Flea beetles

Start here

Prevention, and the changes that stop the problem returning. Most of these pages should end at this tier.

Get the plant through the seedling stage as fast as you can

This is the whole game. A cotyledon losing a third of its area is a seedling that may not survive, and the same beetles doing the same feeding on a plant with six true leaves are a cosmetic problem. Sow into warm soil rather than cold, water and feed to keep growth moving, and never sow a salad crop into a dry seedbed and hope. Every other measure on this page is a way of buying time for this one.

Raise transplants in modules and plant them out at size

A rocket or brassica seedling raised under cover and set out with four or five true leaves walks past the stage at which flea beetles can kill it. Direct sowing is quicker and it hands the beetles the vulnerable fortnight. For anything you have lost before, use modules.

Keep the surface damp

Flea beetles feed hardest in hot dry conditions and go quiet in cool damp ones, and the difference is large enough to see across a bed. Watering overhead in the morning through a dry spell, and mulching between the rows, reduces the damage on its own and helps the seedlings grow away at the same time.

Sow to miss the peak

Adults come out of overwintering and feed heavily as the weather warms, then the new generation emerges and feeds again in late summer. A sowing that germinates between the two flushes takes far less damage. In a temperate spring that usually means holding salad sowings back a fortnight and putting the main crop in early, or sowing in late summer for autumn once the second flush has passed.

Sow a trap crop and let it take the damage

Flea beetles prefer some brassicas strongly over others, and a sacrificial row of mustard, Chinese cabbage or rocket sown a fortnight before the main crop and placed upwind of it draws a good share of them away. Pull and bin the trap row rather than leaving it to breed, and do not put it next to the crop you are protecting.

Do not count on rotation

The adults fly and the overwintering sites are hedge bottoms and leaf litter rather than the bed itself, so moving the brassicas to the far end of the plot achieves much less here than it does against clubroot or cabbage root fly. Rotate for the other reasons and expect nothing from it against this.

Grow the crops that shrug it off, and the varieties that do

Damage varies a great deal between species and between cultivars. Red and purple-leaved brassicas are generally less attacked than green ones, and hairy or waxy leaves take less than smooth soft ones. Rocket, mizuna and radish suffer worst because the leaf you are growing is the leaf being holed.

Tidy the crop debris and leave the rest of the garden alone

Clearing spent brassica stumps and fallen leaves from the bed in autumn removes feeding and shelter. Stripping every scrap of cover from the wider garden costs you the ground beetles and rove beetles that eat the eggs and larvae, so leave the hedge bottom, the log pile and the long grass where they are.

Then reach for a barrier

Netting, hand removal and water. Nothing here harms anything except the target.

Use fine mesh, and check the hole size before you buy it

This is where most gardeners go wrong. An adult flea beetle is 2 to 3 mm long and around a millimetre across, so it walks through the standard insect netting sold for carrot fly and cabbage white, which has a mesh around 1.3 mm. You need an ultra-fine grade at roughly 0.8 mm or below. Read the specification rather than the words on the packet, since "insect proof" is sold against several different insects.

Put the mesh on at sowing and seal every edge

Cover the bed the day you sow, not the day you notice the holes, and bury or weight the edges all the way round. A cover thrown over a crop that already has beetles under it keeps them in and the predators out. Where flea beetles bred in that bed last year, put the mesh over fresh ground instead.

Fleece an early sowing

Horticultural fleece excludes the beetles and warms the soil, which pushes the seedlings through the vulnerable stage faster at the same time. It is the better choice than mesh for the earliest sowings and the worse one once the weather turns hot, since it will cook a salad crop.

Run a greased board over the row

An old trick that works because of how the animal escapes. Smear a piece of board or stiff card with grease or petroleum jelly, hold it just above the plants, and sweep it along the row. The beetles jump as it passes and stick to it. Two or three passes on a warm afternoon take a noticeable share of the population off a small bed, and a yellow sticky trap laid flat between the rows does a slower version of the same thing.

Then bring in the predators

Encourage what already lives in the garden, and buy predators in where the problem is under glass and nothing will arrive on its own.

Keep the ground beetles you already have

Ground beetles and rove beetles hunt flea beetle eggs and larvae in the top few centimetres of soil, and they need cover to live in. A permanent edge of rough grass, a log pile and an undisturbed hedge bottom near the vegetable beds do more against this pest over several seasons than anything you can apply in one. Birds take the adults.

Leave the parasitoid wasps to it

Several small braconid wasps attack flea beetles, laying into the adults so that the female stops laying. They arrive on their own where the garden carries flowers they can feed at, so an umbellifer or two allowed to flower near the vegetables is the practical way to encourage them. None is sold as a released control.

Expect little from bought nematodes

Steinernema and Heterorhabditis are sold against vine weevil and leatherjackets and are sometimes suggested for flea beetle larvae in the soil. The evidence for it is thin, the larvae of the brassica species do little damage anyway, and the money is better spent on ultra-fine mesh.Widely sold across Europe and North America. Live agents are regulated nationally and New Zealand requires approval under the HSNO Act before any new organism is released, so check what is permitted where you garden.

Biological controls are live organisms and their sale and release are regulated country by country. An agent sold freely in one place may be unapproved in another, so buy from a licensed supplier where you live and never import or move one yourself.