Estimating the Relative Abundance of Flush Shoots in Citrus with Implications on Monitoring Insects Associated with Flush

Attention to the management of insects that feed on foliar flush growth has increased in Florida citrus as a result of the establishment of invasive plant diseases associated with insects that develop exclusively on flush. Citrus can be monitored to identify peak periods of flush abundance to time insecticide applications for these insects; however, guidelines for quantifying flush abundance are lacking. We therefore investigated sampling procedures for estimating flush abundance. A sampling method was devised to enable a quantitative estimation of flush shoots, defined as any shoot with immature leaves. A sample unit was the area within a 15 x 15 x 15-cm frame slipped into the outer edge of a tree with the end of a branch inside the frame. The number of flush shoots originating within the sample unit was counted. Three sample units were examined per tree in 45 randomly selected trees weekly during 2005 in each of two blocks of trees, one containing young 'Marsh' grapefruit (Citrus paradise Macf.) and one containing mature 'Temple' orange (tangors) [C. reticulate Blanco x C. sinensis (L.) Osbeck]. A pronounced abundance of flush was generally indicated by means of one or more flush shoots per sample in the particular trees studied. Variation in numbers of flush shoots per sample was similar within and among trees, differed significantly among sample dates, and did not differ significantly between the two blocks of trees overall sample dates. Taylor's power law coefficients indicated that, over all sample weeks, flush shoots were randomly distributed within the young grapefruit trees and only weakly aggregated within the block of mature oranges. Projections indicated that a sampling plan consisting of 40 trees (one sample per tree) would provide density estimates acceptable enough for general estimates at mean densities of one or more shoots per sample. An index of pest abundance based on mean pest density per flush shoot and mean density of flush shoots per sample is proposed.