Holes in Your Seedlings: Slugs, Cutworms or Flea Beetles?
Seedlings are at their most vulnerable in the first two or three weeks after they come up or go out. A plant with four leaves can lose all of them in one night, and a whole row of transplants can be felled before breakfast. The good news is that the damage usually tells you exactly who did it, and the fix for each culprit is cheap and needs no poison.
The Garden Pest Identifier covers the three pests behind most seedling losses: slugs and snails, cutworms and flea beetles. This guide reads the damage for you and gives a protection plan for each, plus the look-alikes that are not pests at all.
Read the damage
| What you find | Most likely | Confirm by |
|---|---|---|
| Irregular, ragged holes in leaves; some seedlings eaten to a stub; shiny trails | Slugs or snails | Silvery slime trails; check by flashlight an hour after dark, especially after rain |
| Seedling cut cleanly through at or just below the soil line, top lying beside it, often barely eaten | Cutworms | Dig gently around the stem: a gray or brown caterpillar curled into a C shape just under the surface |
| Many tiny round holes, like shot from a shotgun, on otherwise intact leaves | Flea beetles | Tiny black beetles that jump away like fleas when you touch the leaf |
The identifier lists their usual targets as lettuce, hostas and seedlings for slugs and snails; seedlings, tomatoes and brassicas for cutworms; and eggplant, radish and brassicas for flea beetles.
Not every collapse is a pest
Before blaming a pest, rule out the common look-alikes:
- Damping off. Young seedlings that pinch and thin at the soil line, then topple and wither, usually have a fungal disease called damping off, not cutworms. It is worst in cold, wet, crowded conditions. There is no caterpillar and the stem is shriveled, not cut.
- Birds. Pulled-up seedlings, especially peas and beans, or leaves torn rather than holed, often point to birds. Netting or a row cover fixes it.
- Rabbits and other mammals. Clean, angled cuts high on the stem and whole plants grazed down suggest a larger animal; a fence is the answer, not a pest treatment.
- Cold or wind damage. Transplants put out without hardening off can bleach, scorch or collapse. Check the forecast against your planting date in the Planting Calendar and harden plants off for a week first.
Slugs and snails
The identifier's options are hand-picking at night, beer traps, diatomaceous earth and copper tape. In practice:
- Night patrols work best. Go out with a flashlight an hour or two after dark, especially after rain, for a week or two while seedlings are small. Collect into a lidded container and dispose of them away from the garden.
- Beer traps sunk into the soil catch slugs overnight. Leave the rim a little above soil level so ground beetles, which eat slugs, do not fall in. Empty and refill every couple of days.
- Remove hiding places near the seedbed: boards, dense weeds and thick mulch right up against young stems. Or use a board deliberately as a trap and check underneath each morning.
- Copper tape around the rim of raised beds and pots is a barrier slugs dislike crossing.
- Diatomaceous earth stops working once it gets wet, which is when slugs are most active, so it is a weak option outdoors. If you use it, follow its label and avoid breathing the dust.
- Raise transplants bigger. A sturdy transplant survives grazing that would kill a newly emerged seedling.
Do not pour salt on slugs in a bed; it damages the soil. If you choose slug pellets, pick a product whose label lists your crops and follow it exactly, scattered thinly, never in heaps, and keep pets and children away from treated areas. Iron phosphate pellets are widely sold for this; metaldehyde pellets are banned for outdoor use in Great Britain and are a known poisoning risk to dogs, so avoid them.
Cutworms
The identifier lists collars around stems, hand-picking at night, Bt and tilling the soil. Collars are the most reliable, because cutworms attack the stem at soil level:
- Make a collar from a cardboard tube, a strip of card stapled into a ring, or a plastic cup with the bottom cut out. It should circle the stem with a little room, sit about an inch into the soil and stand an inch or two above it.
- Search at the scene. When you find a felled seedling, dig gently in the top inch of soil within a few inches of the stem. The cutworm is usually there, curled up. Remove it, or it will take the next plant tonight.
- Prepare the bed early. Clearing weeds and loosening the soil a couple of weeks before planting exposes larvae to birds and removes the plants that host them.
- Night checks with a flashlight also find cutworms feeding at the soil surface.
Bt (Bacillus thuringiensis) is an organic pesticide that kills caterpillars. It only works when eaten, so it is less useful against soil-dwelling cutworms than collars, and it kills caterpillars generally, including those of butterflies. If you use it, choose a product labeled for your crop and pest and follow the label.
Flea beetles
The identifier suggests row covers, diatomaceous earth, sticky traps and trap crops. Flea beetle damage looks dramatic, but it matters mostly while plants are small:
- Row covers from day one. Lay floating row cover over the seedbed or transplants immediately, with the edges buried or weighted so beetles cannot get under. For crops that need pollinating, such as eggplant, remove the cover when flowers open.
- Help plants grow fast. A seedling growing strongly outpaces the damage; once plants have several true leaves, a scattering of shot-holes rarely affects the harvest. Steady water helps.
- Trap crops. A few radishes sown around the edge can draw flea beetles away from more valuable brassicas.
- Yellow sticky traps catch some adults and show you when they are active.
The label rules for any product
Insecticidal soap, neem oil, Bt, diatomaceous earth and slug pellets are all pesticides, organic or not, and the identifier shows this note beside every pest. Use only a product whose label lists your plant and pest; follow the label for rate, protective gear, how long to keep people and pets away and how long to wait before harvest; never mix products or make your own from household chemicals; keep products away from children, pets and drains; and do not apply anything to open flowers where bees are working.
What the identifier cannot do
- It does not examine your seedling. You pick a pest from ten; identification from a description is a best guess.
- It does not cover disease (damping off), birds or mammals.
- It gives no product rates; those come only from the label.
If you lose plants and cannot find a culprit, ask before you treat. In the US your local cooperative extension service can identify samples and photos; in the UK the RHS offers advice to members; many garden centers run plant clinics.
A seedling protection routine
- Clear weeds and debris from the bed a couple of weeks before sowing or planting.
- Collar every transplant of tomatoes, peppers and brassicas.
- Cover brassicas, radish and eggplant with row cover at planting.
- Check at dusk for the first two weeks, especially after rain.
- Sow a few spares; the Plant Spacing Calculator tells you how many fit, so a few extra in a corner costs nothing.
For the sap-sucking pests that show up later in the season, see aphids, whiteflies or spider mites.