Larval Diapause in Chymomyza costata (Diptera: Drosophilidae) II. Frost Avoidance

Chymomyza costata is frost susceptible. The diapausing larvae do not exhibit any stage-specific low supercooling point, but they posess the ability to lower the supercooling point by rapid defecation at low temperature and by dehydration both being effective to maintain a high viability under low temperature. Introduction The present knowledge on the overwintering strategies of insects provides us the two alternative ways of cold resistance, frost avoidance and frost resistance (1, 9, 10). The numerous contributions have been published to find out the mechanisms that enhance these two ways (1, 4, 7, 8, 11). As the study of diapause in Drosophilidae is a relatively new field, their overwintering strategies have been poorly known. Chymomyza costata Zetterstedt is one of few drosophilid species so far known to pass winter by the final (3rd) larval instar (5), in Hokkaido under bark of trees and stumps. Although its detailed habits during winter is still unknown, the experimental study on its cold tolerance may be not vain as only the diapausing individuals confront fairly low temperatures under natural conditions (3). The purpose of the present article is to report some aspects of frost avoidance using the artificially induced diapausing larvae. Materials and Methods T he stock and culture techniques used for th is study is mentioned previously (3). The larvae were reared at lS"C/under LD 10: 14 or 16: S. The last instar larvae in the former condition were regarded to be committed diapause and the latter not. Three items were studied as follows. 1) The su.percooling point at every developmental stage In order to confirm whether diapausing larvae have stage-specific ability to tolerate low temperatures, the supercooling point, a widely accepted criterion to study the cold resistance of poikilotherms, was compared at every developmental stage. The insect attached to the tip of a cooperI Received for publication November 3D, 1981. Contribution No. 2408 from the Ins!. Low Temp. Sci. The Institute of Low Temperatllre Science, Hokkaido University, Sapporo, 060