Abstract: In the present paper, insect and mite pests of crops are divided into direct and indirect pests. The percentage reduction in yield is lower than that in organ or tissue consumed by indirect pests. Leaf eating insects constitute a large part of indirect pests. It is found that the most important factor affecting the relationship between leaf damage and crop yield was the crop. growth stage. The major crops examined here include cereals, cotton, soybean, cowpea, groundnut, potato, sunflower and sugarbeet. In general, the percentage yield reduction was highest when a given percentage of leaf area reduction occurred at the middle stage of crop growth. At this stage, the rate of yield formation approached the maximum and the leaf area index was at the maximal level. The other factors included percentage of leaf area lost, leaf position, feeding duration, crop varietal height, soil water and fertility, weather, and distribution of damaged leaves. Yield reduction would be decreased, or more even, yield increase would be occurred if some of these factors favoured the vegetative growth. This conclusion drawn from a variety of research fields is needed in improving crop loss assessment and developing IPMprogram for a region planted with different crops.

Key words: Pest-yield relationship, Crop loss assessment, Leaf eating insect, Indirect pests, IPM