DEGREE-DAY MODEL FOR PREDICTING EMERGENCE OF PEAR RUST MITE (ACARI, ERIOPHYIDAE) DEUTOGYNES FROM OVERWINTERING SITES

Emergence times of deutogynes of the pear rust mite, Epitrimerus pyri Nalepa, from overwintering sites on branches of pear, Pyrus communuis L., were compared between laboratory-stored (constant 1-degrees-C) and field-wintered branches at 2-4-wk intervals over 20 wk. Until wk 16, which corresponded to 1 March, 1992, the mean time to 50% emergence of mites declined steadily and was not significantly different between the two groups of branches. After wk 16, the mean time to median emergence of mites from field-wintered branches continued to decline, but there was no further decline iii the emergence time of mites from laboratory-stored branches, resulting in a significant difference between the two groups. This divergence of the emergence times of mites from laboratory-stored and field-wintered branches after wk 16 indicated that field-wintered mites had begun ''post-diapause'' development or reactivation after almost-equal-to 1 March. Emergence rates of individual mites from branches held at 1-degrees-C for 20 wk were compared at seven constant temperatures: 5.3, 7.8, 10.6, 13.3, 15.5, 17.6, and 20.8-degrees-C. Weighted, least squares linear regression was used to describe the relationship between emergence rate and temperature (rate = - 0.1008 + 0.0162[temp]) for prediction of emergence in the field. The base threshold temperature for emergence was 6.2-degrees-C, as determined by extrapolation of the regression line to the x-axis. The relationship predicted that median (50%) emergence of mites requires 62 +/- 1 degree-days (+/- SE) above the base temperature. The degree-day model was validated using field phenology data collected at one and four sites in 1991 and 1992, respectively. Using a 6.0-degrees-C threshold temperature for emergence, and daily minimum-maximum air temperatures beginning 1 March, the model accurately (+/- 2 d) predicted the 50% emergence point. This article discusses the use of the degree-day model to improve current pear rust mite management practices in British Columbia.