There are three generations of cabbage root fly during the summer but it is the first generation in late spring-early summer that is often the most damaging.
Adult cabbage root flies resemble house flies in size and appearance.
The larvae are white, legless and headless maggots that are up to 9mm long. They feed on the roots and can kill seedling and recently transplanted brassicas. Later generations are less damaging to cabbages and other leafy brassicas, as older plants have larger root systems and are better able to tolerate the damage. Host plants where the root is the edible part, such as radish, turnip and swede, are damaged by any of the generations.
When fully fed, the larvae go into a brown pupal stage in the soil, either emerging as adult flies a few weeks later or remaining in that state overwinter.