Pear psylla (Cacopsylla pyricola) is an important pest of pear in some areas, including the Pacific Northwest. Honeydew produced by pear psylla causes fruit russet, and serious infestations can stunt and defoliate trees. After overwintering, they gather on newly formed leaf buds where they feed and mate. Researchers have developed a growing degree day model for Pear Psylla to help predict when overwintered adults will lay eggs, when nymphs will emerge, and when subsequent generations will happen.

Calculating Growing Degree Day Units

The Pear Psylla growing degree day model uses a lower threshold of 5C for calculating growing degree day units. The growing degree day units can be calculated from daily maximum and minimum temperatures using these thresholds, or by using the Pest Prophet app, which features this model and is easy to set up and use.

Setting the Biofix date

In order to begin growing degree day accumulation, first a “biofix” date must be set which is the first date, after which growing degree day units will begin to accumulate. In the Pest Prophet app, the biofix date can be adjusted for any observed life cycle stages.

For the Asian Citrus Psyllid model, the biofix date should be set at January 1st for the Northern Hemisphere. Approximately 80 GDD(C) after the biofix date, peak overwintering adult spring counts will occur.

Sources:

Horton, D. R., B. S. Higbee, T. R. Unruh, and P. H. Westigard. 1992. Spatial characteristics and effects of fall density and weather on overwintering loss of pear psylla (Homoptera: Psyllidae). Environ. Entomol. 21: 1319-1332.