Pest: Allium Leafminer (Phytomyza gymnostoma)
Pest/disease identification and lifecycle, most common damage symptoms and crops affected:
Allium, or onion, leafminer (ALM) is an invasive pest of alliums.
Adult allium leafminers are small (approximately 3 millimeters) gray/black flies with a distinctive orange/yellow spot on the tops of their heads, and yellow “knees” (Fig. 1). ALM have two generations in a year. Adults that overwintered in the soil from the previous year begin to emerge in late winter through early spring (March through May) and lay eggs into plant stems. That first generation of eggs then hatches as larvae which mine the leaves, moving downward towards the bulb or base of leaves. They then pupate in plant tissue or in the soil through the summer (Fig. 2), until they emerge as adults in the early autumn, and quickly begin to lay a second generation of eggs into the leaves of various alliums, whose larvae will once again mine the leaves of these plants. It’s this second generation of ALM which is often most damaging to crops. The second generation of pupae will then overwinter in the soil.
Fig. 1. Left: Puncture damage from ALM. Right: ALM adult with distinctive orange head. Courtesy of B. Lingbeek, Penn State Extension
Allium leafminer can infest any species in the genus Allium, including onion, garlic, shallot, green onion, and ornamental and wild alliums. Leeks are often the most damaged host, likely because the second generation has the greatest potential to damage crops and most other allium crops have been harvested by the time the larvae are hatching.
Fig. 2. ALM pupae under leaf sheaths near the base of the plant. Courtesy of Lawrence Barringer, Pennsylvania Department of Agriculture
The first sign of damage from ALM is typically the appearance of repeated punctures in leaf tissue (Fig. 1). Adult females make these punctures with their ovipositors to lay eggs within leaf sheaths, and both female and male adults feed on the plant exudates from these wounds. Once hatched, larvae mine leaves moving towards the plants’ bulbs and roots. Punctures and mines create opportunities for bacterial and fungal pathogens to enter plants. In heavily infested fields, up to 20-100 pupae can be found in individual plants, with 100% of plants infested.
To monitor for ALM, scout for telltale repeated punctures in allium leaves in September-October. Adults have also been found on yellow sticky cards.
To exclude ALM from plantings, cover allium plants in early spring, during the first adult emergence. Covering fall plantings to exclude the second generation of ALM may also be effective.
There are no known effective biocontrols at this time.
Organic pesticides (as a last resort):
Azadirachtin (e.g., Aza-Direct for farmers) and spinosad (e.g., Entrust for farmers or Captain Jack’s Deadbug Brew for gardeners) products can be effective. Follow label instructions for leafminers.
Written by Mariam B. Taleb.
Modified from University of Maryland Extension Resources, written by Gerald Brust September 2021, and “Major Pests of Onion,” University of Massachusetts Extension, May 2021.