Binomial Sequential Sampling Plans for Adult Female Twospotted Spider Mites (Acari: Tetranychidae) on Hops

Abstract

Binomial sequential sampling plans, based on Wald’s sequential probability ratio test, were developed for adult female twospotted spider mites, Tetranychus urticae Koch, on main stem hop leaves at the 2 m elevation for early season sampling and at the 2 + 4 m heights combined for mid- to late season sampling. The 2 + 4 m sampling plan was tested by sampling data sets, which were generated by the Monte Carlo method. Increasing the tally threshold improved the operating characteristic curves and sampling plan robustness and caused the average sample number and percent of maximum sample termination curves to be more peaked around the critical density. Increasing the tally threshold generally reduced the magnitude of sequential sampling errors. Sequential decision errors made at means above the critical density were more serious than errors made below it. When the maximum of 100 samples was taken, the data set mean was generally above the critical density. Therefore, when 100 samples have been taken and no decision has been reached, the sampler should be aware that the field mean is near, and likely greater than, the critical density. The sampling plans were based on a nominal threshold of 5 adult female spider mites per leaf. The plans could be modified using data in this article to develop sampling plans based on an economic threshold when one is determined.