Growing Degree Days (GDD)

Target Numbers (based on Maine averages from 2007 – 2016, with the ‘start date’ being the point at which the bed is first free of water, ice or snow; base temperature of 50°F used for the following Growing Degree Day targets):

  • Cranberry Tipworm eggs first appearing: ~247 GDD (but crop stage is a more dependable predictor; eggs generally appear as soon as tiny new shoots begin to grow up from the runners, and the eggs are deposited almost exclusively on the tips of these tender, new shoots). Tipworm populations have declined significantly in recent years; populations have been so low, in fact, that the first generation of the season has been difficult to detect. Some Sweepnet ‘First Dates’ (for catching tipworm flies): 5/14/07 in Mt. Vernon; 5/28/07, 5/16/08, and 5/13/09 in Troy; 5/16/08 in Lincolnville.
  • Blackheaded Fireworm larvae – The majority of first-generation larvae hatching from their eggs which have overwintered: ~194 GDD

Growing Degree Days (GDD) are a measure of the “heat units” (related to the external temperature and the amount of time per day that an insect spends actively growing) that accumulate over time during a given year.