Why are there holes in my plant's leaves, and what made them?

Searching for the creature is the slow way round, because most of them feed at night or drop off the moment the leaf moves. The damage stays put, and it is specific. A slug and a caterpillar leave visibly different holes, and telling them apart in ten seconds means you spend your evening doing the right thing.

Step by step

1. Start at the leaf edge and work in.Large irregular bites taken from the margin, working inwards, with a silvery dried slime trail somewhere nearby on the leaf, the pot or the soil, are slugs and snails. It is the commonest single answer and it happens after dark.
2. Look for neat notches in a row along the margin.Small rounded scallops repeated along the edges of several leaves, especially on evergreens, are adult vine weevil. A single perfectly smooth semicircle cut out cleanly, as if punched, is a leafcutter bee, which is harmless and protected.
3. Look for small round holes in the middle of the blade.A pepper-shot of tiny round holes through young brassica, rocket and radish leaves in dry spring weather is flea beetle. It is worst on seedlings and largely cosmetic on an established plant.
4. Look for grazing that has not gone through.A patch where the soft green tissue has been eaten away leaving one transparent leaf surface and the veins behind — a window pane that later tears into a ragged hole — is sawfly larvae or very young caterpillars, which cannot yet bite the full thickness. Look on the underside; they are usually still there in a row.
5. Look for large ragged holes with pellets.Irregular holes in the middle of the blade with dark green or black grains of frass caught in the leaf joints and on the leaves below are caterpillars. The frass is the giveaway, and it tells you the caterpillar is still on the plant and above the pellets.
6. Look for a pale winding tunnel.Not a hole at all, but a mine — the larva is between the two leaf surfaces, both of which are intact. Hold the leaf up to the light.
7. Check the timing.Cover the plant or check it at dusk and again at dawn. Damage that appears overnight is slugs, snails, earwigs or vine weevil adults. Damage that appears during the day is caterpillars, sawfly larvae and beetles. This one observation halves the list.
8. Go out after dark with a torch.About an hour after dusk, on a damp evening. This is the only way to see slugs, snails, earwigs and vine weevil adults actually feeding, and it converts guessing into knowing. Check the undersides of leaves and the rim and base of pots.
9. Match the response to the culprit.Slugs and snails: barriers, evening patrols, nematodes, and protecting the vulnerable plants specifically rather than the whole garden. Caterpillars and sawfly: pick them off, which is genuinely effective on a garden scale. Flea beetle: fine mesh and keeping seedlings watered. Vine weevil adults: the notches do not matter, but the grubs in the compost below do.
10. Decide whether it needs anything at all.Holes in the leaves of an established, vigorous plant in midsummer are cosmetic. A plant loses a good proportion of its leaf area every year to something. Intervene where seedlings are being destroyed, where the crop is the leaf, or where the plant is small enough that the loss matters.

Tips

• Photograph the damage and look again in three days. If no new holes have appeared, whatever it was has gone — very common with a single caterpillar or a passing slug.
• Damage concentrated on one plant among many usually means a specialist feeder; damage spread across everything means slugs and snails.
• Lay a plank or an upturned half grapefruit near the plant and look under it in the morning. Slugs shelter there and it makes the nightly patrol quick.
• On brassicas, check the underside for neat yellow egg clusters before the caterpillars hatch. Rubbing the eggs off takes seconds and saves the plant.

Common mistakes

• Spraying an insecticide at unidentified holes — Slugs and snails are not insects and nothing you spray on the leaf will touch them, so the commonest cause of holes is the one a general insecticide cannot address at all.
• Assuming the hole means the pest is still there — Much of the damage you find was done days or weeks ago. Check for fresh damage before treating anything.
• Treating flea beetle shot-holing on mature plants — It looks dramatic and costs an established plant almost nothing. The intervention that matters is protecting the seedling stage with mesh.

If it doesn't work

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