Binomial sequential sampling for management of aster leafhopper (Homoptera: Cicadellidae) and aster yellows phytoplasma in carrot: Impact of tally threshold on the accuracy of treatment decisions

We sampled aster leafhopper, Macrosteles quadrilineatus (Forbes), populatio ns in 1996 and 1997 from central and southern Minnesota to develop a binomi al sequential sampling plan for the leafhopper in carrot, Daucus carota (L. ). Both conventional and organic farms were sampled with a sample unit cons isting of 10 sweeps, Mean density ranged from 0.13 to 16.15 leafhoppers per 10 sweeps. Resampling simulation software was used with the Wald sequentia l probability ratio test to validate a binomial sampling plan from field da ta. Action thresholds were chosen based on aster yellows infectivity level (the proportion of individuals vectoring the phytoplasma) and varietal tole rance combinations. We used 2 infectivity levels (1 and 5%) and 3 varietal tolerance levels (resistant, intermediate, and susceptible) to represent 6 scenarios encountered in the field. Results from resampling validation anal yses were used to select a anal tally threshold (number of insects in a sam ple needed to consider the sample infested), based on the probability of ma king a decision to treat (i.e., operating characteristic). The operating ch aracteristic was analyzed to determine which tally threshold provided the h ighest proportion of correct decisions. A tally threshold of 3 was selected as the overall optimum across infectivity level and varietal tolerance com binations. For the 6 possible infestation scenarios, 4-10 samples (10 sweep s each) were required to make a treat or no treat decision. Stop-line graph s are provided for field use for all 6 scenarios. Use of binomial sequentia l sampling plans should provide effective management of M. quadrilineatus i n carrots and minimize sampling time and cost.