Cultural control of cabbage root fly

Place brassica collars around recently planted brassicas. The collar keeps female flies from laying eggs in the soil. 

Grow brassica plants under fleece or cloches. 

Practice crop rotation. Use a crop that doesn't host cabbage root fly. Overwintering pupae in the soil will have nothing to feed on when they emerge in spring.

There are no chemical controls for cabbage root fly.