How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Adults are small black to gray flies with yellow markings. Females puncture leaves to feed on plant sap and lay eggs within the leaf tissues. After 2 to 4 days eggs hatch and larvae feed between the upper and lower surface of the leaves, making distinctive winding, whitish tunnels or mines that are often the first clue that leafminers are present. Larvae emerge from the mines and pupate on the leaf surface or, more commonly, in cracks in the soil. Many generations occur each year and the entire life cycle can be completed in less than 3 weeks when the weather is warm.

DAMAGE

Larvae mine between upper and lower leaf surfaces, creating winding, whitish tunnels that are initially narrow, but then widen as the larvae grow.

MANAGEMENT

Biological Control
Natural enemies, especially parasitic wasps in the genus Diglyphus, commonly reduce populations of leafminers, unless killed off by insecticides applied to control other pests. Choose selective pesticides for treating other pests, if possible, to avoid this problem. Other parasites attack leafminers, but the leafminer feeding habit protects them from most predators.

Cultural Control
Liriomyza leafminers attack a wide variety of vegetable crops often grown in proximity to lettuce. Where possible, avoid planting next to infested fields, especially those near harvest.

Organically Acceptable Methods
Biological and cultural controls and sprays of azadirachtin or the Entrust formulation of spinosad are organically acceptable, but spinosad is very detrimental to populations of syrphid flies.

Monitoring and Treatment Decisions
Begin checking young seedlings for leafmines. Most mines occur on cotyledons and the first true leaves, and damage is worse in older leaves, which can be stripped in the field at harvest. Some mines are most obvious from the underside of the leaf. If leafminer populations build to high levels when seedlings have four to five leaves, a chemical treatment may be necessary. Treat if you find more than an average of one mine per leaf in your overall field sample. To be effective, sprays must be applied to the larval stage.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
N. C. Toscano, Entomology, UC Riverside

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