The threshold temperature for postdiapause development in overwintering eggs of the European red mite, Panonychus ulmi (Koch), was determined after exposing the eggs to various constant temperatures (5, 10, 15, 20, and 25°C) in the laboratory. The mean number of days to 50% hatch in each temperature was >120, 44.5, 22.0, 14.5, and 8.1 d, respectively. From the regression of postdiapause developmental rate to temperature, it was estimated that 50% of egg hatch coincides with the accumulation of 154.6 degree-days (DD) above the threshold of 7.4°C. In the field (1991 and 1993–1996), 50% egg hatch occurred from the end of March to 20 April after the accumulation of a mean sum of 129.4 ± 4.5 DD above the threshold of 7.4°C from 10 February onward. The starting date of 10 February for heat accumulation was chosen because earlier experiments had shown that diapause is terminated in 50% of the eggs in the first half of February. Using the theoretical sum of 154.6 DD, the predicted dates for 50% egg hatch deviated from field observations by an average of 3.7 ± 0.5 d over the 5 yr of experiments.
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