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

Abstract

We sampled aster leafhopper, Macrosteles quadrilineatus (Forbes), populations in 1996 and 1997 from central and southern Minnesota to develop a binomial sequential sampling plan for the leafhopper in carrot, Daucus carota (L.). Both conventional and organic farms were sampled with a sample unit consisting of 10 sweeps. Mean density ranged from 0.13 to 16.15 leafhoppers per 10 sweeps. Resampling simulation software was used with the Wald sequential probability ratio test to validate a binomial sampling plan from field data. Action thresholds were chosen based on aster yellows infectivity level (the proportion of individuals vectoring the phytoplasma) and varietal tolerance 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 analyses were used to select a final tally threshold (number of insects in a sample needed to consider the sample infested), based on the probability of making a decision to treat (i.e., operating characteristic). The operating characteristic was analyzed to determine which tally threshold provided the highest proportion of correct decisions. A tally threshold of 3 was selected as the overall optimum across infectivity level and varietal tolerance combinations. For the 6 possible infestation scenarios, 4–10 samples (10 sweeps each) were required to make a treat or no treat decision. Stop-line graphs are provided for field use for all 6 scenarios. Use of binomial sequential sampling plans should provide effective management of M. quadrilineatus in carrots and minimize sampling time and cost.