A relative scale of Leaf Damage Indices (LDI) from 0 to 5 describes the visible injury to leaves of cassava, Manihot esculenta Crantz caused by the cassava green mite, Mononychellus tanajoa (Bondar). As the scale is ordinal and thus not quantitative, the observed LDIs are converted individually to relative loss of chlorophyll on a ratio scale before an average injury is determined. This calibration is required because the ordinal and the ratio scales are not linearly related. A calibration curve was established on the basis of laboratory experiments to determine the chlorophyll content, c, of leaves representing various leaf damage indices. Several monotonously decreasing functions were fitted to the experimental data yielding the following relation  , where c o is the chlorophyll content of unijured leaf tissue and a is a constant describing the steepness of the curvilinear relation. This means that LDIs could be converted to relative loss of chlorophyll, d, where  . The photosynthetically active leaf area of plants can be estimated by combining the relative loss of chlorophyll with leaf area assessments and adds the effects of defoliation and suspended growth to the chlorophyll depletion. The difference in photosynthetically active area that arises between uninjured and injured plants over a period of time provides a measure of spider mite injury that can be related to growth and yield. The method integrates the injury inflicted over a period of time, allows successive observations of the same plants, and is rapid and reasonably precise considering the time savings.