PSI: The low-down on leafminers

Meet your foe

Leafminers are common pests of ornamental plants in greenhouses, nurseries and landscapes. The most common leafminers are the larvae of flies, in the family Agromyzidae. These include serpentine leafminers in the genus Liriomyza, columbine leafminers in the genus Phytomyza and the relatively new daylily leafminer, Ophiomyia kwansonis.

Adult Agromyzid leafminers are stout black or dark gray and yellow flies that resemble shore flies. Adults lay eggs on or inside leaves. The white larvae grow to about 2 millimeters long but you will never see them because they are hidden inside leaves. Larvae feed between the upper and lower leaf surfaces in the mesophyll. As they feed, a light-colored, gradually expanding mine forms on the leaf.

Mines are the primary damage caused by leafminers. Serpentine leafminers create pale circuitous mines that start small, when the larva is small, and get wider as the larva grows. Daylily leafminers create pale mines that are parallel with leaf veins. Stippling — tiny white flecks on leaves and flowers — can be caused by adults that puncture plant tissue to feed on sap and lay eggs. Mines can reduce plant growth and make plants unsalable.

Some leafminers like the columbine leafminer and daylily leafminer, feed on just one plant species. However, serpentine leafminers (Liriomyza spp.) are generalists that feed on many plant species. The most common greenhouse hosts of Liriomyza leafminers are gerbera and chrysanthemum. They can also feed on asters, calendula, carnation, dahlia, gypsophila, primrose and other plants.
 

Monitoring

Leafminers can exist in greenhouses year-round as long as a plant host is present. Scout for adult leafminers with yellow sticky cards placed just above the crop canopy. Use your hand lens to distinguish leafminers from other greenhouse flies. Inspect most crop leaves for pale serpentine veins, or on daylilies inspect for pale veins parallel with leaf veins.
 

Prevention

Preventing leafminers starts with sanitation. Leafminers can pupate inside leaves or they can exit leaves to pupate in soil or debris. Therefore, adult flies can emerge from discarded leaves on the greenhouse floor or in trash cans. They can also pupate in weeds and debris under benches. Some weeds can also be hosts in which larvae develop. Remove pruned leaves or infested plants and dump them far from the greenhouse. Thrips screens covering vents and other openings will help keep leafminers out, as well as thrips and other pests. Inspect new plants for adults and larval mines.
 

Treatment

Infested plants should be thrown out or pruned of infested leaves before adults emerge. Parasitoids are the primary biological control agents used for leafminers, and are generally used for the Liriomyza species. There are no commercial biological control agents available for other leafminers. The two primary parasitoids available are Diglyphus isaea and Dacnusa sibirica. These parasitoids lay eggs inside leafminer larvae. The parasitoid larva develops within the leafminer larva and kills it. The parasitoid emerges as an adult ready to parasitize more leafminers. These, like most biological control agents, should be released preventively, especially in highly susceptible crops like gerbera. There are insecticides available to target leafminer adults and larvae. Insecticide resistance has developed in some regions, so check with your local Extension personnel or consultant for advice.

Steven D. Frank is Associate Professor and Extension Specialist at North Carolina State University. He conducts extension and research related to greenhouse pest management.