Delia radicum, Cabbage Fly
Delia radicum can be challenging to tell apart from a small house fly. Cabbage Root Flies are pests of plants in the Brassicaceae family. They're frequent in gardens and allotments but tend to be poorly recorded. The maggot-larvae are the most damaging. Gardeners typically find them in and around the plant roots, where they bore into the main stems at the plant base. The adult flies drink the nectar of plants, so aren't as intrusive as the larvae. Cabbage Root Flies are capable of giving rise to 3 generations per year, meaning that the flies can be problematic throughout summer.
Can stunt the growth of plants, sometimes causing death.
Adult flies are pollinators.
Adult: The adult is under 1cm, is similar in appearance to a housefly. They are active from spring through to autumn. Larvae: The larvae maggot of the Cabbage root fly is white in colour and will be found around the root of the plant in the growing media. Pupae: The pupa is brown in colour and up to 10mm in length. Eggs: These are laid in dense clusters and are white in colour.
The larvae maggot of the Cabbage root fly eat the roots of the Brassica group of plants. A young plant grown for foliage crops, may appear to be struggling to establish and growth may be stunted or the plant may possibly die. Tiny holes in crops. Plants look sickly. Cabbages fail to form hearts. When dug up, small white maggots evident around the roots.
Monitor plants from spring to summer. Any plants which appear to look sickly should be dug up and inspected. Good practice of yearly crop rotation is a must. When planting out young vulnerable plants, a Brassica collar can be used around each plant. These can be purchased from a local garden centre or can be homemade. Use thick cardboard or carpet underlay to make your own, cut out a square or circle 15cm across and make a slit to the middle so it can be placed around the plant at soil level. The Cabbage root fly will lay eggs on to this instead of the soil, and the eggs will dry off in the open air, or the birds will eat them. Young plants may be grown under a fine insect-resistant mesh or horticultural fleece to give protection. Floating row covers or make-shift plastic bottles can be used to protect young seedlings from having eggs laid on them. It's also possible to buy root fly mats for plants online or via garden retailers. If available, you can purchase beneficial nematodes online, which will attack any larvae in the soil. Carefully read the instructions and ensure you are buying the correct species for the target pest. They are supposedly attracted to soil with high organic matter content. Likewise, they are believed to dislike rhubarb, so this would be an excellent companion plant for your veg patch. Be sure to practise crop rotation every year. This will confuse the flies which emerge the following year.
It's possible to apply pesticides; however, if used at the wrong time, it can be ineffective. This is why chemicals usually are discouraged with this pest.
Brassica oleracea (Capitata Group) 'Cabbages'
Brassica oleracea (Botrytis Group) 'Cauliflowers'
Pink Spotted Begonia
Brassica rapa (Rapifera Group) 'Atlantic'
Brassica oleracea var. italica (Italica Group)
Erysimum cheiri 'Sunset Primrose'