SOME EFFECTS OF TEMPERATURE ON THE PRODUCTION AND ELIMINATION OF DIAPAUSE IN THE WHEAT STEM SAWFLY, CEPHUS CINCTUS NORT.

Abstract

Fully grown Cephus cinctus larvae go into an obligatory diapause, the ending of which involves the elimination of two separate factors. The first, or x factor, is eliminated only at subdevelopmental temperatures and has a positive temperature coefficient in a temperature range from the undercooling point up to the developmental threshold. The second, or y factor, is eliminated at either low or moderate temperatures, having a positive coefficient in a temperature range from the undercooling point up to about 30 °C. When the x factor reaction is complete, diapause may be said to be 'broken', but it is not 'eliminated' until the y factor reaction is complete.When the x factor reaction is complete but the y factor is still present, the insect can be returned to diapause by either high temperatures or a lack of adequate moisture or a combination of these. This sometimes occurs in nature, resulting in a two-year life cycle. Once the y factor has been eliminated the insect starts postdiapause development and can no longer return to a state of diapause.Postdiapause development under constant favourable conditions is rather uniform after it has once started, but the time of its initiation is variable, dependent on the y factor. Evidence is presented that indicates that the y factor is eliminated faster in large larvae than in smaller ones, accounting for the rather wide variability in development in samples uniformly treated but not selected as to size.At 10 °C. roughly 40 to 110 days were required to break diapause, each larva requiring a definite conditioning period. Thus the process ends abruptly in individuals, gradually in a group. Under natural conditions diapause was broken as early as Oct. 19, 1945, in a few cases, and was broken in all cases by the end of January 1946.