Why Are My Cabbage Seedlings Covered in Tiny Holes?

Cabbage seedlings with multiple tiny, perfectly round or slightly irregular holes — as if the leaves have been peppered with a pin — and no visible caterpillar, have been attacked by flea beetles (Phyllotreta species). These are small (2–3 mm), shiny black or blue-black beetles that jump vigorously when disturbed — the jumping is the key identification feature that gives them their name. Flea beetles attack brassica seedlings (and rocket, radishes, and other brassica family members) most aggressively in dry, warm conditions from April to June when seedlings are small and vulnerable.

When flea beetles are most damaging

The critical vulnerability window is from germination until the plant has four to five true leaves with individual leaves at least the size of a 50p coin. At this stage, the seedling has a small total leaf area and flea beetle damage represents a large proportion of that area, sometimes causing seedlings to be so perforated they cannot photosynthesize effectively and they die. Once past this stage, the same level of flea beetle activity produces the same number of holes, but on a plant with twenty times the leaf area — the impact becomes cosmetic rather than lethal.

Why dry weather makes it worse

Flea beetles are most active in dry, warm, sunny conditions and least active in cool, wet weather. In a dry April and May, populations explode and damage is severe. In a cool, wet spring the same beetles are much less active and seedlings often escape with minimal damage. This is why timing seedling emergence with predictably cooler conditions (raising under cover and transplanting as larger, established plants) avoids the worst of the problem.

Control

Cover seedbeds and young transplants with fine mesh netting (as used for caterpillars) immediately after sowing or transplanting — this is the most effective control. For direct-sown crops, cover the row immediately after sowing. Yellow sticky traps positioned at plant level catch significant numbers of the beetles and give a useful indication of population levels. Adequate watering keeps seedlings growing quickly — fast-growing seedlings pass through the vulnerable window faster and are less stressed by the feeding damage they do sustain. Kaolin clay powder applied as a dust confuses the beetles, though this is more commonly used on rocket and mustard crops than cabbage.