Why Are There Tiny Holes All Over My Brassica Seedling Leaves?

Young broccoli and cauliflower seedlings that appear to be peppered with small round holes — so-called shot-hole damage — have been attacked by flea beetles. These tiny jumping beetles (1.5–3mm long) are most active in warm, dry spring weather and can shred seedling leaves so quickly that the plant struggles to survive. Established plants are much less vulnerable, but young seedlings in their first few weeks can be severely set back.

Identifying flea beetles

The beetles themselves are small, dark and shiny — usually black or blue-black, sometimes with yellow or white stripes depending on species. If you tap the seedling, the beetles jump away visibly. They feed during the day in warm, sunny conditions and are least active in cool, cloudy weather. Their larvae live in the soil and feed on roots, but it is the adult feeding on leaves that causes the visible shot-hole damage.

How much damage matters

A seedling with a few scattered holes in its leaves will recover once it grows past the young, tender stage. A seedling with more than about 30% of its leaf area destroyed may struggle to photosynthesise adequately and grow slowly or die. Very small seedlings at the cotyledon (seed-leaf) stage are most vulnerable; once a plant has four to six true leaves and a developed root system, flea beetle damage becomes cosmetic rather than critical.

Protecting seedlings

Fine insect-proof mesh laid over transplants immediately after planting provides complete protection. The mesh must be fine enough — under 0.8mm — to exclude flea beetles, which are smaller than most other pests. Remove it once plants have four or more true leaves. Mulching around seedlings and keeping soil moist also discourages flea beetles, which prefer warm dry conditions. Transplanting in the evening reduces stress and gives seedlings time to establish before warm daytime beetle activity begins.

Organic sprays

Kaolin clay suspended in water and sprayed over seedlings creates a physical barrier that irritates flea beetles and discourages feeding. Pyrethrin spray kills beetles on contact but requires repeated application. Neem oil has some repellent effect. None of these are as reliable as physical netting but are useful in combination or when netting is not available.