Cone, Wyatt W.

Wright, Larry C.

Undetermined

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Section VIII.Mites & Sap-Sucking InsectsA SAMPLING PLAN FOR TWOSPOTTED SPIDER MITES ON HOPSL.C. Wright and W.W. ConeIrrigated Agriculture Research and Extension CenterWashington State University24106 N Bunn RoadProsser, WA 99350-9687509/786-9274FAX 509/786-9370lwright@beta.tricity.wsu.eduTwospotted spider mites, Tetranychus urticae, were sampled on hop, Humuluslupulus, main stem leaves at the 0, 1, 2, 3, 4 and 5 m heights to determine whichelevation(s) would provide the best estimate of the number of mites over the wholevine. The number of mites at the 2 and 4 m heights had the highest correlations withwhole vine means but the number at 2 m overestimated the vine mean and thenumber at 4 m underestimated the vine mean. The number of mites on main stemleaves at the 2 and 4 m elevations combined had the highest correlation (r = 0.980)with the mean number of mites over all the heights sampled. The mean number ofmites at 2 and 4 m was also the closest to the vine mean of the leaves sampled.Therefore, we developed a sampling plan based on the total number of adult femalemites on main stem leaves at the 2 and 4 m heights combined. The sampling planwas based on the adult female spider mite because it is easier to count in the field andshould give a good indication of the potential for change in mite numbers. Additionalsampling was only done at the 2 and 4 m elevations to increase the number ofobservations used to develop the sampling plan.A binomial sequential sampling plan was developed to make sampling faster and easierthan a numerical plan. The binomial sampling plan requires only that the samplerdetermines if a sample of main stem leaves from the 2 and 4 m heights has more than3 adult female twospotted spider mites. Counting the total number of mites is notnecessary. A running total of the number of samples with > 3 adult females is kept.After each sample, the sampler checks the total against the numbers representing thesampling plan upper and lower decision lines. When the total is greater than the valuefor the upper line, a control treatment is indicated and the sampler can quit sampling.If the total is below the lower decision line value, sampling can cease and no controlis needed at that time. If the total is between the two sampling plan values, moreinformation is needed and another sample must be taken. The sampling plan wastested for accuracyand precision by sampling computer generated data sets that hadknown means and variances.105

A Sampling Plan for Twospotted Spider Mites on Hops

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A Sampling Plan for Twospotted Spider Mites on Hops

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