Spatial distribution of San José scale Diaspidiotus perniciosus (Hemiptera: Diaspididae) on an apple tree

Abstract

Five distinct infestations of San José scale (Diaspidiotus perniciosus) were identified on a single vase-shaped ‘Red Delicious’ apple tree, and a stratified random sampling programme of the population was carried out from September 1969 to September 1970. All live and dead scale insects, except crawlers, were counted fortnightly on samples of bark, fruit and leaves from the trunk, leaders, main branches, twigs, fruit spurs and new shoots within the strata (50 samples/sampling occasion). The strata were below, within and above the five infestations, and the stages recorded were settled 1st instar, 2nd instar, prepupal male, pupal male, adult male and adult female (with and without crawlers). Density was expressed as numbers per cm2 of bark, and numbers per fruit and leaf. At harvest, total scale insects (but not the separate stages) were counted on each fruit within each stratum. Analyses showed that, with the exception of adult females on the leaves, all stages were aggregated. Density varied greatly within the tree. Highest densities occurred within the infestations compared with above and below the infestations; two of the leader infestations had higher densities than the others and the trunk infestation; densities on the bark>fruit>leaves. Sex ratios were male-dominated on the leaves. Regressions of scale insects per fruit on their density on the bark were always significant, but scale density on the bark of the fruit spurs was the best predictor of scale insects per fruit, which was further related to percentage fruit infestation by logistic regression. These analyses confirmed infestation rates of fruit spurs as the most reliable and practical sampling unit for determining the need for control measures.