A method of timing pest activity (and therefore controls) is ornamental plant development. Using Plant Phenological Indicators (PPI) involves observing certain plants whose bloom time (flowers, leaves, fruit, etc.) coincides with a life stage event of a specific pest. As temperatures rise in the spring, both plants and insects/mites begin development and continue throughout the growing season in response to this accumulation of heat units (GDD). Therefore, the development of plants can be correlated to insect activity.
Information from "Accurately Timing Pesticide Applications by Using Plant Phenological Indicators (PPI)" by Steven K. Rettke, Ornamental IPM Program Associate, in the May 14, 2009 edition of the Plant and Pest Advisory, Landscape, Nursery and Turf Edition from Rutgers University.