Within-Plant Distribution of Two-Spotted Spider Mites, Tetranychus urticae Koch (Acari: Tetranychidae), on Strawberries: Decision of an Optimal Sampling Unit

Abstract

It is known that two-spotted spider mite (Tetranychus urticae), a major pest of strawberry, is difficult to manage. This study was conducted to determine the optimal sampling unit to increase management efficiency of T. urticae in strawberries. The sampling unit was determined by character-izing within-strawberry distribution of T. urticae and by comparing coefficient of variation (CV) and correlation coefficient (r2) among potential sampling units. There was a significant (p < 0.05) difference in densities within a strawberry according to the leaf age. However, there was no significant difference in its density within a trifoliate leaf. More T. urticae were found on young-fully-opened (moderately old) leaves than on young and old ones. Moreover, these leaves had lower CV and r2 values than others. More specifically, optimal sampling units of T. urticae were fifth, fifth or sixth, and sixth oldest leaves for motiles (immatures and adults), all stages (motiles and eggs), and eggs, respectively. The required sampling number should be determined depending on the density. However, for management purposes, one and two leaflets would be acceptable for eggs and motiles, respectively. By using this small number of suitable sampling unit, the sampling and management for T. urticae in strawberries could be more efficient than before.