Allium leafminer (ALM), is an invasive fly from Poland that was first detected in Lancaster County, 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 five- to seven-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 five- to seven-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 two 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 Pennsylvania, 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.
Upcoming webinars, events
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Herbs from Garden to Kitchen, Saturday, April 23, an informal workshop with the Master Gardeners’ herb experts, 9:30 to 11:30 a.m. in the Ag Heritage Center, 185 Franklin Farm Lane, lower-level meeting room 7/8. This class is currently full; you can visit https://extension.psu.edu/herbs-from-garden-to-kitchen for more information and to be added to the wait list if there are any cancellations. Participants will learn about different types of herbs, including when and how to harvest them from the garden for use in culinary dishes, how to include herbs in the landscape, and how to preserve them.
The Garden Hotline starts for the season Monday, April 25, and will continue through Sept. 30. Master Gardeners will be available at the Extension office, 181 Franklin Farm Lane, to assist you with your home garden questions on Mondays, Tuesdays, and Thursdays from 10 a.m. to 1 p.m., except on those holidays when the office is closed, Memorial Day, Independence Day, and Labor Day. Call 717-263-9226, e-mail email@example.com, or stop in the Extension office for help with your garden problems. You may bring in samples of plants and insects for identification, diagnosis, and recommendations.
Master Gardener Plant Sale, Saturday, May 21, from 9 a.m. to 1 p.m., located along the stream at the Extension office and Ag Heritage Center, 181-185 Franklin Farm Lane. The Master Gardeners are excited to host the plant sale after a two-year hiatus and to share their gardening expertise with plant sale customers. Plants available for very reasonable prices include greenhouse-grown annual flowers, herbs, tomatoes, peppers, and other vegetables, including heirloom and unusual varieties, as well as perennials, groundcovers, shrubs, and trees. New this year, “plug” plants, small perennials good for mass planting and drifts, will be available, including some great late-season bloomers for pollinators. Also new this year, we will be accepting Visa and MasterCard credit and debit cards, along with cash and check, for payment. Proceeds from the plant sale support Master Gardener educational outreach activities in Franklin County. Bring your gardening questions, and get information about plants and planting from friendly, knowledgeable Penn State Master Gardener volunteers.