The Spruce Spider Mite (Oligonychus ununguis) is an important pest on spruce trees. The Spruce Spider Mite overwinters as eggs with mites emerging later. Insecticide treatments targeting 1st generation Mites are typically the best way to control Spruce Spider Mite populations for the entire growing season. The Spruce Spider Mite 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 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. Mites typically emerge at 150 GDD (F) after the Jan. 1st biofix. Mites at this stage can be targeted for insecticide treatment.

Source:

Herms, D.A. Using degree days and plant phenology to predict pest activity. Chapter 11. Tactics and Tools for IPM.