The Arborvitae Leafminer (Argyresthia thuiella) is an important pest on Arborvitae. Overwintered Arborvitae Leafminer larvae typically emerge in spring and the first adults may be seen soon after. There can be 2-3 generations in a year, depending on the weather. Insecticide treatments targeting 1st generation adults, pre-egg laying are typically the best way to control Arborvitae Leafminer populations for the entire growing season. The Arborvitae Leafminer 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 insecticides 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 360 GDD (F) after the Jan. 1st biofix. These adults can be targeted for traps. Larvae emerge at approximately 170 GDD(F) following adult emergence. The next generation of adults emerge at approximately 167 GDD (F) following this. These adults can be targeted by insecticide treatments.

Source:

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