The following is information on scouting and monitoring for spider mites in the greenhouse.
Monitoring for Two Spotted Spider Mites
Obviously, since spider mites are unable to fly during any life stage they will not be observed on sticky traps. Hence, when scouting for mites it is necessary to inspect individual plants within the crop. Looking for symptoms and signs such as leaf stippling and webbing help to indicate which plants to inspect more closely with a 15x magnifying hand-lens.
Some specific thresholds of two-spotted spider mites on ivy geraniums have been determined through research. It was shown that action thresholds of 7 mites per leaf are reached on plants greater than 5 weeks in production. Alternatively, action thresholds of only 2 mites per leaf are reached on plants less than 5 weeks in production. Estimated pest mite counts are required when releasing beneficial predatory mites (e.g., Phytoseiulus persimilis). Release one (1) predatory mite for every 4 to 10 twospotted mites counted
Two spotted spider mites. Photo by Frank Peairs, Colorado State University, Bugwood.org
Information from "Pest Counts and Action Thresholds in the Greenhouse" by Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University in the August 20, 2009 edition of the Plant & Pest Advisory, Landscape, Nursery & Turf Edition; A Rutgers Cooperative Extension Publication