THE DIAPAUSE AND RELATED PHENOMENA IN GILPINIA POLYTOMA (HARTIG): II. FACTORS INFLUENCING THE BREAKING OF DIAPAUSE

Abstract

Field and laboratory experiments have shown the importance of a period of "cold-rest" at a temperature below the threshold of development as a requirement for overcoming diapause in the spruce sawfly, especially in stock from a one-generation area. After cold-rest, maximal development results at a temperature of 74° to 75° F. or higher, and after contact with water. Temperatures in the field are lower and fail to promote so high development as may be obtained in the laboratory; however, temperature variations between 65° and 45° F. evidently have little influence on the degree of emergence from the diapause condition, though speed of development is directly affected. The benefit of contact with water is reduced or lost if contact occurs only while soil temperature remains below the threshold of development, and if the moisture taken up in the cocoon wall is lost by evaporation before it can be absorbed by the larva. The role of the cocoon in water exchanges, and differential effects of abnormal weather conditions upon intracocoon development in stocks in one-generation and two-generation areas, are described.