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

Leafminers can cause several types of damage. Female adult flies puncture the leaves with their ovipositors both to create feeding sites and to oviposit. These punctures cause a stippling pattern on the wrapper and cap leaves. Puncture sites where eggs are laid eventually develop winding, whitish tunnels 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 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.

Common name		Amount/Acre**	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
A.	ABAMECTIN*
	(Agri-Mek 0.15EC)	8–16 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/growing season. Apply in at least 20 gal water/acre.
B.	CYROMAZINE
	(Trigard WSP)	0.166 lb	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 2 sequential applications to reduce potential for resistance to develop. Check label for plantback restrictions.
C.	AZADIRACHTIN#
	(Neemix 4.5)	4–7 oz	12	0
	(Aza-Direct 1.2%)	8.3–40 oz	4	0
	(Agroneem 0.15%)	48 oz	4	0
	MODE OF ACTION GROUP NUMBER1: 18B
	COMMENTS: This material 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 material in an organically certified crop.
D.	SPINOSAD
	(Entrust)#	1.25–2.5 oz	4	0
	(Success)	4–8 oz	4	0
	MODE OF ACTION GROUP NUMBER1: 5
	COMMENTS: Not recommended when lettuce aphid is present because of its negative impact on syrphid fly larvae.
**	Mix with enough water to provide complete coverage.
*	Permit required from county agricultural commissioner for purchase or use.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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). For additional information, see their Web site at http://www.irac-online.org/.

PUBLICATION

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

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