Many crop protection chemicals are unstable (see 10.6) in that they behave like vertical resistance. The parasite is able to produce a new strain that is unaffected by that chemical. In other words, the protection mechanism is within the capacity for micro-evolutionary change (see 10.5) of the parasite. DDT-resistant houseflies are the classic example. The use of that vertical resistance, or that pesticide, must then be abandoned, and a new one must be found. This has happened so frequently with modern cultivars, and with modern crop protection chemicals, that many people now believe that there is no limit to the capacity for micro-evolutionary change of our crop parasites.

It is clear that both horizontal resistances and some crop protection chemicals are beyond the capacity for micro-evolutionary change of crop parasites. Examples of such chemicals include Bordeaux mixture and sulphur fungicides and, among insecticides, both the natural pyrethrins (extracted from Chrysanthemum cineriifolium) and rotenone (extracted from Derris elliptica). Insecticidal soaps and diatomaceous earth are also beyond the capacity for change of insects. And mosquito larvae cannot change to overcome the effects of a film of oil on water.

In practice, this accumulation of pesticide resistance in crop parasites is often quantitative. This means that the recommended rates of pesticide application become inadequate. These rates are then increased but, in their turn, these too become inadequate. This gradual increase in the use of a pesticide can continue until the rates of application are absurd. This quantitative loss in effectiveness is a prime cause of the pesticide overload.

Most synthetic crop protection chemicals eventually succumb to new strains of the parasite, either qualitatively or quantitatively. Vertical resistance also breaks down to new strains of the parasite, but horizontal resistance does not.

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