Factors Affecting the Attacks of Sucking Insects on Cotton

Abstract

It is often convenient to distinguish between insect pests with biting mouthparts (locusts, caterpillars, beetles, etc.) and those with sucking mouth-parts (Thrips, and the many divisions of the Hemiptera, such as Aphids, scales, Capsid bugs, etc.). The cotton crop is attacked by a large variety of pests of both groups, but in the present article attention is confined to the latter. In a sense the biting insects are more straightforward to deal with, since the result of their feeding is usually obvious; the amount of damage done is correspondingly easy to assess, and often their activities are little affected by the health of the plant. On the other hand, with sucking insects it is frequently necessary to do experiments to determine the effect of their feeding; it is usually difficult to assess even roughly the amount of damage they cause, and any estimate of loss is almost bound to be speculative and open to controversy; finally, they may be much influenced by the health of their host plants, so that it is nearly always a matter for argument whether they are doing any damage at all. It is sometimes maintained that the presence of sucking insects is always a symptom of ill-health in a crop, and that any loss incurred is in fact due to the ill-health of the plants, or at least that the only way to reduce insect damage is to improve the health of the plants.'In answer to this it may be said that. while the health of the plant undoubtedly influences the degree of insect attack in many cases, it is by no means the only factor involved; that, even where the degree of insect attack is largely governed by factors operating through their effect on the health of the plant, the pests concerned often cause severe damage to crops which woulo otherwise have produced fair yields in spite of somewhat adverse conditions; and lastly that it is frequently easier to control such pests by direct methods than by treating the plants. It is worth examining the effect of plant health on insect attack in some detail. The first point which arises is whether the health of the plant affects the insects feeding on it as much as is often made out. Often the severity of insect attack may be correlated with poor soil, waterlogging, or other unfavourable factors which would be reflected in yield, even in the absence of insects. Here it seems legitimate to say that the health of the plant does influence the amount of insect attack. On the other hand there are cases where the extent of insect'attack may be correlated with other factors, such as varietal susceptibility, climate, or other ecological circumstances, which may not be connected with the health of the plant. Examples of both sorts are discussed below, under the individual pests. Secondly, it is necessary to distinguish between the effect of climate and weather, and the effect of plant health, on the insect population. It is of course obvious that climate has a profound effect on the health of the plant. At the same time it has an equally great effect on insect population, having a direct influence on the activities of the insects, on their reproduction, on their natural enemies.