Vegetable farmers of the El Rahad Scheme (a newly developed scheme situated between latitude 13°31'-14°25' north and longitude 33°31-34°32' east) used to extend irrigation frequency for onion production as they believed it would hamper and suppress thrips incidence. Thrips, T. tabaci, is the only major insect pest of onion in the El Rahad Scheme and the influence of irrigation intervals on the population density of the pest and on onion yield was not quantified. Irrigation is a factor in the development of crop pests and the levels of the pest population are related to the commencement of irrigation. The effect of irrigation frequency on the development of onion thrips and yield was investigated and the response was found to be a significant increase in the population density of the pests from February to March with shorter irrigation frequency. A steady increase of thrips population was noted from February and March and a sharp decline was recorded in April during both the 1992/93 and 1993/94 seasons. At wider irrigation intervals, levels of the pest population were significantly less from February to March during both seasons. Total bulb yield and average bulb weight were significantly higher at shorter irrigation frequencies when compared with extended frequencies. The same pattern of results existed throughout the course of the experiment.

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