Leafminers

Adult Liriomyza spp. are about 1/12 inch long and mostly black and yellow flies with clear wings. Phytomyza spp. adults are mostly blackish or gray on top with a white to yellowish abdomen. Females of both genera insert tiny eggs in leaves of numerous crop and weed hosts where the larvae feed within leaf tissue. Adult females also puncture leaf surfaces to feed on the oozing plant juices.

There are many types of leafmining insects in addition to Liriomyza and Phytomyza spp. flies (Agromyzidae). Leafminer pests of azalea and rhododendron include Caloptilia azaleella, a brown, gray, and silvery moth (Gracillariidae) with larvae up to 1/2 inch long. Chrysanthemum leafminers include Chromatomyia syngenesiae, Liriomyza spp., and Phytomyza atricornis, all agromyzid flies. Columbine leafminers include Phytomyza spp.; adults are commonly grayish. Manzanita and madrone are hosts to Marmara arbutiella (Gracillariidae), small brown and whitish moths with yellowish larvae with distinct, triangular-shaped segments. Orchids and cymbidium are attacked by larvae of a moth, Pyroderces badia (Cosmopterigidae). If it is uncertain what species of leafminer are the problem several can be collected, such as with an aspirator, and taken to the local office of the county department of agriculture or University of California Cooperative Extension for identification.

Larvae of Liriomyza and Phytomyza spp. are orangish or yellow and develop through three increasingly larger instars. Larvae widen and lengthen their mine as they grow up to about 1/10 inch long. Liriomyza trifolii mines are readily observed from the top of the leaf, while Liriomyza huidobrensis mines may only be apparent from the underside of the leaf. Liriomyza huidobrensis mines tend to follow the veins of the leaf.

At maturity, leafminer larvae commonly exit mines and pupate in growing media or topsoil. Liriomyza pupae are orange or yellow and those of Phytomyza are gray. Uncommonly pupae are observed on leaves. Liriomyza leafminers have multiple generations per year. For example, Liriomyza trifolii can complete 1 generation in about 1 month at common greenhouse temperatures, or in 14 days or 64 days at 95° and 59°F, respectively.

Adults' feeding punctures in leaves turn white, resulting in numerous round pale blotches in foliage. The most obvious damage is from the larval mines that make foliage unsightly and when abundant render plants unmarketable. Although high populations of leafminers rarely if ever kill hosts, mining may slow crop growth and cause infested leaves to drop prematurely.

Biological and cultural controls and insecticide applications are used to manage leafminers.

Parasitic wasps generally keep leafminer populations low in field-grown plants unless biological control is disrupted. Outbreaks of leafminers commonly occur when broad-spectrum, persistent, insecticides applied for other pests kill leafminers' natural enemies. To conserve natural enemies, control ants, reduce dustiness, and where feasible rely on cultural controls and selective and semi-selective insecticides for all pests as listed in Relative Toxicities of Pesticides Used in Floriculture and Nurseries to Natural Enemies and Honey Bees.

The Liriomyza larval parasites Dacnusa sibirica and Diglyphus spp. are commercially available and effective for controlling leafminers in greenhouses and shade structures that are adequately screened to contain the parasites and exclude in-migrating adult leafminers and other pests. For more information, see Biological Control, Protecting Natural Enemies and Pollinators, and Natural Enemy Releases for Biological Control of Crop Pests.

Carefully inspect plants to ensure they are pest free before moving them to production areas; discard infested plants or leaves or effectively treat plants. Steam treat planting beds or otherwise pasteurize growing media to the recommended pasteurization temperature immediately after removing any infested crop to kill any leafminer pupae in the soil before replanting beds. Steam is difficult to use in field soils but can be applied using a plowlike steam rake to raise the topsoil temperature to levels sufficient to kill most pests.

Keep growing areas and sites bordering crops free of weeds, which can host leafminers; Liriomyza and Phytomyza spp. leafminers feed on a wide variety of plant species and readily move among hosts. Promptly dispose of plant debris in covered containers. Exclude migrating adults by properly screening doors, vents, and other openings to greenhouses and screenhouses. Effective screens have an opening width of 600 microns (0.6 mm, or about 1/40 inch) or smaller and sufficient surface area to provide adequate ventilation.

Biological and cultural controls are organically acceptable management methods. Certain formulations of the botanical azadirachtin (Azatin) and the fermentation product spinosad (Entrust Naturalyte, Entrust SC) are organically acceptable.

Deploy yellow sticky traps in greenhouses to attract and capture adults. Place at least 1 trap per 10,000 sq. ft. of growing area. Monitor traps weekly; count and record the results. Change traps when they are fouled or have trapped too many insects to readily count. For side-by-side comparisons to help identify the captured insects, see Sticky Trap Monitoring of Insect Pests. For more information, see Monitoring with Sticky Traps and Establishing Action Thresholds.

Insecticide resistance is widespread in populations of Liriomyza; some products listed below may not be effective depending on the exposure history and pesticide tolerance of local leafminer populations. Rotate between effective products with a different mode of action every 1 to 2 months if ongoing applications are targeting leafminers. See Managing Pesticide Resistance for more information.

Selected Products Registered for Greenhouse or Nursery Ornamentals