About Cabbage Root Fly
You will have seen the cabbage root fly many times in your life as there are lots of them out there. They just look like normal flies. However, they are attracted to all brassicas and lay their eggs at the base of brassica plants. When the eggs hatch the larvae bury into the soil and feed on the roots of the brassica plants (as seen in the photo here). As they eat the roots, the plants start to struggle. Growth will slow right down and eventually the plant will wilt and die.
Protecting Your Plants
If plants are damaged, there is unfortunately nothing you can do to fix it, and worst of all, the pupae can overwinter ready to hatch out the following spring, too.
Prevention is the way forwards, and we would always recommend the following:
1. Place a brassica collar around the base of brassica plants when you plant them out
2. Cover all brassicas with horticultural fleece or insect-proof mesh netting
3. Rotate crops each year and grow brassicas in a different part of the plot to avoid any overwintering larvae from eating brassicas in early spring when they hatch.