Damage thresholds for cabbage root fly [Delia radicum (L.)] in cauliflowerassessed from pot experiments

The influence of cabbage root fly [Delia radicum (L.)] on cauliflower dry m atter production was investigated in three experiments during 1992 and 1993 . Cauliflower plants grown outdoors in pots were inoculated with different numbers of cabbage root fly eggs, 1, 2, 3 or 4 weeks after transplanting as seedlings. A non-destructive Method was used to estimate dry matter produc tion during the growing season. Regression analyses of plant dry matter aga inst egg number demonstrated that control measures against cabbage root fly larvae are needed to protect young seedlings during the first 3-4 weeks af ter planting, whereas 100 eggs or more were required to cause a 5% reductio n in mean plant dry matter when plants were inoculated 4 weeks after transp lanting. Egg and larval mortality varied with means in the range from 47 to 61% and density-dependent larval mortality was demonstrated in two out of four combinations of experiment and inoculation time.