Leafminers on Vegetables

Leafminer damage on pumpkin leaves

Updated: August 21, 2023

Leafminers are insects (flies) that feed between the surfaces of leaves during their juvenile (larval) stage of life. Spinach and beet leafminer (Pegomya hyoscyami and P. beta), vegetable leafminer (Liriomyza sativae), and American serpentine learfminer (Liriomyza trifolii) are commonly found in vegetable gardens. Allium (onion) leafminer (Phytomyza gymnostoma) has become more common in home gardens since it was first detected in Maryland in 2017. Leaf miners are considered minor pests, but heavy infestations of leafminers can reduce vegetable quality and yield.

Symptoms and signs of leafminers

Leafminers produce wavy lines, tunnels, trails, mines, or blotches just under the surface of leaves and soft stems. These markings usually appear white, gray, or paler in color compared to healthy leaf tissue.
Allium Leafminers produce white spots in a straight line on leaves of onions, garlic, leeks, and chives.
Worm-like larvae and frass (excrement) sometimes may be visible inside of the tunnels or blotches.

Leafminer damage on beets

Leafminer damage will reduce the quality of beet greens. Photo: University of Maryland Extension

Allium leafminer

Allium leafminer pupae inside of an onion. Photo: Lawrence Barringer, Pennsylvania Department of Agriculture, Bugwood.org

Allium Leafminer Symptom

Egg-laying by female allium leafminer flies produces rows of white spots on leaves. Photo: C. Carignan

Leafminer on basil

Trails left on a basil leaf as a result of leafminer feeding. Photo: Jon Traunfeld, UME

Typical behavior of leafminers

Female leafminer flies puncture leaves and insert eggs into them, producing small wounds in the plant. The flies typically feed and lay eggs around mid-day.
After the eggs hatch, the juvenile insects (larvae) are found in mines, tunneling through and feeding inside of leaves or soft stems
The larvae go through several stages (instars) while feeding then turn into pupae (the non-feeding, resting stage). Leafminer pupae may drop into the soil or can be present in plant parts (e.g., garlic bulbs).
Adult flies emerge from the pupae to begin a new cycle.

Common leafminers in a vegetable garden

allium leafminer damage and adult on foliage — Allium leafminer symptoms and adult leafminer. Photo: B. Lingbeek, Penn State Extension

Host Plants & Damage:
- All Allium species can be hosts. This includes garlic, leeks, onions, green onions, shallots, and chives (also wild and ornamental Alliums).
- Small round white dots (ovipositing/feeding scars) in a row appear on the middle to end of leaf blades.
- Larval feeding may cause leaves to be curled or wavy.
- Plants can become heavily infested with >20 larvae or pupae per plant.
- Both the leaf punctures and mines serve as entry routes for bacterial and fungal pathogens.
- Bulbs may be smaller than usual; portions of plants may become mushy and inedible.
Appearance:
- Adults: Small (~ 3 mm) long grey or black flies with a distinctive yellow or orange area on the top and front of head. Wings are held horizontally over the abdomen when at rest. Legs have distinctive yellow “knees.”
- Eggs: White, 0.5 mm long, and slightly curved
- Larvae: White, cream, or yellowish maggots, headless, up to 8 mm long at their final instar (larval growth stage)
- Pupa: Dark brown oval, 3.5 mm long

Allium leafminer feeding damage — ALM larva (red arrow) and feeding damage (black arrows). Photo: S. Spichiger, Penn State Extension

Life cycle
- Allium Leafminer (ALM) overwinters as pupae in plant tissue or surrounding soil.
- Adults (flies) emerge in late winter (mid-March) into spring (through early May), and mate.
- Female flies puncture leaf surfaces with their ovipositor to lay eggs in rows. Male and female flies feed on the plant sap at the puncture sites.
- Larvae hatch, mine leaves (feeding between upper and lower leaf surfaces), and move downward into the base of leaves or into bulbs where they pupate. Pupae may also move into the soil.
- The first generation pupae undergo a dormant phase that lasts throughout the summer and develop into adults that emerge in fall (September/October). The second generation of adults lays eggs on Allium species, larvae feed on available Allium plants and then develop into pupae that overwinter in plant tissue or soil.
- Spring and fall adult flight occurs over a 5-7 week period.

Host Plants & Damage:
- Wide host range includes bean, beet, carrot, celery, cucumber, eggplant, lettuce, melon, onion, pea, pepper, potato, squash, and tomato (also flowers such as chrysanthemum and marigold)
- Pale to white leaf-mining trails, meandering randomly just below the leaf or stalk surface and readily visible.
Appearance:
- Eggs: Tiny (1.0 mm long and 0.2 mm wide) oval white eggs inserted into the tissue just below the leaf surface
- Larvae: Worm or maggot-like, clear to yellowish; goes through three stages (instars) of feeding activity
- Pupae: Yellow to golden brown to dark brown, non-feeding pupal cases (puparium)
- Adults: Very small flies (less than 2 mm in length) with yellow markings and abdominal stripes and transparent wings
Life cycle: 21 to 28 days; multiple generations per growing season

Host Plants & Damage:
- Feeds mostly on spinach, beets, and Swiss chard (and weeds in the same family, e.g. lambsquarters).
- Causes irregularly-shaped botch mines in leaves
Appearance:
- Eggs: White, oblong, on undersides of leaves
- Larvae: Whitish, cone-shaped
- Pupae: Reddish-brown, overwinter in soil
- Adults: Small flies, gray-brown, about 8 mm in length
Lifecycle: 30-40 days; 3-4 generations per season

Host Plants & Damage:
- Wide host range includes beans, eggplant, potato, peper tomato, squash – and weeds (black nightshade)
Appearance:
- Eggs: White, elliptical
- Larvae: Worm-like instars that change from pale to greenish to yellow with black mouthparts; about 2 mm in length; three stages (instars)
- Pupae: Reddish-brown, 1.5 mm in length
- Adults: Yellow and black, about 2 mm in length
Life cycle: Egg to adult in 15 days. Many overlapping generations per year

Prevention & monitoring

Remove weeds around the garden; some weeds serve as alternate hosts for leafminers.
For severe, yearly infestations, consider tilling or cultivating the top 2-in. of soil in early spring to disrupt the life cycle and kill overwintering pupae.
Use row cover to prevent egg-laying. Row covers should be placed on spring garlic and other Alliums by late February and on other vegetable crops before egg-laying begins in April.
On uncovered plants, look for pale to white leaf-mining trails meandering randomly just below the leaf or stem surface. On Alliums (garlic, leeks, onions), look for rows of small white dots on the leaves.

Leafminer eggs on a beet leaf. Scout for eggs and handpick them to reduce populations

Management

For Allium Leafminers

Apply row cover or netting over Allium plants in February, prior to the emergence of adults; keep plants covered during spring emergence. When covering a fall-planted crop, like garlic, in early spring, make sure there are no ALM in the crop, otherwise, you’ll trap the flies under the row cover.
Cover fall plantings during the 2nd generation flight (September).
Consider delaying spring planting until mid-May to avoid the 1st generation egg-laying period and planting garlic in late October to avoid ALM if it’s active in your area.
Remove and discard all Allium plant debris after the growing season to eliminate overwintering sites.
Organic insecticides: Neem oil or spinosad. The latter is absorbed into leaf tissue so it will control larvae after they hatch and start to feed. Apply spinosad after white dots (oviposition marks) appear. Make a second application two weeks later. Since Allium leaves are waxy, it is recommended to add a surfactant (e.g. insecticidal soap) to help insecticides stick to leaves.

For Other Vegetable Leafminers

Pinch mined leaves to crush the larvae inside or remove and destroy them. This is the primary control.
Many beneficial insects (parasitoids) attack leafminers and kill them. Avoid pesticide use, which suppresses parasitoid activity. Leafminers are resistant to many insecticides and difficult to spray directly.
Use row cover over plants to prevent egg-laying.
Remove all host weed species around the garden, such as chickweed, lambsquarters, nightshade, and plantain.
For severe, yearly infestations, consider tilling or cultivating the top 2-in. of soil in early spring to disrupt the life cycle and kill overwintering pupae.

Allium Leafminer

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