Of the fruits grown in the Kootenay Valley of British Columbia, pears are third in importance; in the more arid Okanagan Valley 300 miles to the wrest they are second. During 1959 the pear psylla, Psylla pyricola Foerster, caused severe losses for fruit growers in the Okanagan Valley. The damage was much greater than had occurred during the previous 17 years. It became clear that the insect was a major economic pest, and that it was not being adequately controlled by the recommended insecticides. Obviously, more information was needed on the biological aspects of the situation. The roie of climatic conditions was particularly in question. In 1960 a start was made on a biological study of the pear psylla at Creston in the Kootenay Valley. This paper deals with observations thnf were made there.