Cottony cushion scale, Icerya purchasi Maskell, is an occasional pest of citrus, especially when insecticides disrupt vedalia beetle, Novius cardinalis (Mulsant) (Coleoptera: Coccinellidae). In two field tests conducted in successive years, navel orange trees were artificially infested with I. purchasi to determine the impact of I. purchasi on fruit yield and quality. In the first year, for which adult scale densities ranged between 0 and > 45 per branch and between 0 and > 500 on trunk counts, there was a highly significant negative linear relationship between the number of I. purchasi adults on branches or the trunk and its effect on fruit number, the percentage of large fruit, and the mass of fruit, and a highly significant positive relationship for the percentage of sooty mold-affected fruit and the percentage of juice grade fruit. In the second year, in which adult scale densities ranged between 0 and 10.5 per branch and between 0 and 35 on trunk counts, only the percentage of sooty mold-affected fruit showed a relationship. Damage to fruit was better predicted by population estimates from branch samples versus trunk counts. Analysis of the two years, estimated significant damage ranging from 2 adults per branch to no detectable effects of insect density, depending on the given metric of fruit number or quality, at the time that treatment decisions would be made. These results support May–June monitoring of branches for adult scales and application of treatments at an action threshold of <2 adult females/branch to prevent damage to navel orange trees.