Locusts and grasshoppers as pests of crops and pasture-a preliminary economic approach.
Abstract
The following is based largely on the author's summary. The amount of damage caused by locusts and grasshoppers is a function of a number of complex variables such as the amount of vegetation eaten daily by a single insect, differences in food preferences between species, the fluctuations of population sizes and the relative mobility of the insect stages. The last factor is governed, in the case of Schistocerca gregaria (Forsk.), mainly by meteorology, which is, in turn, influenced locally by topography. The quantitative loss in yield for many crops depends upon the stage of growth of the crop when attacked. In estimating the effect of crop damage upon the economy of a country, a distinction must be made between subsistence and cash crops, which require different evaluations. Cost-benefit ratios for control campaigns can be evaluated only when both the immediate and longer-term effects of control upon locust population dynamics have been measured and crop damage more accurately estimated. New locust and grasshopper problems are created and existing ones aggravated by agricultural development in areas of natural vegetation. A system of estimating the vulnerability of a crop to damage by a locust species in a unit area is described. It amalgamates some of the major variables upon which the vulnerability of a crop depends into a single ' crop vulnerability index ' value for each unit area and is exemplified by means of data for S. gregaria, the species for which it was initiated, damaging cotton in India and Pakistan.