Cabbage Root fly
Cabbage Root Fly is a pest that affects most brassicas e.g. Broccoli, Brussels Sprouts, Cabbage, Calabrese, Cauliflowers, Kale, Swedes and Turnips.
The larvae of the fly live in the soil and eat the roots of the plants.
The females lay their eggs close to the stems of the brassicas,these hatch into maggots that burrow into the stem and roots to feed.
About three weeks later the maggot is fully grown and leaves the plant but remains in the soil where it becomes a small brown pupa, about 1cm long.
A week or so later, another generation of flies emerge and fly off in search of more brassicas to repeat the cycle.
Three generations of adults can develop during the summer and attacks can persist until mid-September.
Initially the leaves of any infested plants wilt and droop, then develop a blueish tinge due to the root damage reducing the passage of nutrients to the leaves.
The symptoms can only be confirmed by pulling up a plant, and inspecting it for larvae or larvae damage.
The damage will appear as tunnels in the roots and lower stem, and sometimes emerging larvae or blackened roots.
There is no chemical insecticide available to the amateur gardener so it means prevention is the only cure.
Young plants that become affected usually die,however, older more mature plants often survive but may fail to produce their optimum size and maturity.
Starting plants in modules and or pots can help!
Plants grown in this manner have a more robust root system prior to planting out and this can compensate for larvae damage.
Once planted out placing a slotted disc of some barrier material around the base of the stem at soil level can prevent the fly laying its eggs cloose to to the plant.
This not as convenient with Swedes and turnips, here one would have to cover the rows of plants with a fleece cloche arrangement.
Digging over beds after harvesting will expose the pupae to the birds, which may reduce the problem the following season.
Some forms of beetle may eat the eggs /larvae during the growing and winter months.
Similarly drenching the soil after planting with a tar oil based liquid can act as a deterrant, as it is thought that the smell confuses the adult fly as they seek out the smell of brassicas.