Flea beetles on radishes, rocket and brassicas

Anyone sowing radishes or rocket in spring knows the sight: within days the leaves are riddled with tiny round holes, as if someone had gone at them with a fine needle. The culprits are flea beetles — small beetles that appear in numbers during dry, warm weather.

Flea beetles rarely kill a crop, but they can weaken young sowings so badly that nothing worthwhile comes of them. And they are easy to manage once you know their weakness: they hate moisture.

In short

Flea beetles are 2 to 3 mm beetles, often glossy black or striped, and they jump away when disturbed.
The typical damage is many small holes, known as shot-hole damage.
They love dry, warm, sunny spots — damp conditions they do not like at all.
Keep the bed evenly moist, mulch, and cover with fine insect netting.
Affected are mainly brassicas: radish, rocket, mooli, mustard, cabbage and kohlrabi.

What are flea beetles?

Despite the name they are not fleas but leaf beetles of the genus Phyllotreta. They grow to 2 to 3 mm and have powerful jumping legs — hence the name. Touch an affected leaf and the whole company springs away.

The beetles overwinter in soil crevices, under leaf litter and in hedges. From about 15 °C they become active and seek out brassicas specifically, because they need their mustard oils. Females lay eggs in the soil; the larvae feed on roots but usually do little harm there. The problem is the beetles on the leaves.

How do I recognise flea beetle damage?

Shot holes: many small round holes of 1 to 2 mm, spread evenly across the leaf. In heavy attacks what remains is a sieve.
Jumping beetles: sweep a hand across the bed — the animals visibly spring up.
Timing: mainly April to June and again in late summer, each time during dry warm spells.
Easily confused: slugs leave irregular, larger damage from the leaf edge; cabbage white caterpillars eat large holes and leave green droppings.

How serious is the damage?

For established plants with plenty of leaf area, shot-hole damage is cosmetic. It becomes critical for seedlings and very young stands: if a radish seedling loses most of its leaf area in the first two weeks, it never grows away properly and the root stays small or fails to form at all.

With rocket and Asian salads the appearance suffers too — riddled leaves are still edible but do not look good in a salad bowl.

Prevention: moisture is your best weapon

Flea beetles love dryness. Keep the surface of the bed evenly moist and you make life so unpleasant that they usually move on by themselves.

Water regularly, especially through dry spring weeks. Better a short daily watering over the top than a rare soaking — the moist surface is the effect we want.
Mulch. A thin layer of grass clippings or fine compost keeps the soil moist and interferes with the beetles landing.
Insect netting. Fine mesh (0.8 mm) laid immediately after sowing and sealed at the edges. This is the only protection that reliably works completely.
Loosen the soil. Regular shallow hoeing disturbs egg laying and exposes larvae.
Shift your sowing date. Sowing radishes very early, or from September onwards, largely avoids the main flight periods.
Keep a rotation. Do not grow brassicas on the same bed year after year — including green manures such as mustard or oil radish.

What to do about an active attack

Hose over thoroughly, ideally twice a day. Many beetles then leave the bed.
Sticky board for collecting: hold a yellow board coated with glue or oil just above the stand and sweep across the plants with your other hand. The beetles jump up and stick. It sounds old-fashioned but noticeably reduces the population.
Add netting even once the attack is under way — hose down thoroughly first so you do not shut the beetles in.
Create distance: harvest or turn in badly affected, hopeless rows so the stand cannot restock itself from them.

Tip

A simple trick for radishes: lay fleece immediately after sowing and leave it until harvest. It keeps the beetles out, creates a moister microclimate and speeds up growth as well — three effects in one move.

Caution

Avoid spray products on leaf crops such as rocket. The growing time is so short that waiting periods are hard to observe — and netting plus water solves the problem better anyway.

Frequently asked questions

Are leaves with holes still edible?

Yes, without restriction. It is pure feeding damage, not a disease. Only the appearance suffers.

Why does it hit rocket so badly?

Rocket is a brassica with a high mustard oil content and is often broadcast densely in a dry spring — a perfect invitation for flea beetles.

Does intercropping with lettuce or spinach help?

It can, because the beetles find their host plants less easily between non-hosts. As a sole measure it is not enough under heavy pressure.

Do flea beetles disappear on their own?

After longer rainy spells and with increasing shade from larger plants the attack usually eases noticeably. The critical phase is the first two to three weeks after sowing.

Setting cabbage and kohlrabi as young plants rather than sowing them skips exactly that critical phase — by then the plant has enough leaf area to shrug off shot-hole damage.