Advice
RHS Help & Advice
Flea beetles on brassicas and allied plants
Plants affected
Cruciferous plants such as cabbage, cauliflower, brussels sprout, kale, turnip, swede, radish, salad rocket, wallflower and alyssum. Other plants, such as nasturtium and Cleome can also be attacked.
Symptoms
Round holes are scalloped out of the upper leaf surface; often they do not go all the way through the leaf. The damaged areas dry up and turn pale brown. Seedling plants are particularly susceptible to damage by the adult beetles and the growth of older plants can also be checked if infestations are heavy.
Biology
Several Phyllotreta and Psylliodes species are responsible. These beetles are 2-3mm long, coloured black or metallic blue, and some have a broad yellow stripe running the length of the wing cases. Flea beetles can be seen jumping off leaves when disturbed.
This pest overwinters in leaf litter as adult beetles that emerge in mid spring to attack the seedlings of brassicas and other host plants. In late summer there is sometimes a significant migration of adult beetles from oilseed rape fields into gardens when damage to mature plants can occur. The larvae feed on the plants' roots, but this does not usually cause severe damage.
Control
Ensure rapid germination and development of seedlings so that they grow through this vulnerable stage quickly. Keep plants watered during dry spells and avoid sowing in to cold soil.
Chemical controls that can be applied include rotenone/derris* (Doff Derris Dust*, Vitax Derris Dust*), or pyrethrum (Py Spray Garden Pest Killer, Py Powder, Scotts Bug Clear Gun for Fruit & Veg). Seedling leafy brassicas, but not mature ones, can be sprayed with the systemic insecticide thiacloprid (Provado Ultimate Bug Killer Ready To Use).
* Rotenone/derris-based insecticides are being withdrawn from sale. Gardeners who have this pesticide may continue using it until 10 October 2009. Any unused products after that date should be disposed of safely by handing them to the staff at a local authority manned waste disposal facility.
Disposing of old chemicals