We investigated the relationship between the cumulative density of twospotted spider mite (Tetranychus urticae) and plant damage, measured using overall plant rating, on ivy geranium, Pelargonium peltatum. Overall plant rating is based on appearance and takes into account leaf browning and distortion, and plant size. Results showed that a linear model, y=-0.012x + 7.53 (r2=0.3, p < 0.0001) adequately described the relationship. We also assessed the use of biological control to reduce twospotted spider mite (TSM) numbers and plant damage. We used six-week-old plants to assess the effectiveness of Phytoseiulus persimilis at three release ratios. We compared ratios of 1:60, 1:20, and 1:4 at two levels of TSM, 50-170 and 200-400 TSM/plant. During the study, P. persimilis was allowed to interact with TSM for four weeks. At a TSM level of 50-170 TSM/plant, a ratio of 1:4 kept TSM numbers significantly low throughout the four-week period, 1:20 significantly reduced TSM numbers in weeks 2 and 3, and 1:60 had no significant effect. At a TSM level of 200-400 TSM/plant, ratios of 1:20 and 1:4 kept TSM numbers significantly low throughout the four-week period; 1:60 significantly lowered TSM numbers during weeks 2 and 4. At both TSM levels, ratios of 1:20 and 1:4 significantly reduced TSM damage whereas 1:60 did not. At both TSM levels, at ratios of 1:20 and 1:4, the number of P. persimilis increased to a maximum in week 3 and then declined; at 1:60 the number of predators increased throughout the four-weeks. This study indicates ratios of 1:20 and 1:4 are effective.
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