What Is Eating My Garlic and Leaving Maggots in the Bulb?

If your garlic is yellowing, wilting and rotting, and you find small white maggots tunnelling into the bulb and roots, the culprit is most likely the onion fly (or, in some regions, the allium leaf miner). These pests target the whole allium family, and their larvae do the damage from inside. Identifying them lets you protect the crop, mainly by keeping the egg-laying adults away. Let me explain.

Onion fly

The onion fly is a small fly, similar to a housefly, whose adults lay eggs at the base of allium plants or in the nearby soil. The white legless maggots that hatch burrow into the roots and bulb, tunnelling and feeding, which causes the plants to yellow, wilt and collapse, and the bulbs to rot — often with the maggots visible inside. There are usually several generations through the season. Garlic is somewhat less attacked than onions, but it is still vulnerable, especially in beds where alliums are grown repeatedly.

Allium leaf miner

The allium leaf miner, an increasingly widespread pest, is another maggot culprit. The adult fly lays eggs on the leaves, and the larvae mine down through the foliage into the bulb and neck, leaving tunnels and lines of damage and rows of distinctive white dots where the adults fed on the leaves. The larvae then pupate as small brown capsules in the bulb and stem. This pest can cause serious losses and opens the way for rot. It has two active periods, typically spring and autumn.

The mesh barrier defence

Because both pests do their damage as larvae inside the plant, the key control is preventing the adults from laying eggs — and the most effective method is a physical barrier. Cover the garlic with fine insect mesh or fleece, sealed at the edges, during the periods when the adult flies are active (especially the leaf miner's spring and autumn flights). This physically denies them access to lay. Insect mesh is the single most reliable defence against these allium maggots, and many growers cover their alliums routinely where these pests are present.

Other measures

Alongside mesh: rotate alliums to fresh ground each year, since pupae overwinter in the soil where alliums grew; remove and destroy affected plants promptly (with the larvae or pupae inside) so they cannot complete their cycle; clear away allium debris; and avoid leaving culled alliums lying about, as their scent attracts the flies. Firming soil and avoiding disturbing the plants (which releases attractant scent) helps with onion fly. Combine a well-timed mesh cover, rotation, and prompt removal of infested plants, and you can keep these maggots from tunnelling your garlic.