POPULATION ECOLOGY Threshold Temperature for Post-Diapause Development and Degree-Days to Hatching of Winter Eggs of the European Red Mite (Acari: Tetranychidae) in Northern Greece Downloaded from

ABSTRACT 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 25ЊC) in the laboratory. The mean number of days to 50% hatch in each temperature was Ͼ120, 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 7.4ЊC. In the Þeld (1991 and 1993Ð1996), 50% egg hatch occurred from the end of March to 20 April after the accumulation of a mean sum of 129.4 Ϯ 4.5 DD above the threshold of 7.4ЊC 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 Þrst half of February. Using the theoretical sum of 154.6 DD, the predicted dates for 50% egg hatch deviated from Þeld observations by an average of 3.7 Ϯ 0.5 d over the 5 yr of experiments. KEY WORDS Panonychus ulmi, European red mite, degree-days, hatching, winter eggs THE EUROPEAN RED mite, Panonychus ulmi (Koch), is an important foliar pest of apple trees feeding on the palisade and spongy mesophyl cells and causing a reduction in the photosynthetic rates of the plant Application of chemical acaricides early in spring against newly hatched larvae is a widely used method for the control of this mite. The development of a degree-day (DD) model for predicting the time of egg hatch in the Þeld could be a useful tool for the proper timing of acaricide applications. In the current study we determined the threshold temperature for postdiapause development and subsequently using laboratory and Þeld data estimated a sum of degree-days for predicting the time of egg hatch. The accuracy of the degree-day model for predicting egg hatch was validated under Þeld conditions. Materials and Methods Threshold Temperature for PostDiapause Development. Twigs bearing diapause-eggs were cut from an apple orchard in the area of Alexandria (Northern Greece) in the second half of February of 1994, and subsequently transferred to the laboratory. The course of diapause termination in the Þeld was determined in earlier experiments by transferring diapause eggs during winter and early spring for 4 yr (1992Ð 1996) from the same and other apple orchards to short days (a photoperiod of 8:16 [L:D] h) at 20ЊC where percentages of diapause termination were determined Every third day the percentage of eggs hatch at each temperature was determined. The data were then used to estimate the mean number of days required for 50% egg hatch (t 50% ) at each temperature. The reciprocal, 1/t 50% , represented the daily rate of postdiapause embryonic development. The threshold temperature for postdiapause development was estimated by the x-intercept metho

Editorial: Ecosystem Services and Disservices Provided by Plant-Feeding Predatory Arthropods

Editorial of the Research Topic issue on Ecosystem Services and Disservices Provided by Plant-Feeding Predatory Arthropod