RELIABILITY OF DEGREE-DAY INDICES FOR PREDICTING SPRING EMERGENCE OF THE SPOTTED TENTIFORM LEAFMINER, PHYLLONORYCTER BLANCARDELLA (LEPIDOPTERA: GRACILLARIIDAE), IN ONTARIO

Abstract

AbstractThe validity of using 40 ± 12 degree-days (DD) above 6.7 °C summed from 1 April for predicting first spring emergence of the spotted tentiform leafminer, Phyllonorycter blancardella (Fabr.), was tested. In Ontario's major apple growing areas from 1978 to 1981, the observed day of first emergence occurred 12–45 days before the predicted day. Reasons for this inaccuracy are discussed.An alternative method, summing DDs from 1 March above a base temperature of 5.5 °C, was also tested using three methods of computation. The number of DDs accumulated to the day of first emergence and to the day of 50% cumulative emergence varied greatly between years at one location and between locations during one year. This variation is likely due to a variable relationship between air temperature and temperature in the leafminer's habitat. It is suggested that a more accurate predictive index could be developed through an understanding of the relationship between habitat temperature and weather factors such as air temperature, insolation, and snow cover.