Thermoperiod Reduces the Thermal Constant Required for Oviposition in the Leaf Beetle Atrachya menetriesi

Adult beetles of Atrachya menetriesi started oviposition in a shorter time in the field than under constant laboratory conditions when the heat sums above the threshold (11℃) were about the same. This might be due to the differential effects of constant and fluctuating temperatures on sexual maturation in the female. To confirm this possibility, females were exposed to thermoperiods of 28 or 32℃ (12h)/16 or 20℃ (12h) superimposed on a photoperiod of LD 16 : 8h. Both the pre-mating and pre-oviposition periods were shortened by thermoperiods compared with those predicted for constant temperatures equal to the mean temperatures of the thermoperiods. Such an effect was particularly conspicuous in thermoperiods with a 16℃ cryophase. Acceleration of sexual maturation was discernible even in an extreme thermoperiod of 32℃(2h)/16℃ (22h) but became less obvious with increasing duration of the thermophase up to 10h. No sharply defined critical thermophase or cryophase was observed for this response, so that the involvement of a clock system might be excluded. These results suggest that experiments using constant temperatures may not be satisfactory to establish a model for predicting seasonal life cycles in insects. Females of a strain selected for producing nondiapause eggs at 25℃ required much less heat for starting oviposition than those of a wild strain producing only diapause eggs.