Mortality of Winter Eggs of the European Red Mite Panonychus Ulmi (Koch), at Low Temperatures, and Its Ecological Significance

Winter eggs of the phytophagous mite Panonychus ulmi (Koch) from Nova Scotia were killed by higher temperatures than were eggs from colder areas in New Brunswick and Quebec. For short exposures the mean lethal temperatures were respectively −24° F and −35° F. In the lethal temperature range the mortality of both forms increased about 10% for each doubling of the exposure period in the range from 15 minutes to 16 hours. Reciprocal crosses of the two forms indicated that the factor for resistance to cold in P. ulmi is recessive. Resistance was increased in the laboratory by selection. A method of estimating the winter mortality from weather records is outlined.