in greenhouses. They represent a controlled environment 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 such as tomato).
Greenhouses are also a suitable environment for control of insect pests by introduced entomopathogens (pathogens which infect only insects). These pathogens tend to be host specific, and can often achieve a high level of control. Moreover, the greenhouse is protected from extremes of temperature, UV light and humidity. This favors the spread of entomopathogens.
Some key greenhouse pests which are now being widely controlled by biological agents are two-spotted spider mites, whitefly, thrips, and plant-parasitic nematodes. A number of control agents are being mass reared in several Asian countries to control these pests. They include the predatory mite Phytoseius persimilis, and the parasitoid wasp Encarsia formosa. Several strains of parasitic bacteria and fungi which infect insect pests and nematodes have also been found to be effective.
Biological control is much more difficult to apply in the open field. However, biological control of field crops is receiving a lot of attention, because in most countries these are the bulk of crops actually grown by farmers. Two species of Orius bug are being tested for control of thrips on crops grown in the open field in Taiwan. Although natural population densities of Orius are low, it was possible to mass produce Orius spp. in the laboratory. The effect tends to be greater if larger numbers of Orius are applied to crops in the field, but the economic cost and returns must also be considered.
Open-air crops also include perennial crops such as fruit trees and tea. Predatory mites such as Amblyseius womersleyi have been effective in controlling spider mites in Asian apple orchards. Population densities of this mite vary widely, from one orchard to another. If growers want to encourage high levels to control spider mites, they must practice the right kind of orchard management, e.g. monitoring pest levels and using selective pesticides.
Even if biological control is effective against one pest, the crops may suffer from other pests or plant diseases which need to be controlled by chemicals. An important aspect of biological control is the use of pesticides which are compatible with natural enemies.
For example, if the fungal pathogen V. lecanii is being used to control whitefly and cotton aphid on cucumber, growers must be careful about applying fungicides. The common fungicide dimethomorph has little effect on either spore germination or mycelial growth of V. lecanii. In contrast, propineb and several other formulations are highly toxic.
A similar approach may be followed to conserve spiders in rice fields. They can help suppress pests such as rice leafhoppers and planthoppers. Some spiders may eat as many as 15 planthoppers a day. It has been found that different types of pesticide have different effects on spider populations. Dust or granule formulations are particularly lethal.
Besides looking for pesticides that are less toxic to natural enemies, researchers are also looking for natural enemies which are relatively resistant to pesticides. A study of the predatory mite Amblyseius womersleyi in tea fields in Japan found strains in central Japan that were highly resistant to both synthetic pyrethroids (SP) and organophosphates. Unfortunately, this resistance was not stable. It did not persist unless the mites were sprayed repeatedly. However, the release of SP resistant A. womersleyi combined with adequate applications of SP gave effective control of Kanzawa spider mite, yellow tea thrips and tea green leafhopper.
Some scientists see chemical control as the cornerstone, on which all biological control strategies must be built. Others see biological control as the best kind of pest control, which should determine how and whether chemicals are used. Whether growers use one or the other will depend on price, safety, and consumer preference.
However, the conservation approach has been found to be remarkably effective. Simply reducing the level of pesticide sprays gives natural enemies the opportunity to increase their numbers and control pests. Sometimes this is enough in itself to control important insect pests. However, it takes time. The question is how long the delay will last, and how long the grower is willing or able to wait.
Moreover, chemical pesticides are a billion-dollar business. The multinational companies which produce most of them have a great deal of influence. At both a national and a farm level, much of the information about pest control comes from chemical companies. They also have a lot of money to spend for promoting their products.
Chemical pesticides have an annual value of US $8,000 million. This can be compared to only US$132 million for microbial pesticides, most of them BT (Bacillus thuringiensis) products.
Biological control tends to be long-term and delayed, whereas chemical control tends to be sudden and immediate. Successful programs of biological control always involve information for farmers, pointing out the advantages (e.g. cost and net returns, safety and persistence). In Malaysia, Vietnam and Thailand, government programs combine the production of natural enemies with extension information for farmers. This is an effective way of promoting biological control among the small-scale farmers of Asia.
In addition to information, it is the cost of biological control which will determine its appeal to farmers. If biological control agents are purchased as commercial inputs, their cost must be related to the value of the crop. Biological control must not only be technically feasible, but give farmers a net profit.
In turn, the cost will depend on efficient methods of mass rearing. Research to find the optimal conditions takes a long time, and regional exchange of such information would reduce the cost of biological control agents. Quality control is a major issue in mass rearing. Storage is another problem. Since biological control agents are living organisms, they tend to have a short shelf life, so they must be mass produced shortly before they are used. This adds to the cost of production and distribution.
Twenty or thirty years ago, researchers could usually import natural enemies from abroad quite freely. There was a general assumption that such agents would be benign. Nowadays there is more concern about the impact of imported species on the indigenous fauna. There is particular concern about generalist species such as predatory mites.
The FAO Guidelines are widely accepted as reasonable and realistic. To some extent, the current restrictions on importing exotic natural enemies could be seen as an opportunity, because they force researchers to pay more attention to indigenous natural enemies. The natural enemies which are widely used today are only a very small proportion of potential biological control agents. Most of these have not yet been studied.
The key to biological pest control is effective combinations of different control methods in an IPM program which can suppress damage from all pests and diseases to below the economic threshold level. This often involves the use of chemical pesticides that are compatible with natural enemies. There is also potential for the selection of strains and species of natural enemies that are resistant to chemical treatments. On the demand side, consumers must be educated to accept agricultural products which may be slightly damaged by pests. However, in some Asian countries, the appearance of fresh products is important and it may be difficult to change this culture. In other countries, the presence of a few pests and/or damage suggests that the product is devoid of pesticide residues and is considered to be a positive sign.
Although scientists may work on the control of a single pest, farmers in reality are faced with a complex or "guild" of pest species. Biological control today is looking at these complexes of pest species, and also combinations of plant pests and pathogens. Reduced pesticide use is emphasized, and the protection of natural enemies, in order to allow the natural balance between pests and their predators and parasites to be restored and maintained.
Cost and returns will be basic to acceptance by growers of the biological approach to pest control. Low costs essentially depend on effective methods of mass production, and a reasonably long storage life. Quality control is an important aspect of mass rearing.
Biological control often needs active participation by growers in monitoring pests and natural enemies. Predatory mites and other biocontrol agents should be applied early in the growing season, before pest populations have had time to build up to high levels. The same natural enemy that is effective against a smaller population of target pests may have no impact at all on a well established population with a high population density.
Held at the Rural Development Administration, Suwon, Korea, May 14 -18, 2001
No. of participating countries: 9 (Japan, India, Korea, Malaysia, Philippines, Taiwan ROC, Thailand, United States, Vietnam)
No. of papers: 16
No. of participants: 16 plus observers
Co-sponsor: Rural Development Administration, Korea
Figure 1 Fungal Pathogen Used to Kill Rice Brown Planthopper
Figure 2 Two (Brown) Weevils Killed by Entomopathogenic Nematodes beside Normal (White) Weevil