Proceedings of the South & East African combined agricultural, cotton, entomological and mycological conference held at Nairobi, August, 1926

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Monograph

Identifikator:: 1738588467

URN:: urn:nbn:de:zbw-retromon-115043

Document type:: Monograph

Title:: Proceedings of the South & East African combined agricultural, cotton, entomological and mycological conference held at Nairobi, August, 1926

Place of publication:: Nairobi

Publisher:: East African Standard

Year of publication:: 1926

Scope:: VI, 337 Seiten; Ill.

Digitisation:: 2020

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Etomology & Mycology

Collection:: Economics Books

PART IV. numbers By that seldom failing and still more seldom understood process known as the Balance of Nature, is able to assume the proportions of a pest. : It is obvious that no crop can be grown without thus affording an abundant supply of food to any insects which may be able to feed on it; but there are other ways of controlling the food supply of a pest. It the crop is an annual one, it is often possible to enforce a close season, so that for some period. of the year the insect is theoretically unable to maintain itself. Again, comparatively few plant-feeding insects are omnivorous or even catholic in their choice of food, while conversely, comparatively few confine themselves rigidly to one species of plant and one only. The majority of insects have a well- founded sense of systematic botany, and feed on all the plants of perhaps one Natural Order, or of one, or it may be two or three genera. The different food plants sometimes affect the life eyele of the insect or its fertility, thus one Egyptian cotton pest (Oxycarenus hyalini- pennis, Costa) which breeds on all the Malvaceous plants found in Egypt, lays an average of twenty eggs when it feeds on cotton, thirty if it feeds on Hibiscus esculentus, and less than ten when its host plant is a species of Sterculia (5, p. 101)+ It is thus clear that it may well be an unending task to try to control a pest if it is left unmolested on wild alternative host plants which may be growing in the vicinity of the crop; and that the control of the food supply may often be perfectly practicable. and a necessary adjunct to other methods. (c) and (d) TeHE UriLiSATION,K OF PARASITIC AND PrepaTORY ENEMIES OF INSECT PESTS. Under natural conditions, it is very largely the Parasitic and Predatory enemies which almost invariably succeed in keeping the numbers of an insect within reasonable limits. Insects are extremely prolific, there are but few species which lay less than 50 to 100 eggs, while with many the number is to be reckoned in thousands. Yet, on the average, only two out of each generation survive to reproduce their kind. But when man has upset the balance that formerly prevailed, the story may be a very different one. Food is there in abundance, and the insect feeding directly on the plant gets a start over its enemies and increases out of all proportion. It is true that under certain circumstances, even without any assistance from man, these enemies may also. increase and in time control the pest. Thus in certain parts of the United States, the Hessian Fly (Cecidomyia destructor) is not reckoned to be serious, because as soon as its numbers increase to any extent, its parasites also increase sufficiently to prevent any appreciable damage being done (4, p. 257)t This is not however always the case, and as will be shown shortly, unaided natural control of the Coffee Mealy-bug in this country is almost certainly impossible. When the balance of nature has been upset by Man, it is therefore necessary for Man to do something artificially to restore that balance. The larger animals, such as birds, toads, .ete., which may be predators on an insect pest, can often be encouraged without much t See list of references on page 194. 186

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