Insects and plants are influenced by environmental conditions, including temperature, moisture, and day length. Temperature has a major impact, as both insects and plants develop faster as temperature increases and more slowly as temperature decreases. Knowing the influence of temperature can be useful in successfully managing insect pests. The key factors involved are growing degree-days (GDD) and plant phenology, which can be used to properly time scouting practices, placement of pheromone traps, and insecticide applications at the most susceptible life stage of many plant feeding insects. These practices can lead to fewer insecticide applications and thus preserve existing natural enemies.

Growing degree-days accumulate when the average high temperature is greater than the base temperature of 50°F because insect growth and activity is reduced below this temperature. The formula for GDD involves adding the maximum and minimum daily temperatures, dividing by two, and then subtracting the base temperature of 50°F. This results in the GDD for a particular day. For example, if for a given day the high is 75°F and the low is 60°F, then the average is 67.5°F. Subtracting the baseline temperature of 50°F results in 17.5 GDD. If the average temperature is less than 50°F, then the GDD is 0. Growing degree-days accumulate over the year, determining the appearance and development of insects. Forecasting models based on GDD have been developed for many landscape insects pests, including the elm leaf beetle, bronze birch borer, flatheaded appletree borer, dogwood borer, lilac/ash borer, Nantucket pine tip moth, bagworm, and pine needle scale. GDDs vary from one location to another due to microclimates.

Plant phenology is used to estimate pest emergence based on the synchronization of plant growth stages (that is, bud swell, leaf emergence, flowering, and/or fruiting). Plants used for phenological observations are often called indicator plants. Indicator plants need to be common to a wide geographical area and easy to grow and recognize. The flowering of a certain plant species indicates when the most susceptible life stage of many insects is present. Following are insects and certain stages that are present during the blooming cycle of Vanhoutte spirea (Spiraea x vanhouttei):

Full bloom: Birch leafminer young larvae, elm leaf beetle young larvae, European pine sawfly feeding larvae, gypsy moth feeding larvae, pine needle scale crawlers (first generation)

Full to late bloom: Lilac/ash borer newly hatched larvae, oystershell scale (brown) crawlers

Finishing bloom: Bronze birch borer newly hatched larvae

Most blossoms brown, a few still white: Flat-headed appletree borer larval hatch; peach tree borer newly hatched larvae; viburnum borer newly hatched larvae

Finished bloom: Oystershell scale (gray) crawlers

Predicting the stage of insect development based on the flowering of particular plant species is well documented in the book Coincide by Donald A. Orton, a retired Illinois Department of Agriculture nursery inspector. This book is published by Labor of Love Conservatory, 468 South President, Suite 103, Carol Stream, IL 60188-2894; phone, (630)668-8597.