The adult cabbage root flies look like, and are the same size as, common houseflies.
The larvae look like typical maggots – white, legless, headless and up to 9mm (1/3in) long.
There are up to three generations per year, during summer. The first generation, from late spring to early summer, is usually the most damaging.
When fully fed, the maggots stop eating and turn into brown pupae in the soil. Depending on the generation, they either emerge as adult flies a few weeks later, or remain as pupae in the soil over winter. These then emerge as adult flies the following spring.