Mortality of eggs, larvae and pupae and larval dispersal of the cabbage moth, Mamestra brassicae, in white cabbage in south-eastern Norway

The mortality of eggs, larvae and pupae and larval dispersal of the cabbage moth, Mamestra brassicae (L.) (Lepidoptera: Noctuidae) was investigated in a series of small-scale field experiments in white cabbage, Brassicae oleracea var. capitata (L.), and in the laboratory during 1990-1992 in south-eastern Norway. The highest mortality was found in young larvae and in hibernating pupae. In 1990, larval mortality in the first instar was 80% (range 9-97% for the individual cohorts). Most larvae died within the first 1-3 days after hatching. The dispersal activity during these days was high, and failure to establish feeding sites and predation were probably the main mortality causes. Pupal mortality during winter was 90% on average for 1990-1993 (range 81-100%). The main mortality factor was probably unfavourable weather conditions, and indications of cold stress were found. The impact from parasitoids and diseases was generally low. Trichogrammma semblidis (Aurivillius) (Trichogrammatidae) was reared from M. brassicae eggs in very low numbers in 1991. Larval parasitism increased from < 1% in 1990 to almost 24% in 1992, and was almost totally caused by the braconids Microplitis mediator (Haliday) and Aleiodes (Aleiodes) sp. Predation of frozen larvae on the soil surface was 75% on average (range 63-96%) during 1990-1992 in first instar larvae and decreased gradually with larval age. The consumption rates of Philonthus atratus (Gravenhorst) (Staphylinidae) and the carabids Bembidion tetracolum (Say), Pterostichus melanarius (Illiger) and Harpalus rufipes (Degeer) on M. brassicae eggs and larvae were investigated in non-choice experiments in the laboratory. A preliminary survival model based on estimates of the mortality factors identified in this study is presented.