Crop Loss Surveys

Crop loss surveys may be undertaken simply to determine the types of losses occurring and their main causes, to determine the distribution of losses in different areas or to actually evaluate losses with a view to forecasting crop production or justifying control measures (Walker, 1987). In addition, surveys may be undertaken to meet more specific objectives such as defining farmers' perception of yield losses (Mulaa, 1995) or to establish baseline data by which to evaluate area-wide control measures (e.g. classical biocontrol introductions; Farrell et al., 1996). Such surveys may be conducted as face-to-face interviews with individual or groups of farmers (e.g. Mulaa, 1995) or by use of postal questionnaires (e.g. French et al., 1992, 1995). The question/interview approach can be supplemented by on-site evaluation of actual yield losses. In the study conducted by Mulaa (1995) the interviews were conducted and then the numbers of maize plants damaged by the stalkborer Busseola fusca were counted on each farm from five areas (10 m X 10 m) selected at random. The percentage of damaged plants per hectare was then determined using a simple formula, taking into account plant spacing, which allowed comparison of the distribution and extent of plants damaged by stalkborers in the Trans Nzoia District, Kenya. Surveys provide useful information on yield loss that it is often impossible to collect by any other means. However, to carry out surveys correctly requires a great deal of resources. In the B. fusca example above, 300 farmers were interviewed in five different agro-ecological zones (Mulaa, 1995).

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