Biological anarchy

Most of our crop parasites encounter various other organisms that keep their numbers down. These include hyper-parasites, predators, competitors, antagonistic micro-organisms, and organisms that trigger host resistance responses.

The overall effect that these biological reductions have on a parasite population is called biological control, and this is an important component of the self-organisation of the pathosystem. However, in modern crop husbandry, the opposite effect is far more common, and far more important. This opposite effect is the loss of natural biological controls because of an excessive use of crop protection chemicals, which also kill hyper-parasites, predators, competitors, and antagonists. Robinson (1996) proposed the term 'biological anarchy' for this loss of biological control.

Biological anarchy occurs most commonly with the insect pests of crops, but the effect can probably be detected, to a greater or lesser extent, with all categories of plant parasite that have been treated with chemical pesticides. There is a clearly established case, for example, with coffee berry disease (Colletotrichum coffeanum). This microscopic fungus is parasitic only on coffee berries. Between berry-bearing seasons, it resides harmlessly in the bark of the coffee tree, constituting about 5% of the innocuous, microscopic, bark inhabitants. When coffee trees are sprayed with a fungicide to control other diseases, many of these competing bark inhabitants are killed, and the coffee berry disease fungus population then increases to occupy much of the bark. In the next season, the severity of the disease is increased accordingly.

A model of aphid asexual reproduction provides a useful illustration. Suppose that every aphid has ten offspring, and that all the offspring survive to produce ten more offspring in each generation. After ten generations, there will be 1010 aphids (i.e., 10,000,000,000). Now suppose that ladybirds are eating half of the aphids, so that only five of each aphid's offspring survive to reproduce in each generation. After ten generations, there will be 510 aphids (i.e., 9,765,625), which is approximately one thousandth of the earlier total. And, if only one aphid survives to reproduce in each generation, after ten generations there will be only one aphid. In practice, ladybirds really do eat many aphids. But if an insecticide kills all the ladybirds, and all the aphids are resistant to that insecticide, there will be many more aphids than if the insecticide had never been used in the first place.

There are thus two biological factors to be considered in any discussion of biological anarchy. The first factor is the biological anarchy itself, induced by a pesticide chemical. The second is the fact that a crop parasite may develop a new strain that is less affected, or even completely unaffected, by that pesticide. This is an effect closely similar to the failure of vertical resistance and the pesticide is described as being 'unstable' (see 10.6). The population explosion of a new pesticide-resistant strain of a major pest is damaging for two reasons. Firstly, there may be no immediately available pesticide to control it and, second, biological anarchy will ensure that it behaves with a ferocity that would be impossible if its natural enemies were keeping its numbers down.

Biological anarchy has two important consequences, which must be taken into account when breeding for horizontal resistance. First, when we abandon the use of crop protection chemicals for the purposes of screening for horizontal resistance, we shall have crop losses that are much higher than normal, until the natural biological controls are fully restored. These crop losses can be so large that they provide a very misleading indication that horizontal resistance is useless.

Second, if we want to measure the level of horizontal resistance in potential new cultivars, we must do this under conditions in which there is no biological anarchy. If we measure horizontal resistance under field conditions, in which the parasite has considerably increased numbers, because of biological anarchy, that level of resistance will appear inadequate. But, once the biological controls are restored, that same level of resistance might be high enough to control the parasite completely.

In practice, this means that field measurements should be made in quite a large area that has been free of crop protection chemicals for several seasons. It may not always be possible to find such an area. The only alternative would then be to use laboratory measurements, which can only be relative measurements. A closely similar problem lies in attempting to assess how much horizontal resistance we are likely to need in a breeding program. To do this, we must assess parasite damage when the biological controls are functioning fully.

The importance of biological anarchy can be assessed by the success of the practice known as integrated pest management (IPM), which is designed to restore biological controls. The movement of agricultural pests around the world provides a further indication of its importance. The prickly pear cactus and rabbits in Australia are classic examples of the damage that can be caused by biological anarchy. This damage was largely controlled by the introduction of biological control agents.

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