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. 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
Adult leafminers have such a preference for cotyledons that seedling growth may be stunted. Larvae mine between upper and lower leaf surfaces, creating winding, whitish tunnels that are initially narrow, but then widen as the larvae grow. Excessive mining renders leaves unmarketable, reduces photosynthetic capacity, and provides easy access for disease organisms.
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. To avoid killing beneficials, choose selective pesticides for treating other pests, whenever possible. Other parasites attack leafminers, but because leafminers feed within the leaf, they are protected from most predators.
Cultural Control
Liriomyza leafminers attack a wide variety of vegetable crops often grown in proximity to spinach. Where possible, avoid planting next to infested fields, especially those near harvest. Postharvest disking of fields destroys pupae and reduces migration of adult flies into susceptible fields.
Organically Acceptable Methods
Biological and cultural controls and sprays of azadirachtin (AZA-Direct, Neemix) are acceptable for use on organically grown produce.
Monitoring and Treatment Decisions
Regularly check young seedlings for leaf mines. Most mines occur on cotyledons and the first true leaves. Some mines are most obvious from the underside of the leaf. If leafminer populations build to high levels when seedlings have 4 to 5 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 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 product being used. | ||||
| A. | CYROMAZINE | |||
| (Trigard WSP) | 2.66 oz | 12 | 7 | |
| MODE OF ACTION GROUP NUMBER1: 17 | ||||
| COMMENTS: Very effective against leafminer larvae. Do not make more than 2 sequential applications. Check label for plantback restrictions. | ||||
| B. | AZADIRACHTIN# | |||
| (AZA-Direct) | 1–2 pt | 4 | 0 | |
| (Neemix 4.5) | 4–7 oz | 12 | 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. In an organically certified crop, check with certifier for restrictions regarding the use of this product. | ||||
| ** | Mix with enough water to provide complete coverage. |
| ‡ | 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. |
| 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). |
| # | Acceptable for use on organically grown produce. |