Vegetable Gardeners: Watch Out for Leafminer Pests!
Something new in your vegetable garden? Allium leafminers (Phytomyza gymnostoma) invaded counties in the NC Piedmont in 2023 and we are now on the lookout for these European pests in WNC! Chives, garlic, leeks, all types of onions (including ornamental flowering onions), shallots, and ramps are potential targets.
What are leafminers?
You’ve likely seen leafminers on plants in your landscape. They infest not only vegetables, but many flowering plants, trees, shrubs, and even weeds! These critters are the larvae of a variety of insects: beetles, flies, moths, and sawflies. The adult insects deposit eggs into leaves, the larvae hatch between the leaf layers, feed until they form pupae, the next stage of their development. The damage from their feeding shows up as trails, usually a lighter color than the leaves. There may be several generations of leafminers between spring and fall with pupae overwintering in the soil.
Vegetable leafminers
Leafminers affect many popular vegetable crops besides Alliums, including beans, beets, cabbages, cucumbers, melons, peas, peppers, radishes, spinach, squashes, and tomatoes.
The good news: Many leafminers do only cosmetic damage to plant leaves. If you avoid using insecticides, beneficial parasitic wasps can keep leafminers under control. Removing affected leaves can disrupt leafminer lifecycles.
The bad news: If leaf damage is extensive, the affected areas may provide refuge for bacteria and fungi. These invaders may then create larger brown spots and “blisters” that can ruin leaf crops, such as beet greens, spinach, and Swiss chard.
Allium leafminers
Allium leafminers are of special concern because their larvae and pupae often damage the plant bulbs as well as leaves, ruining much of the crop. How do you know if you have Allium leafminers? You may be able to identify the adult flies, but the more obvious signs are the distinctive damage to leaves and bulbs.
[Photos in the gallery above were taken by Debbie Roos, NC Cooperative Extension Agent, Agriculture-Sustainable/Organic Production, Chatham County.]
When are Allium leafminers active?
Areas of New York where the Allium leafminers are endemic have similar hardiness zones to much of WNC, so we might expect one generation of these pests in spring and another in fall.
Prevention:
- Do not buy Alliums from areas infested with leafminers.
- Planting “sets” of onions and scallions between the spring and fall generations has helped growers avoid crop damage.
- Covering your Allium crops with row covers can preclude infestation by preventing the adult flies from inserting their eggs into the leaves.
- Recent research suggests that planting chives as a trap crop may reduce damage to other Alliums.
If Allium leafminers infest your crops:
- Destroy any infested plants. Dispose of them away from your compost area and garden bed.
- Solarize soil where infested plants were grown.
- Plant Alliums in different garden areas in following years as the adult flies do not travel long distances.
Here’s hoping we can learn from the experience of those who’ve studied this new invader, and we’ll continue to enjoy our Allium crops with little or no damage from leafminers!
Article by Deborah Green, Extension Master GardenerSM Volunteer
For more information:
Allium leafminers:
NC Extension – Growing Small Farms – Allium Leafminers
University of MD Extension – Allium (Onion) Leafminer
Frontiers in Insect Science – Allium leafminer (Diptera: Agromyzidae) host preference: implications for developing a trap cropping strategy
Vegetable leafminers:
NC Extension – Insect and Related Pests of Vegetables
University of MD Extension – Leafminers on Vegetables
University of California – IPM: Vegetable Leafminers
General information on leafminers:
Maine Entomological Society: Backyard Leafminers (Webinar)
Soil solarization:
NC Extension – Hoke County: The Perfect Weather for Solarizing Soil