Flea beetles (Phyllotreta) riddle garden plants with tiny holes and like their namesakes, jump away when disturbed. There are over 100 types but the best known are those that attack the brassica family, including cabbages, Brussels sprouts, sprouting broccoli, cauliflowers, land cress, kale, kohlrabi, rocket, radish, mustard, swedes and turnip (they used to be known as turnip flies). Some can also affect flowering plants too, including wallflowers, nasturtiums, alyssum, anemone, stocks, cleome and godetia.
What are flea beetles?
Flea beetles are small, shiny, black, brown or blue beetles that are 2-3mm long. They can be hard to spot, and jump out of sight when disturbed. Flea beetles appear in spring, after hibernating around the garden. They feed on plant foliage (their larvae feed on plant roots, but rarely do much damage) and lay eggs at the base of host plants. These pupate by midsummer, so there are then two generations of flea beetle attacking plants. Flea beetles can be particularly prevalent in late summer, when fields of oil seed rape are cut. They begin hibernating from early autumn.
Identifying flea beetle symptoms
The upper leaves of brassicas and various ornamentals have small holes, some of which may not go all the way through the leaf. Each hole is surrounded by pale brown scar tissue. This is usually not a problem for larger plants as they can cope with attacks (and the leaves are still edible – when cooked, the holes largely disappear) but seedlings may be stunted or wiped out. You may spot the tiny beetles jumping away when disturbed.
Controlling flea beetles
Make a sticky trap
Exploit the beetles' habit of jumping by catching them with a sticky trap. Coat a piece of card with grease, such as insect barrier glue, leaving a clean strip along one edge. Brush the clean edge of the card over the top of your plants – when the beetles hop into the air they'll stick to the grease. Repeat as necessary but don't leave the sticky trap among your plants as it could trap bees and other pollinators, and even birds and bats.
Use an organic insecticide
Try one that contains pyrethrins. Do not spray onto plants in flower, as this endangers bees and other pollinating insects.
Commercially available bug sprays should be a last resort and should not be sprayed onto plants in flower, as this endangers bees and other pollinating insects. Bear in mind, also, that a bug spray could potentially contaminate food you want to eat.
Preventing flea beetles
Grow under insect-proof mesh
Growing crops under insect-proof mesh is the best way to keep flea beetles at bay. Cover the area with this as soon as possible after sowing.
Remove overwintering spots
Flea beetles overwinter in leaf litter, so remove any close to susceptible plants.
Remove the weeds that flea beetle also attack, such as shepherd's purse and hairy bittercress.
Delay planting out seedlings
Seedlings are especially vulnerable to attack, so protect them by waiting until they're a good size before planting out. Crops sown later in July or August may also avoid attack.
Encourage flea beetles' natural predators into your garden – birds, frogs and ground beetles will eat them.