Two Kinds of Stability

A host protection mechanism is either within or beyond the capacity for micro-evolutionary change of the parasite (see 10.6).

If it is within the capacity for micro-evolutionary change of the parasite, it is liable to stop functioning on the appearance of a new strain of the parasite. It thus provides a temporary protection. The obvious examples are the breakdown of vertical resistance, and the appearance of DDT-resistant houseflies. Such protection mechanisms are described as 'unstable'.

However, if the protection mechanism is beyond the capacity for micro-evolutionary change of the parasite, it will not fail in this way and it will provide a durable protection. The obvious examples are horizontal resistance, and crop protection chemicals such as Bordeaux mixture, natural pyrethrins, and rotenone. Such mechanisms are described as 'stable'.

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