Comparing Chemical and Biological Control Strategies for Twospotted Spider Mites in Mixed Production of Ivy Geranium and Impatiens

Biological and chemical control strategies for the twospotted spider mite (TSM; Tetranychus urticae) were evaluated in a greenhouse experiment replicated over time in mixed production of ivy geranium (Pelgargonium peltatum 'Amethyst 96') and two impatiens cultivars (Impatiens wallerana 'Impulse Orange' and 'Cajun Carmine'). Chemical control using the miticide bifenazate was compared with two release strategies for biological control using the predatory mite, Pbytoseiulus persimilis. Specific treatments included 1) a single application of bifenazate at 0.3 g(.)L(-1) formulation (22.6% a.i.); 2) a single release of predatory mites at a 1:4 predator to pest ratio based on sampled pest density; 3) a weekly release of predatory mites at numbers based on the area covered by the crop; and 4) an untreated control. TSM populations were monitored for 4 weeks. After another 4 weeks, when plants were ready for market, plant quality ratings were recorded. The number of TSM per leaf dropped for all treatments on all genotypes but increased in the untreated plants. On ivy geranium, the fact that there were significantly more TSM on untreated plants was not reflected in average plant quality, but it did reduce the proportion of containers rated as salable at full price compared with both chemical and biological control. On impatiens, both treatment and cultivar had significant effects on the mean plant quality rating and on the proportion of containers rated as salable at full price. The use of a sampling plan to determine the appropriate number of predators to release was as effective as the currently recommended management treatments for TSM in bedding plants.