The Pandemis Leafroller (Pandemis pyrusana) is an important pest on apples and other fruits in some regions. Overwintered Pandemis Leafrollers larvae typically emerge in late winter/ early spring and the first adults may be seen soon after. There can 1 or 2 generations in a year, depending on the weather.  The Pandemis Leafroller Growing Degree Day Model, developed by researchers, and available on the Pest Prophet app, is a tool growers can use to predict adult emergence, based on 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 Pandemis Leafrollers, this biofix is usually set to March 1st for the Northern Hemisphere. Once adult moths have been trapped for a few weeks in a row, the date of the first catch can be used to reset the biofix, and improve accuracy of later predictions.

Growing Degree Day Accumulation and Treatment

After the biofix is set, growing degree day accumulation can begin, using a lower threshold of 41F and an upper threshold of 85F. This can be done using daily high and low temperatures, or using hourly temperature data using the Pest Prophet app. The first adults from overwintered larvae will occur at about 930 GDD (F) and peak at approximately 1160 GDD. First egg hatch will begin at approximately 1280 GDD after the Jan. 1 biofix date. First moth catch of 2nd flight will occur at approximately 2250 GDD (F). The biofix should be adjusted after each stage is observed in order to improve accuracy of later predictions.

Source:

Brunner J. 1991. Leafroller pests of fruit crops in Washington State. In New Directions in Tree Fruit Pest Management. Good Fruit Grower. pp 185-198.