Spatial Dispersion and Binomial Sequential Sampling for Citricola Scale (Homoptera: Coccidae) on Citrus

Abstract

A binomial (presence-absence) sequential sampling plan for citricola scale, Coccus psclldomagnoliarum (Kuwana), was developed and validated for infestationson orange trees. Population densities from 200 collections of 25 leaves in both 1991 and 1992 were used to determine the Green coefficient ( Cx ), and the intercept and slope values for both the Taylor power law and Iwao regression of mean crowding on the mean. All measures of dispersion agreed in both years that scale populations were aggregated; Cx was > 0.3 and slopes from the regressions were significantly > 1 ( P < 0.001). Because the fit of the regression lines was best using the Taylor power law, a binomial sampling plan was developed incorporating power law coefficients into Wald's (1947) approach with upper and lower thresholds, a critical action level of 1 scale per leaf, and a truncation at 200 samples. A minimum sample of 25 leaves collected from different trees is recommended to minimize potential incorrect treatment decisions caused by among-tree variation. In 11 validation trials in 1993, the binomial plan compared favorably with a conventional technique where citricoia scales on 125 leaves from 5 trees were counted. Both techniques reached the same conclusions regarding the need for treatment in 10 of 11 trials, but the binomial plan required less average time while allowing more trees to be examined.