Population dynamics of cabbage root fly in Canada and England.
Life tables for populations of Hylemya (Erioischia) brassicae (Bch.) on cabbage crops in Canada [cf. RAE/A 60, 538] and England [50, p. 286] are analysed by the method of key-factor analysis [55, 1163]. The key factor determining population change in Canada is failure of the observed adult females to attain the potential maximum egg production. The key factor in England is probably egg predation. Populations are stabilised by density-dependent pupal mortality. In Canada, this is due to parasitic and predatory beetles, but in England to the death of the food-plant. Population models are developed to predict mean density and also population change. The reasons for differences in mean density in Canada and England are discussed in relation to the population models.