White Maggots Are Tunnelling into My Onion Bulbs

Small white maggots — typically 5–8 mm long — found tunnelling through the base of onion plants, inside the stems near the base, or within the bulb itself, are almost certainly the larvae of the onion fly (Delia antiqua). Onion fly is a significant pest of onions, garlic, leeks, and shallots across northern Europe. The damage it causes goes beyond just the maggots' direct feeding — the tunnels create entry points for bacterial soft rot, which can then destroy much of the bulb even after the maggots have moved on.

The onion fly life cycle

The adult onion fly resembles a small grey housefly and is active from April to August, with two to three generations per year in the UK. The first generation is most damaging: adults lay small white eggs at soil level near the base of onion plants, or directly on the set, in late April and May. The eggs hatch within a week and the larvae (maggots) burrow into the bulb base or stem, feeding for two to three weeks before pupating in the soil. The damage from the feeding is compounded by the bacterial rots that colonise the tunnels. Plants with maggot damage often wilt and collapse at soil level; pulling them reveals a rotten, maggot-filled base.

Physical exclusion — the best defence

Fine mesh insect netting (with a mesh size of 1.3 mm or less) placed over the bed immediately after planting and kept in place until late July prevents the adult fly from reaching the plants to lay eggs. The netting must be well sealed at the edges — flies will find any gap. This is the most reliable organic control and, in a year with high fly pressure, the difference between a full crop and a total loss. Remove the netting for weeding and replace it immediately. Fleece gives slightly less protection but also works if netting is unavailable.

Finding and treating active infestations

If you find collapsing plants with maggot damage in early summer, remove and destroy the affected plants immediately — do not compost them. Check neighbouring plants by gently clearing soil from around the base; remove any additional maggots you find. There is no approved insecticide for home garden use against onion fly larvae in the UK. Once maggots are in the bulb, removing the plants is the only option. Healthy plants nearby may still be protected if you cover them with netting immediately to exclude the next generation of adults.

Prevention next season

Rotate alliums to a fresh bed each year to reduce the pupae overwintering population in the soil of frequently used beds. Cover the crop with insect mesh from planting. Avoid damaging sets during planting — the scent of damaged onion tissue attracts egg-laying females from a distance. Some growers interplant onions with carrots, as the scent of each plant masks the other (though evidence for this is mixed). Clearing and disturbing the soil thoroughly after harvest exposes pupae to birds and frost.