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

SIMPLE SUMMARY: Although two-spotted spider mite (Tetranychus urticae) is a major pest for strawberry, a popular fruit, there is limited information about the sampling unit for T. urticae in strawberries. Thus, the objective of this study was to suggest an optimal sampling unit of T. urticae in strawberries based on characteristics of their within-plant distribution. The sampling unit was changed according to developmental stage of T. urticae: the sixth oldest trifoliate leaf (eggs), the fifth oldest leaf (immatures and adults), and the fifth or sixth oldest leaf (all stages). For management purposes, the required sample numbers within a strawberry were one leaflet and two leaflets for eggs and motiles, respectively. Any leaflets in a trifoliate leaf could be selected among three leaflets of the selected sampling unit. However, the sample number for the research purpose should be determined by counting the number of T. urticae on a few leaflets of the suggested sampling unit. By using the optimal sampling unit for T. urticae and its required sample number, its sampling could be more precise to estimate T. urticae density in strawberries. Moreover, this precise sampling could improve the efficiency of T. urticae management. 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 characterizing within-strawberry distribution of T. urticae and by comparing coefficient of variation (CV) and correlation coefficient (r(2)) 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 r(2) 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.