Agriculture: Lettuce Pest Management Guidelines

Leafminers

Description of the Pest

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 can occur each year and the entire life cycle can be completed in less than 3 weeks when the weather is warm.

Damage

Leafminers can cause several types of damage. Female adult flies puncture the leaves with their ovipositors (egg-laying organs) both to create feeding sites and to lay their eggs. These punctures cause a stippling pattern on the wrapper and cap leaves. Puncture sites where eggs are laid eventually develop mines when the egg hatches and the larvae feed between upper and lower leaf surfaces. Larvae exit the mines to pupate and can cause contamination of lettuce head even if the mined leaves are removed at harvest.

Management

Biological Control

Natural enemies, especially parasitic wasps in the genus Diglyphus, commonly reduce numbers 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

Use biological and cultural controls and sprays of azadirachtin or the Entrust formulation of spinosad in organically certified crop. Note that spinosad is detrimental to beneficial syrphid flies and parasitoids.

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 numbers build to high levels when seedlings have four to five leaves, a chemical treatment may be necessary. Apply insecticide if you find more than an average of one mine per leaf in your overall field sample. To be effective, insecticides must be applied to the larval stage.

Common name Amount per acre** REI‡ PHI‡
(Example trade name) (hours) (days)
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
A. ABAMECTIN*
(Agri-Mek SC) 1.75–3.5 oz 12 7
MODE-OF-ACTION GROUP NUMBER1: 6
COMMENTS: Is effective at killing larvae in the mines. Do not apply at less than 7-day intervals or exceed 48 fl oz/acre per growing season. Apply in at least 20 gal water/acre.
B. CYROMAZINE
(Trigard) 2.66 oz 12 7
MODE-OF-ACTION GROUP NUMBER1: 17
COMMENTS: An insect growth regulator that is very effective against leafminer larvae. Do not make more than two sequential applications to reduce potential for resistance to develop. Check label for plantback restrictions.
C. AZADIRACHTIN#
(Neemix 4.5) 4–7 oz 12 0
(AzaGuard 3%) 8–21 fl oz 4 0
MODE-OF-ACTION GROUP NUMBER1: 18B
COMMENTS: This insecticide is consumed by the larvae but does not kill the leafminer until it finishes feeding, drops from the plant, and forms a pupa; consequently, it doesn't prevent damage from current generation but it can prevent the production of a following generation. Kills leafminer after pupation. Restricted use pesticide in an organically certified crop.
D. SPINOSAD
(Entrust)# 1.25–2.5 oz 4 1
(Success) 4–8 oz 4 1
MODE-OF-ACTION GROUP NUMBER1: 5
COMMENTS: Not recommended when lettuce aphid is present because it harms beneficial syrphid fly larvae.
** Mix with enough water to provide complete coverage.
* Permit required from county agricultural commissioner for purchase or use.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for organically grown produce.
1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
Text Updated: 04/17
Treatment Table Updated: 04/17