Sequential Sampling for Thrips tabaci on Onions

Abstract

A binomial sequential sampling procedure based on the presence of five or more Thrips tabaci per onion was as precise as Iwao’s sequential sampling method that requires counting all thrips present. Acceptance boundaries were calculated for economic thresholds of 0.9 and 2.2 thrips per leaf and the plans were validated in fields containing 0.01 to 32.33 thrips per leaf. For both procedures the correct decision was reached >83% of the time by sampling 12 to 14 onions. Error rates were <4%, which is below the theoretical error levels of 10% used for the Iwao procedure and 5% for the binomial procedure. In the remaining situations no decision could be reached because thrips densities were near the economic threshold. Binomial sampling was more time efficient than Iwao’s sampling method, reducing up to 88% of the counting time in no-decision situations. Thrips in onion fields exhibited a contagious distribution.