Q What are flea beetles?
A Flea beetles are tiny beetles 2-3mm long, which jump like fleas when disturbed. Although they used to be called turnip flies, they are actually true beetles. It is hard to see them in the soil and among vegetation. The larvae feed on the roots of plants, but the real damage is done by adults attacking the leaves. They are a pest of cabbage-family vegetables, including Brussels sprouts, cabbages, calabrese, cauliflowers, Chinese cabbage and other oriental greens like pak choi, kale, sprouting broccoli, kohlrabi, swedes and turnips.
Closely related crops like radishes and rocket or ornamentals such as alyssum, stocks and wallflowers are also vulnerable. Occasionally, certain species will attack other plants, such as irises and anemones. Cabbage-family weeds such as shepherd’s purse and hairy bittercress also support flea beetles.
Caption: Flea beetles attack the leaves of members of the cabbage-family
Q Is there more than one kind of flea beetle?
A There are at least six kinds that feed on cabbage-family plants. All do similar damage and are controlled in the same way.
Black flea beetle (Phyllotreta atra). Its colour makes it difficult to spot against dark soil.
Large striped flea beetle (Phyllotreta nemorum). This is black with two slightly irregular yellow stripes down its back. At 3mm it is slighty larger than the small striped flea beetle.
Small striped flea beetle or turnip flea beetle (Phyllotreta undulata). Similar to but smaller than the large flea beetle, it has a pair of wide yellow stripes down its back.
Mangold flea beetle (Chaetocnema concinna) is a glossy, bronzed black with indented wing cases. It attacks beetroot, spinach and spinach beet.
Q How do I recognise flea-beetle damage?
A The adult beetles eat small circular holes in the leaves and stems of seedlings. The affected parts become covered in tiny round scars. The attacks start when the shoot appears above the ground, and continue even when the plants mature. Larger plants usually shrug off attacks as long as they are growing well. Seedlings, however, can be severely checked or even killed.
You have to have sharp eyes to see the adults. Often you will only notice them when they spring away as you pass by. In severe attacks, you may hear rustling as large numbers move.
The larvae also cause damage as these feed on the roots of plants. Fortunately, this is usually much less serious than the damage done by the adults to the plant above ground.
Caption: Large plants usually shrug off flea-beetle damage
Q What diseases do flea beetles spread?
A When they feed, flea beetles can carry turnip rosette virus, turnip yellow mosaic virus and turnip crinkle virus from plant to plant. But the virus is usually much less important than the physical damage to seedlings.
Q When should I expect to see flea beetles?
A Typically, attacks start in April and May. Fine, sunny weather, when the soil warms up, is the most likely time. Look out for the adults' feeding marks on emerging seedlings. The adults will have overwintered in hedges, rubbish heaps, in mulch and under tree bark. Gardens are ideal winter habitats.
In May and June, eggs are laid around host plants. Large striped flea-beetle larvae crawl up into the stems and tunnel into the leaves, creating 'mines'. Some flea-beetle larvae attack roots.
It takes about a month for the larvae to reach full size and pupate. About three weeks later a new generation of adults emerges; these survive to lay eggs next spring and summer. This new generation causes damage to young wallflowers and late-sown cabbage-family vegetables, such as Chinese cabbage, in late summer.
Q Are they worse in some years than others for flea beetles?
A Flea beetles were once extremely common and serious pests. They are much less severe now, though the reason for this is unknown. Severe attacks were seen on brassicas sown late in the season and especially on wallflowers.
Large numbers build up in oilseed-rape crops. When these are cut, the beetles migrate downwind, and if they end up in gardens, they can do much damage in late summer. They thrive in sheltered sites with plenty of cover.
Q Could I mistake flea beetles for anything else?
A Larvae around roots are more likely to be cabbage root fly. Birds and caterpillars damage leaves but don't make the characteristic puncture marks. Mottled foliage may be the result of thrips (thunder flies).
Q Can plants affected by flea beetles recover?
A If you act quickly, you can rescue the plants. Watering, adding 70g a sq m of nitrogen-rich fertiliser, firming the soil around the plants and covering plants with fleece to boost their growth in spring, will all help them recover.
Q Can flea beetles be sprayed?
A Spray with an insecticide based on pyrethins such as PY Insect Killer. Spray or Scotts Bug Clear Gun for Fruit & Veg or, better still, dust the emerging seedlings with PY Powder. Repeat every week if the problem persists.
Bayer Sprayday Greenfly Killer and Westland Plant Rescue Fruit & Vegetable Bug Killer will also kill flea beetle.
For a totally pesticide-free option, you can use flea-beetle traps. These are rectangles of cardboard, coated with grease. Attach these to a broom handle and move it quickly along the rows of plants. As the flea beetles jump, they stick to the grease.
Q Can flea beetles be avoided?
A You can discourage flea-beetle attacks by clearing away as much shelter as possible. You should also keep down cabbage-family weeds, like shepherd's purse.
Plants raised indoors won’t usually be affected. When they’re planted out in June and July, they’ll be too big to be damaged badly and the worst of the flea-beetle attacks will be over by then. Plants raised in moist, well-manured, fertile soils with a fine tilth or texture, will not be as badly damaged as plants raised in poorer soils.
Raising cabbage family plants in seed beds protected by fleece or insect-proof mesh will exclude flea beetles, if put on soon after sowing. They will also keep the seedlings free of cabbage root fly, cabbage aphid and other pests. Fleece captures too much warmth for summer sowings. To avoid cooking your plants, use the insect-proof mesh in summer.
Late sowing in June can avoid attacks, but this will often lead to the plants failing to crop well. Sowing swedes, turnips, Chinese cabbage and other oriental vegetables at this time will result in good crops and fewer flea-beetles.
The final sowings in July and August of turnips and Chinese cabbage, for example, can be protected by watering the seed bed thoroughly before sowing. When the excess water has drained away, sow, and the seedlings should grow fast enough to resist the flea-beetle damage. Alternatively, raise the plants in pots or modular trays indoors.
Q Is it safe to replant in a spot where flea beetles have recently attacked?
A There should be no problem, as long as you protect the emerging seedlings with an insecticide dust or spray. Alternatively, planting out seedlings raised in pots or trays should be safe.