Forecasting emergence of adult yellowheaded spruce sawflies (Hymenoptera:Tenthredinidae) [Pikonema alaskensis, Picea glauca]

The authors determined in a laboratory study that emergence of adult yellowheaded spruce sawflies, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae), from the overwintering cocoon stage required 343 degree-days (SE = 13.1). The last date of snow cover was used as the starting date for accumulating degree-days. The accumulation was based on soil temperatures predicted from daily minimum and maximum air temperatures. Using a lower developmental threshold of 1.6 degree C (SE = 0.5), the median dates of emergence for the years 1978 to 1983, inclusive, were predicted. Predicted dates of emergence averaged 0.3 days earlier than actual dates, using the predicted soil temperature at a depth of 0.1 cm. Yearly differences between predicted and observed dates ranged from 0 to 6 days, but actual emergence extended over 40 days.