Within-plant distribution of twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), on ivy geranium: development of a presence-absence sampling plan.

The twospotted spider mite, Tetranychus urticae Koch, is an important pest of ivy geranium and other ornamental plants. As a part of our long-term goal to develop an integrated crop management program for ivy geraniums, the focus of this study was to produce a reliable sampling method for T. urticae on this bedding plant. Within-plant mite distribution data from a greenhouse experiment were used to identify the young-fully-opened leaf as the sampling unit. We found that 53% of the mites on a plant are on the young-fully-opened leaves. On average 22, 37, and 41% of the leaves belonged to the young, young-fully-opened, and old leaf categories, respectively. We then developed a presence-absence sampling method for T. urticae in ivy geranium using generic Taylor's coefficients for this pest. We found the optimal binomial sample sizes for estimating populations of T. urticae at densities of between 0 and 3 mites/leaf to be quite large; therefore, we recommend the use of numerical sampling within this range of T. urticae densities. We also suggest that population estimates of T. urticae on ivy geranium be done based on mite density/unit area of greenhouse space, both for conventional greenhouse pest management, and for determining how many phytoseid predators to release when using biological control.