The Cooley Spruce Gall Adelgid (Adelges cooleyi) is an important pest on spruce. Adults emerge from galls early in the year and can be targeted by insecticide treatments. The Cooley Spruce Gall Adelgid Growing Degree Day Model, developed by researchers at Michigan State University, and available on the Pest Prophet app, is a tool growers can use to predict egg hatch based on adult trap catches and field-specific temperature data.

Initiating the Biofix

To properly use the model, the biofix must first be set. This is the date that will serve as the starting point for growing degree day accumulation. For this type of model January 1st is the standard biofix to start accumulation (for the Northern Hemisphere). This can be used to predict when adults will emerge and pheromone traps can be used.

After adults are observed and confirmed, the biofix can be adjusted in order to improve accuracy of the model for predicting later life stages.

Growing Degree Day Accumulation and Treatment

After the biofix is set, growing degree day accumulation can begin, using a lower threshold of 50F. This can be done using daily high and low temperatures, or using hourly temperature data using the Pest Prophet app. Adults typically emerge at 25 GDD (F) after the Jan. 1st biofix. Larvae emerge approximately 95 GDD (F) after that will galls becoming apparent approximately 80 GDD (F) later.

Source:

Observations made by Michigan State University Pest Management extension, (https://www.canr.msu.edu/ipm/agriculture/christmas_trees/gdd_of_landscape_insects)