Eclosion time and spatial distribution of overwintering spruce spider mite (Acari: Tetranychidae) eggs on Colorado spruce

Abstract

No ovipositional preference was shown by spruce spider mites, Oligonychus ununguis (Jacobi), with respect to compass direction or the upper, middle, or lower strata of Colorado spruce, Picea pungens (Engelmann). However, along the axes of host plant limbs, eggs were deposited significantly farther from the center of the plant on the south and east sides than on the north and west sides. Eggs were deposited significantly farther from the center of the host plant in the lower stratum than in the upper and middle strata. Degree- day accumulations, using 2 lower developmental thresholds, were used to described eclosion of diapausing eggs. Mean degree-day accumulations (±SD) from 1 March to eclosion were 143.3 ± 30.1 base 7.1°C and 170.1 ± 29.6 base 5.6°C. These means coincided well (≃5 d) with observed hatching times in the field in 1993 and 1994. A In-probit transformation of the data and regression analysis revealed no significant difference between the accuracy of either model relative to field observations. However, the model using the lower of the 2 developmental threshold (5.6°C) provided a better fit with data gathered in the lab (r2 = 0.94, MSE = 0.114), suggesting that 5.6°C is a better base temperature for predicting emergences based on degree-days.