Integrated Pest Management

Integrated pest management (IPM) aims to make the maximum use of natural biological controls. It depends entirely on the self-organisation that occurs within the agro-ecosystem when the use of crop protection chemicals is reduced or stopped entirely.

Integrated pest management developed out of the insecticide overload, when it was observed that many beneficial insects were being killed. These beneficial insects included predators and hyper-parasites of the crop pests in question. The concept of integrated pest management was simply to reduce insecticide use to the minimum, in order to encourage biological control to the maximum. This was achieved mainly by careful crop monitoring, so that insecticides need not be applied until absolutely necessary.

IPM is very much a technique of the entomologists, and its importance in the control of other crop parasites has been seriously neglected. In addition to predators and hyper-parasites, other biological control organisms include competitors, antagonistic micro-organisms, and various organisms that trigger an induced resistance response.

Biological anarchy is the loss of biological control, and it occurs when these various organisms contributing to biological control are killed by crop protection chemicals.

One of the more extraordinary aspects of integrated pest management was the manner in which the role of host resistance has been ignored. It seems that the practitioners of integrated pest management were aware of vertical resistance, but were totally unaware of horizontal resistance.

Robinson (1997) commented on the use of host resistance in IPM as follows.

"In principle, host resistance is the most important tool in the practice of integrated pest management (IPM) in crops. As a matter of historical fact, however, the co-operation between the practitioners of IPM, and the plant breeders, has been minimal. Vertical resistance has proved of little value to the practitioners of integrated pest management. Being qualitative, vertical resistance either provides a complete protection, in which case IPM is not required, or it provides no protection at all, in which case it is of no use to IPM."

"The role of horizontal resistance in IPM is entirely different from that of vertical resistance, and its role is critically important. There is a direct correlation between the level of this quantitative resistance, and the effectiveness of IPM. With low levels of resistance, IPM will be difficult, even impossible. With high levels of resistance, IPM will be both easy and effective, and possibly unnecessary. It should perhaps be emphasised that the differences between the extremes of low and high levels of horizontal resistance are usually very great. In the absence of crop protection chemicals, these two extremes are commonly represented by a total loss of crop, and no significant loss of crop."

"A characteristic of horizontal resistance is that the breeding for it is cumulative and progressive. Because the resistance is durable, a good cultivar need never be replaced except with a better cultivar. Breeding for horizontal resistance thus makes all other aspects of IPM progressively easier, safer, cheaper, and more effective. And, at its higher levels, horizontal resistance may eliminate the need for IPM entirely."

"It is now clear that, unlike vertical resistance, the role of horizontal resistance in IPM is crucial. Indeed, this quantitative resistance must be regarded as fundamental to all IPM work. The first step in any IPM investigation, therefore, should be a consideration of the level of the horizontal resistance of the crop host in question. If there is too little horizontal resistance for IPM to be effective, breeding for an increased level of horizontal resistance will be essential. And, however much horizontal resistance may be present, breeding for more horizontal resistance will make all aspects of IPM more effective, until, finally, there will be no further need for IPM at all. Consequently, breeding the crop host for horizontal resistance should be the first consideration in all cases of IPM."

Horizontal resistance restores biological control by reducing or eliminating the use of crop protection chemicals. Biological control enhances horizontal resistance by reducing the reproductive capacity, and the epidemiological competence, of the parasites. The two phenomena are mutually reinforcing. When horizontally resistant cultivars are issued to farmers, the resistance may well be inadequate because there is considerable biological anarchy. Provided the farmers use no crop protection chemicals, the levels of horizontal resistance will apparently increase. More accurately, the levels of parasitism will decrease. This effect may be as dramatic as the results of successful integrated pest management. This is also a clear example of self-organisation in a plant pathosystem.

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