Effects of Temperature on Diapausing Cocoons of the Yellowheaded Spruce Sawfly, Pikonema alaskensis12

Abstract

This study sought optimal laboratory temperature conditions for obtaining adult yellowheaded spruce sawflies, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae), from cocoons of field-collected larvae. One month prechill at 23°C, 4 to 8 months chill at 0°C, and 15°C postchill temperature resulted in greater than 70% emergence within 8 weeks of warming. Extending the prechill period or using 8°C for the initial and final months of chill did not yield earlier or significantly higher emergence. Adult emergence was greatly reduced after 1 month prechill at 23°C and 1.5 to 3 or 9+ months chill at 0°C. When the prechill and chill periods totaled less than 3 months, emergence was extended, spanning 12 to 20 weeks, but when these periods totaled 7 months or more, emergence was very uniform, spanning 3 to 4 weeks in most cases. When the prechill and chill periods totaled 3 to 6 months, the effects of prechill duration and chill temperature on emergence pattern were complex. The relationship between emergence pattern and diapause is discussed. Diapause occurred under all the temperature regimes, and its duration at 15°C would prevent fall emergence in the field. Lower temperatures did not generally accelerate diapause development. Zero-degree cold prevented postdiapause development, which required ca. 4 to 5 weeks at 15°C.