Description of the Pest
Adults are small, black flies. They are about 0.06 inch (1.5 mm) long, with a bright yellow triangle on the thorax between the base of the wings and yellow on the face and underside. Eggs are laid in the leaf tissue and hatch in 2 to 4 days during warm weather.
The yellow maggots (larvae) of leafminers grow to be up to 0.1 inch (2.5 mm) long as they feed between the upper and lower surface of leaves. Feeding by the maggots causes distinctive winding, whitish tunnels that increase in length and width as the larva grows. Mature maggots emerge to pupate on the leaf surface or in or on soil.
Leafminers have many generations per year. One generation can be completed in less than 3 weeks during warm weather.
Damage
Females puncture leaves to feed on plant sap. When high numbers of females lay eggs in leaf tissue, numerous pale spots will be present on leaves. Leafminer feeding can reduce the plant's photosynthetic capacity, render edible leaf portions unmarketable, and provide entrance sites for plant pathogens.
Management
Leafminers are primarily seedling pests. Regularly monitor seedlings and young plants to determine if an insecticide application is necessary.
Cultural Control
Where possible, avoid planting next to infested fields, especially when such fields are near time to harvest. Leafminers attack a wide variety of vegetable crops, and readily migrate to cole crops from nearby hosts. Cultivate or destroy infested weeds and crop residue after harvest.
Biological Control
Natural enemies, especially Chrysocharis and Diglyphus spp. parasitic wasps, commonly control leafminers. Choose selective pesticides when managing other seedling pests to avoid disrupting leafminer biological control. Consider installing insectary plants to attract natural enemies of leafminers.
Organically Acceptable Methods
Use cultural controls and the Entrust SC formulation of spinosad in an organically certified crop.
Monitoring and Treatment Decisions
Regularly check young seedlings and transplants for leaf mines. Most mines occur on the cotyledons and first true leaves. If leafminers are numerous when seedlings have five or fewer true leaves, an insecticide application may be necessary. Apply an insecticide if there are an average of one or more mines per leaf in the overall field samples in the early stages of crop growth.
While leafminer infestations at early stages of growth can be detrimental to yields, broccoli and cauliflower with six or more leaves are rarely damaged by leafminers, regardless of their numbers. Parasitic wasps usually keep leafminers below damaging numbers as long as disruptive pesticides (such as pyrethroids) are not applied.
For cabbage, consider applying insecticide if edible leaves are mined.
| 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. | SPINETORAM | |||
| (Radiant SC) | 6–10 fl oz | 4 | 1 | |
| MODE-OF-ACTION GROUP NUMBER1: 5 | ||||
| COMMENTS: Toxic against some natural enemies (predatory beetles, syrphid fly larvae, and predatory thrips) when sprayed and 5 to 7 days after. Control improved with addition of an adjuvant. | ||||
| B. | SPINOSAD | |||
| (Entrust SC)# | 4–10 fl oz | 4 | 1 | |
| MODE-OF-ACTION GROUP NUMBER1: 5 | ||||
| COMMENTS: Toxic against some natural enemies (predatory beetles, syrphid fly larvae, and predatory thrips) when sprayed and 5 to 7 days after. Use higher rate for heavy infestations. | ||||
| C. | CYROMAZINE | |||
| (Trigard) | 2.66 oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NUMBER1: 17 | ||||
| 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 the two intervals is the minimum time that must elapse before harvest. | |
| 1 | Rotate insecticides 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; insecticides with a 1B group number should be alternated with insecticides 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 |