Spatial dispersion and sampling of citrus whitefly, Dialeurodes citri, for control decisions in a citrus orchard

1. Monitoring the citrus whitefly Dialeurodes citri (Ashmead) in citrus orchards in Israel is necessary for control decisions in IPM in the period before the establishment of natural enemies. The main damage caused by D. citri is due to sooty mould, which colonizes the sweet secretions of this pest. 2. Using Taylor's Power Law to describe the variance-mean relationship, the dispersion of the citrus whitefly was found to be aggregated at all developmental stages. The action threshold was estimated using regression of sooty mould level on egg or larval counts. 3. Two-stage sampling plans were constructed in different pest generations, for eggs and larval stages, on the basis of the variance between leaves on a tree and the variance between trees in a plot. 4. Fixed sample size plans were prepared, with possible numbers of trees and the numbers of leaves per tree to be sampled, in order to attain a given precision level. 5. Sequential sampling plans were also drawn up, on the basis of four leaves per tree, where trees are sampled until a decision can be reached. The maximum number of trees to be sampled was set according to the corresponding fixed sample size plan. 6. Simulation of the sequential sampling plans showed them to be highly reliable in terms of the decision reached with economy in sample size for high and low infestation levels.