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 ca bbage moth, Mamestra brassicae (L.) (Lepidoptera: Noctuidae) was inves tigated in a series of small-scale field experiments in white cabbage, Brassicae oleracea var. capitata (L.), and in the laboratory during 1 990-1992 in south-eastern Norway. The highest mortality was found in y oung larvae and in hibernating pupae. In 1990, larval mortality in the first instar was 80% (range 9-97% for the individual cohorts). Most l arvae died within the first 1-3 days after hatching. The dispersal act ivity during these days was high, and failure to establish feeding sit es and predation were probably the main mortality causes. Pupal mortal ity during winter was 90% on average for 1990-1993 (range 81-100%). Th e 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 num bers in 1991. Larval parasitism increased from < 1% in 1990 to almost 24% in 1992, and was almost totally caused by the braconids Micropliti s mediator (Haliday) and Aleiodes (Aleiodes) sp. Predation of frozen l arvae on the soil surface was 75% on average (range 63-96%) during 199 0-1992 in first instar larvae and decreased gradually with larval age. The consumption rates of Philonthus atratus (Gravenhorst) (Staphylini dae) and the carabids Bembidion tetracolum (Say), Pterostichus melanar ius (Illiger) and Harpalus rufipes (Degeer) on M. brassicae eggs and l arvae were investigated in non-choice experiments in the laboratory. A preliminary survival model based on estimates of the mortality factor s identified in this study is presented.