CHAMBERSBURG, Pa. — Allium leafminer (ALM), is an invasive fly from Poland that was first detected in Lancaster Co., Pennsylvania in December 2015. This pest attacks plants in the Allium genus including onion, garlic, leek, scallions, shallots, and chives. It spends winter as a pupa in tissue at the base of plants in or adjacent soil, then emerges as adults in the spring over a 5 to 7-week period. After mating, females puncture leaves with their ovipositor to lay eggs and both males and females will feed on leaf sap. The oviposition marks are often the easiest method to scout for this pest, as they occur as a series of round wounds in a line, often found near the tip of leaves. The number of wounds can range but their placement in a line makes them fairly easy to recognize. Once hatched, larvae will mine their way down towards the base of the allium plant, feeding as they go and leaving a white streak of leaf mining damage. Once larvae completely develop, they will enter the pupal stage. This first generation will undergo summer aestivation and wait out the heat of summer as pupae, before emerging again in late September for a second 5 to 7-week flight period.
Knowing when adults have started flying helps with management. Row covers made of fine netting can provide protection as long as they are secured to prevent flies from entering. Inspect your netting for any holes prior to use and remove from plants once the flight period has concluded. Insecticides targeting adults and developing larvae can be an additional option if needed. Products with systemic activity tend to work best because the larvae are mining inside the leaf tissue, and this allows the insecticide to reach where they are feeding. Since Allium leaves are very waxy, a surfactant is recommended whenever applying insecticides to allium crops. Researchers at Penn State and Cornell Universities have found the most consistent control of ALM occurred using foliar applications of dinotefuran (Scorpion), cyantraniliprole (Exirel) and spinetoram (Radiant) for conventional production, and spinosad (Entrust) was the most effective OMRI-labeled option. Applications can begin as soon oviposition marks are detected or up to 2 weeks after first detection and still achieve effective control.
ALM typically begins to emerge a week or so after plants such as forsythia, daffodils, and ornamental pears have bloomed. In southeast PA, we have reached the time to start scouting your Allium crops and prepare for management. Adults are easiest to spot in the cool temperatures of early morning when they are not yet active for the day, by looking at the tops of the leaves. Additionally, inspecting leaves for oviposition wounds can be an additional, often easier, method of scouting. ALM will oviposit and feed on weedy alliums, such as wild garlic, so keeping an eye out on these plants along fence lines or forested borders of farms can help detect earliest emergences. For cultivated Alliums, scallions or green onions tend to be the hardest hit in the spring emergence, so these will be important to scout and manage ALM on this spring. Garlic may also be vulnerable at this time of year. During the fall generation, keep a close eye on leeks.
If you’re a commercial grower needing assistance scouting for ALM damage, feel free to reach out to your local horticulture extension educator.
Note: article written by Karly Regan, Commercial Horticulture Educator, Penn State Extension Franklin County; Tim Elkner, Commercial Horticulture Educator, Penn State Extension Lancaster County; and Shelby Fleischer, Professor Emeritus of Entomology, Penn State University
–Penn State Extension