About Cabbage Root Fly

Cabbage Root Fly Lifecycle

There are three generations of cabbage root fly during the season. The first generation in late spring to early summer that is often the most damaging.

Adult cabbage root flies resemble normal black house flies in size and appearance.

The larvae are white maggots that are up to 9mm long. They feed on the roots and can kill seedlings and recently planted out brassicas. Later generations are less damaging to cabbages and other leafy brassicas, as older plants have larger root systems and are better equipped to withstand the damage. Host plants where the root is the edible part, such as radish, turnip and swede, can be damaged by any of the three 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 the same over winter.

Eggs hatch in two to seven days and larvae immediately begin feeding on the roots of nearby host plants. Feeding continues for three to four weeks before larvae pupate in the soil. A second generation of adults emerges in late June and lays eggs which hatch producing additional larvae that feed and eventually pupate.

The life cycle is driven by temperature, and activity is earlier in warm years or locations. In the UK, there are generally two to three generations of adults in the South and only two generations in the North.

In some areas, a proportion of Cabbage Root Flies emerge later in the spring than would be expected. These are called ‘late-emerging’ flies and they are genetically different from ‘early emerging’ flies.

When plants have large root systems, damage may not be apparent until harvest. Even at this stage, damage to aerial parts may be difficult to identify.