Allium leafminer
Phytomyza gymnostoma
A recent arrival spreading through the northeast: rows of white dots down an onion leaf, and pupae in the stem that let soft rot in behind them.
How to know it
The first sign is a neat vertical line of small round white dots near the top of a leaf, where the female has punctured it to feed. Beneath the surface, a white maggot mines downward toward the bulb. The pupae are the confirmation: dark red-brown, rice-grain shaped, embedded in the leaf bases and the neck of the bulb.
What you'll see
- A tidy row of white dots running down the upper part of a leek, onion or chive leaf.
- Leaves wavy, curled and distorted.
- Winding mines visible in the leaf when it is held to the light.
- Dark red-brown pupae like grains of rice in the leaf sheaths and the neck of the bulb.
- Soft, foul rot spreading into the bulb or the leek stem behind the mining.
Where it comes from
Two generations a year, with a long summer gap. The first flight is in spring, the second from late summer into autumn, and the pupae oversummer and overwinter in the plants. The autumn generation is the damaging one for leeks and overwintering onions, and the rot that follows the mining usually does more harm than the maggot itself.
First found in Pennsylvania in 2015 and spreading through the northeast and mid-Atlantic. Overwintering leeks, spring onions and chives left in the ground carry it from one year to the next.
Dealing with it
- Cover the crop with fine insect mesh during both flight periods, sealed at the ground — the only reliable control.
- Remove and bin plants with pupae in them; do not compost them.
- Strip the affected outer sheaths off a leek at harvest; the heart is often perfectly good.
- Harvest and clear affected alliums promptly rather than leaving them standing.
- Watch for the second flight in September, which is the one that ruins leeks.
Keeping it out
- Rotate alliums to a bed well away from last year and mesh them from planting.
- Do not leave chives, wild garlic or overwintering onions standing next to the crop; they are the reservoir.
- Clear all allium debris at the end of the season.
- Time plantings to miss the flights where you can, using local extension flight reports.
- Inspect and reject any transplants showing the row of white dots.
What it goes for
Leeks, onions, garlic, chives, shallots, spring onions and ornamental alliums.
Trouble it brings with it
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com