Why Do My Brussels Sprout Seedlings Have Tiny Holes?

A seedling that was perfectly healthy yesterday and today looks like it has been peppered with a fine shotgun — dozens of tiny round holes scattered across the leaves — has almost certainly been visited by flea beetles. This is one of the most common early-season problems on Brussels sprouts and all other brassicas. Understanding the beetle's behaviour helps you protect plants at their most vulnerable stage.

What Are Flea Beetles?

Flea beetles are tiny beetles — typically 2–3 mm long — that jump when disturbed, which is how they get their name. Several species attack brassicas; the most common in UK and European gardens are the brassica flea beetle (Phyllotreta nemorum), which is black with yellowish stripes, and the cabbage stem flea beetle (Psylliodes chrysocephala), which is bronze-green. Adults emerge in spring and immediately seek out young brassica leaves on which to feed, chewing countless small holes through the upper surface of the leaf.

How Serious Is the Damage?

A few holes on a large, established plant cause no lasting harm — the plant grows away from the damage quickly. The real risk is to seedlings in the first two to four weeks after germination or transplanting. At this stage the total leaf area is so small that heavy feeding can halt growth entirely, cause the seedling to wilt and, in a severe attack during hot dry weather, kill the plant. Large transplants tolerate flea beetle much better than freshly germinated seeds sown in situ.

Why Dry Conditions Make It Worse

Flea beetles are most active and most damaging when the weather is warm and dry. Dry soil stresses seedlings, making them slower to grow through the vulnerable stage. Beetles themselves are more mobile and appetite-driven in warm conditions. Watering seedlings consistently keeps them growing strongly so they pass through the high-risk period as quickly as possible. A seedling that grows vigorously produces new leaves faster than beetles can damage them.

Fleece and Fine Mesh Protection

Covering seed beds and transplants with horticultural fleece or fine insect-proof mesh immediately after sowing or planting is the most effective physical barrier. Flea beetles cannot fly or jump through the material and adults already present in the soil can be trapped out by the time of emergence. Keep the cover on for the first four to six weeks, or until plants have developed at least four true leaves and are growing vigorously. Ensure the edges are sealed to the ground.

Neem Oil and Other Organic Sprays

Neem oil diluted in water with a few drops of dish soap and sprayed over seedlings acts as a deterrent and mild insecticide. Apply in the evening when bees are not foraging. Diatomaceous earth dusted around and over seedlings creates a physical barrier that damages the beetles' exoskeletons and reduces feeding. Reapply after rain. These measures are most useful as supplements to physical barriers rather than as standalone solutions.

Timing Sowing to Avoid Peak Risk

Brussels sprouts are typically sown indoors in early to mid spring and transplanted outdoors from late spring onward. Growing on indoors until plants are sturdy — at least 10–15 cm tall with several true leaves — means they are transplanted at a size that tolerates flea beetle feeding without lasting harm. Starting plants under cover also allows early sowing without exposure to the first flush of beetle activity outdoors.