Two Kinds of Biological Control

We must make a distinction between biological control and biological anarchy. Biological control is the reduction of the parasite population by hyper-parasites, predators, antagonistic micro-organisms, competitors, and organisms that trigger a resistance response in the host. Biological anarchy is the loss of biological control that results from the use of crop protection chemicals that unintentionally kill the agents of that biological control. Biological anarchy can also occur when a parasite is moved to a new continent and the agents of its biological control are left behind in the centre of origin. The classic examples of this latter category involved the introduction of rabbits and cactus to Australia.

Biological control may be natural or applied. Applied biological control is a deliberate attempt to overcome biological anarchy.

There are three categories of applied biological control. The first involves a new-encounter parasite or weed that has been introduced from abroad, such as rabbit and cactus in Australia. This category of biological control involves the import of control agents from the place of origin of that parasite or weed. The second category is the deliberate propagation of biological control agents, and their introduction into an infested crop. This form of biological control is at its most effective in glasshouses, and against insect parasites. The third is integrated pest management (IPM) in which the natural biological control is encouraged by reducing the use of crop protection chemicals as much as possible. IPM is at its most effective when there has been a serious pesticide overload, and when the crop in question has relatively high levels of horizontal resistance.

Biological anarchy (Robinson, 1996) occurs when biological control is severely diminished, or is absent, because the agents of biological control have been reduced or eliminated by crop protection chemicals. Biological anarchy has remained largely unrecognised and it is important in three ways.

First, biological anarchy means that the severity of the damage from parasites is considerably increased. The importance of this increase is indicated by the widespread effectiveness of IPM.

Second, it is clear that horizontal resistance is the most important tool in the practice of IPM. The greater the horizontal resistance, the less is the need for crop protection chemicals, and the more effective is the biological control. Horizontal resistance leads to an increase in biological control. Equally, biological control makes horizontal resistance more effective, in the sense that less resistance is required when the biological control is fully functional. Horizontal resistance and biological control are mutually reinforcing.

Third, it is impossible to make accurate measurements of horizontal resistance when there is biological anarchy. This is a serious problem when breeding for horizontal resistance, because it is impossible to imitate biological control in a plant growth chamber, and considerable biological anarchy reigns in most agro-ecosystems. In practice, this difficulty can be resolved only by a combination of relative measurements of horizontal resistance among cultivars, and long-term experience of cultivar behaviour in farmers' fields, in the absence of crop protection chemicals, and in the absence of all biological anarchy.

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