Pyrrhalta viburni (Paykull), a new landscape pest in the United States, is univoltine and overwinters in the egg stage, feeding as larvae and adults on the foliage of plants in the genus Viburnum. Experiments were conducted to determine chilling requirements for completion and breaking of diapause and development rate of larvae at different temperatures in the laboratory. Egg hatch required at least 4 mo for induction of diapause, and ≈200 growing degree-days (GDD) at a calculated base temperature of 8.2°C for breaking of diapause in the laboratory. Development of all larval instars was significantly faster at 22 than at 17°C and faster at 27 than at 22°C, but 27°C appeared to be nearing the upper developmental threshold, because mortality was uniformly high, and deformities of adults were observed. We confirmed that larvae pass through three instars and found that head capsule width increased geometrically with instar. We calculated the larval developmental threshold to be ≈11.7°C at constant temperatures in the laboratory. Development under ambient conditions, where temperatures fluctuate, was considerably faster than that observed in the laboratory at constant temperatures. Thus, rearing experiments conducted in the laboratory at constant temperatures provide useful estimates of threshold temperatures but unrealistic estimates of GDD thresholds for P. viburni and other insects whose developmental rate differs between constant and fluctuating temperatures.
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