EFFECT OF TEMPERATURE AND PHOTOPERIOD ON DIAPAUSE DEVELOPMENT IN A DOUGLAS-FIR SEED CHALCID, MEGASTIGMUS-SPERMOTROPHUS

Douglas fir seeds infested by diapausing larvae of a chalcid, Megastig mus spermotrophus, were transferred under different temperature and ph otoperiod regimes at various dates during autumn and winter. Both wint er diapause and prolonged diapause were compared to the diapause patte rns observed in control lots kept under natural conditions. The result s showed that exposure to low temperatures in autumn and winter is req uired for the completion of diapause development. Median adult emergen ce dates and the date of transfer of larvae to 20 degrees C were negat ively correlated. The frequency of prolonged diapause of larvae transf erred to 20 degrees C decreased significantly after November. When lar vae were submitted to different constant temperatures, the longer the daylength, the higher and faster the reactivation. These results sugge st that chalcid larvae can perceive daylength through the seed coat. T his phenomenon is probably related to the fact that most of the infest ed seeds falling in autumn are usually covered by a layer of leaf litt er during the winter.