The Fall Webworm (Hyphantria cunea) is an important pest on a variety of trees. Larvae emerge in spring from overwintered eggs. Insecticide treatments targeting 1st generation larvae are typically the best way to control Fall Webworm populations for the entire growing season. The Fall Webworm 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 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. Larvae typically emerge at 850 GDD (F) after the Jan. 1st biofix. Larvae at this stage can be targeted for insecticide treatment. Tents become apparent 1000 GDD (F) after that.

Source:

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