Something Is Eating My Onion Seedlings and Sets

You plant a neat row of onion sets or freshly germinated seedlings, come back the next day or within a few days, and find them gone — pulled out, eaten, or simply vanished. This is a frustrating experience, especially when you are working with limited seed or expensive sets. Several different pests are responsible for disappearing onion seedlings and sets, and the clues left behind usually make identification straightforward. Here is what to look for and what to do.

Birds pulling out sets

Birds — particularly starlings and blackbirds — pull freshly planted onion sets out of the soil, apparently attracted by the smell of the onion and the ease of extracting a small object from freshly worked earth. You will often find sets scattered on the surface nearby rather than eaten. The solution is simple and reliable: push sets deeper (at least 2–3 cm of soil over the top) and cover the bed with netting or fleece immediately after planting, even if only for the first two weeks while the sets establish and their roots anchor them. Once rooted, birds lose interest and the covering can be removed.

Slugs

Slugs eat young onion seedlings — particularly seed-raised onions with their thin, fragile first leaves — at night, leaving the characteristic slime trail as evidence. They rarely eat onion sets (the sulphur compounds in mature onion tissue deter many slugs) but will target the young growth emerging from newly planted sets. Evening torch-light inspection around the base of plants will confirm slug activity. Control with ferric phosphate pellets (wildlife-safe and approved for organic use), copper tape around container edges, or by going out at night and physically removing slugs. Good soil preparation that avoids leaving large clods under which slugs shelter reduces their population near the crop.

Cutworms

Cutworms — the soil-dwelling larvae of several moth species — cut through seedling stems at or just below soil level, leaving the top of the plant lying on the surface, seemingly unharmed but severed at the base. If you find seedlings lying flat with a clean cut at soil level and no sign of surface activity at night, dig around the base of the affected plant and you may find a plump, greasy-looking caterpillar (typically 3–4 cm long, grey or brown) curled up just below the surface. Disturbing the soil around the base of plants exposes cutworms to birds. Encourage natural predators; there is no effective organic spray treatment once cutworms are established in the soil.

Onion fly

Young seedlings can also be killed by onion fly larvae (maggots) within a week or two of planting, before you realise there is a problem. Unlike the above pests, the seedling wilts and collapses in place rather than disappearing. Check the base — if there is soft rot and tiny maggots, onion fly is responsible. Cover the bed with fine mesh insect netting immediately to exclude further egg-laying.