Leafminers are the larvae or immature stages of insects that feed between the surfaces of leaves. Spinach and vegetable leafminer (Liriomyza sativae), and American serpentine or Chrysanthemum leafminer (Liriomyza trifolii) are commonly found in the vegetable garden.
- Eggs: Tiny oval (L. trifolii) or elliptical (L. sativae) white eggs inserted into the tissue just below the leaf surface.
- Larvae: Three tiny, active maggot-like instars that change from pale to greenish to yellow with black mouthparts in all stages (L. sativae), or golden brown turning darker brown (L. trifolii).
- Pupae: A tiny reddish/brown non-feeding puparium.
- Adults: Very small black flies with yellow markings and abdominal stripes, and transparent wings.
- Leafminers overwinter in the pupal stage in soil, and adults emerge in spring.
- There are multiple overlapping generations each year.
- Female flies puncture leaf undersides and insert eggs individually, producing many small wounds. Female flies are typically most active feeding and laying eggs near mid-day.
- Larvae are found in mines, tunneling through and feeding in leaves or soft stems (e.g., onion scape).
- Mature larvae make a slit in the upper leaf surface, emerge and drop to the soil, burrow in shallowly, and form brown pupal cases. Flies emerge in about 9 days.
- Spinach leafminer feeds mostly on spinach, beet, and Swiss chard.
- The vegetable leafminer has a wide host range, including bean, cantaloupe, celery, cucumber, eggplant, onion, pepper, potato, squash, tomato, watermelon.
- American Serpentine leafminer is also a significant pest of chrysanthemums and is common in greenhouses.
- Leaf-mining mars the leaves of leaf crops, and heavy infestations can reduce crop yields.
- Intensive insertion of eggs may produce slight leaf stippling damage.
- Look for pale to white leaf-mining trails, meandering randomly just below the leaf or stalk surface and readily visible.
- Pinch mined leaves to crush the larvae inside or remove and destroy them. This is the primary control.
- Many parasitoids attack leafminers, to the extent that leafminer outbreaks are associated with pesticide use, which suppresses parasitoid activity.
- Pesticides are ineffective. Leafminers are resistant to many insecticides and difficult to spray directly.
- Use floating row cover to prevent egg-laying.
- Till the soil in early spring to disrupt life cycle and kill overwintering pupae.
- Remove all host weed species around the garden, such as chickweed, lambsquarters, nightshade, and plantain.