The Elongate Hemlock Scale (Fiorinia externa) is an important on Hemlock and other plants. Elongate Hemlock Scale crawlers typically emerge in spring from overwintered nymphs. After emergence they can be targeted by traps or insecticide treatments. The Elongate Hemlock Scale Growing Degree Day Model, developed by researchers, and available on the Pest Prophet app, is a tool growers can use to time treatment of dormant nymphs and to predict crawler emergence for the first and second generations in a season.

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 the biofix date should be set to March 1st for the Northern Hemisphere.

Growing Degree Day Accumulation and Treatment

After the biofix is set, growing degree day accumulation can begin, using a lower threshold of 50F. Calculating growing degree days can be done using daily high and low temperatures, or using hourly temperature data using the Pest Prophet app. Dormant treatment is recommended between 8-120 GDD(F). First generation crawler emergence occurs between 260-700 GDD(F) and second generation between 2515-2625 GDD (F). These adults can be targeted with traps or insecticides.

Source:

Growing Degree Day Information. Nathan W. Siegert, Deborah G. McCullough and Jeffrey A. Andresen. Michigan State University, 2 October 2015. Web. 25 February 2016.

Using Growing Degree-Days for Insect Pest Management. Thomas Kowalsick and Scott Clark. Cornell Cooperative Extension in Suffolk County, March 2012. Web. 25 February 2016.