Development of biological control of twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) on greenhouse ivy geranium, Pelargonium peltatum (L.) l'Her ex Ait., using the predatory mite, Phytoseiulus persimilis Athias-Henriot (Acari: Phyt

I investigated the effect of different irrigation regimes and ivy geranium varieties on populations of the twospotted spider mite, Tetranychus urticae. Plants of the ivy geranium cultivars ‘Amethyst 96’ and ‘Sybil Holmes’ were irrigated when pot weight dropped 15, 30, or 45% from container capacity. My results showed that irrigation frequency had no effect on T. urticae populations, but that the variety ‘Amethyst 96’ is more susceptible to T. urticae than is ‘Sybil Holmes’. The latter finding suggests that host plant resistance may be a viable pest management option for growers.; I also developed a presence-absence sampling plan to more efficiently estimate T. urticae population densities. Estimates can be made by examining leaves from just one stratum of the plant for the presence of T. urticae. However, I found that the optimal binomial leaf sample sizes for estimating population densities in the range of 0 and 3 mites/leaf were quite large; therefore, I recommend the use of numerical sampling within this range of T. urticae densities.; A third objective was to assess the effectiveness of Phytoseiulus persimilis as a biological control agent of T. urticae at release ratios of 0, 1:60, 1:20, and 1:4. My experiments demonstrated that P. persimilis may be able to effectively control T. urticae at a moderately low predator release rate of 1:20 over a range of initial pest densities. However, a release ratio of 1:4 may provide a level of control that is consistently the greatest.; Finally, I assessed the degree to which different indices of ivy geranium damage could be correlated with twospotted mite density and overall marketability. I found leaf distortion and plant appearance to be correlated with both cumulative T. urticae density and plant marketability. I computed preliminary action thresholds for spider mites on ivy geranium to be 2 spider mites per leaf for the first five weeks and 6 spider mites per leaf for the last five weeks of the production period. Combinations of 24 mM nitrogen and 0.64 or 1.28 mM phosphorus produced plants that showed the least T. urticae damage. Therefore, I recommend these concentrations of nutrients to growers.