INFLUENCE OF DATE OF OVIPOSITION ON EGG HATCHING AND EMBRYO SURVIVAL IN THE RED TURNIP BEETLE, ENTOMOSCELIS AMERICANA (COLEOPTERA: CHRYSOMELIDAE)

Embryological development and hatching for the red turnip beetle, Entomoscelis americana Brown, were studied at 20 °C. In eggs kept at this temperature for > 12 days, embryological development proceeded until a late stage embryo was formed, at which time the embryo entered diapause for the winter. The time required for 50% of the eggs to reach the late embryo stage was estimated to be 11 days. The late embryo stage was not reached before winter in eggs kept at 20 °C for < 11 days, but most of them survived and hatched. Embryogenesis proceeded normally after winter without any apparent adverse effects, and hatching was delayed by the amount of time required to complete embryogenesis. Although it was not essential for the eggs to reach the late embryo stage to overwinter successfully and hatch, a critical minimum stage of development had to be reached during embryogenesis for the eggs to survive. The critical minimum stage of development occurred in the germ band formation stage and was reached during the first 1.5 days of embryogenesis. Most of the eggs laid from 1 to 15 September 1980 and kept outdoors reached the late embryo stage before winter and hatched the next spring; those laid from 15 September to 15 October reached the critical minimum stage of development before winter and also hatched successfully; and those laid after 15 October did not hatch, because they apparently had not reached the critical minimum stage. An evaluation of the data for the eggs kept outdoors and of atmospheric air temperature data suggested that embryo death due to eggs not reaching the critical minimum stage of development before winter normally should not be an important factor in the population dynamics of E. americana in the rapeseed- and canola-growing areas of western Canada. The ecological significance of the data is discussed.