Initiation of oviposition after winter diapause in the spider mite Tetranychus urticae (Acari: Tetranychidae): prediction and historical patterns

The rate of development from winter diapause to initial oviposition for Tetranychus urticae on strawberry, Fragaria × ananassa , in southwestern British Columbia, Canada, was linearly related to temperature above 9.4 ± 0.71°C. Initial oviposition (IO) in the field, defined as the day at which half of the leaflets with T. urticae also had T. urticae eggs (IO 0.5 ), was determined for 21 populations between 1988 and 2006 by logistic curve fitting to sequential field samples. Prediction of IO 0.5 required adjustment of air temperatures for the effects of cloud opacity on microclimate, otherwise predictions were 20–30 days late. A thermal summation start date of 27 November was determined by iteratively summing day‐degrees (>9.4°C) from different start dates until IO 0.5 , for 10 calibration populations, and observing when the coefficient of variation for the mean thermal requirement was minimal. The thermal requirement for the 10 calibration populations was negatively correlated with accumulated cold‐rest hours (<4.0°C). Model predictions of IO 0.5 were not significantly different from the validation data. Predictions for 1954–2003 were positively correlated with the mean monthly Southern Oscillation Index (SOI) during September–December and negatively correlated with year ( R 2 = 0.36, P < 0.0001), indicating some potential for prediction of IO 0.5 2–3 months in advance. Estimated IO 0.5 in 2003 was 12.8 days earlier than in the mid‐1950s, and variation in IO 0.5 related to the SOI was 25.2 days. These results may be generally applicable to other temperate arthropods that emerge at the same time as T. urticae , such as the aphids Chaetosiphon fragaefolii on strawberry, and Ericaphis fimbriata on blueberry.