White Blotch Mines Inside My Leek Leaves

Pale, whitish or yellow-white blotchy patches visible inside leek leaves — where the leaf tissue has been consumed but the outer leaf skin remains — are the feeding galleries of allium leaf miner larvae (Phytomyza gymnostoma). This is a relatively recently established pest in the UK, first confirmed here in 2002 and now widespread across England and Wales. It has become one of the most damaging pests on leeks in many gardens, particularly in two flight periods (spring and autumn) when adult flies are active.

How to confirm allium leaf miner

The adult fly is small (about 3 mm) and makes characteristic rows of white puncture marks along the leaf, used for feeding and egg-laying — these tiny white dots in a line along a leaf are often the first visible sign. Larvae mine inside the leaf tissue producing irregular white blotch mines. As larvae mature they move down through the leaf sheaths toward the centre of the plant, eventually pupating in the stem or leaf base. The orange-brown puparia (pupae cases) are sometimes visible inside split leaf sheaths at harvest. Affected plants may be noticeably distorted, with twisted or bent leaves.

The two flight periods

Allium leaf miner has two distinct generations per year in the UK. The spring flight occurs from approximately March to April; the autumn flight from approximately October to November. These are the periods when adult flies are actively laying eggs on leek and allium plants. Leeks growing during these windows without protection are vulnerable. Plants that have completed these windows are no longer at risk of new infestation — the larvae already inside will pupate in the plant tissue but no new eggs will be laid until the next flight period.

Physical exclusion

Fine mesh insect netting (1.3 mm mesh or less) placed over the bed before and throughout the two flight periods is the only reliably effective protection for organic growers. The netting must be sealed at the edges to prevent adult flies crawling underneath. If you have already experienced allium leaf miner, covering the crop from transplanting in spring through to the end of April and again from October through November is the practical approach. There is no approved insecticide for home garden use against this pest.

Affected crops — can you still eat them?

Yes — leeks with leaf miner damage are edible. Remove the outer, most damaged leaves and the inner shank is clean and usable. Check inside leaf sheaths when trimming to remove any puparia you find. Heavy infestations that have reached the central growing point may cause significant distortion, but in most cases the majority of the leek is unaffected. Remove and dispose of all plant material off-site at harvest to avoid providing overwintering sites for pupae.