Quick facts about leafminers
Leafminers are insects that feed within leaves, producing large patches or winding tunnels of dead tissue. These mines are opaque initially and then later turn brown.
- The most commonly encountered leafminer species in Minnesota vegetable gardens are the spinach leafminer, Pegomya hyoscyami, and the vegetable (serpentine) leafminer, Liromyza sativae.
- The spinach leafminer feeds on spinach, Swiss chard, tomato, cucumber and celery.
- The vegetable leafminer feeds on bean, eggplant, pepper, potato, squash, tomato, watermelon, cucumber, beet, pea, lettuce and many other plants.
- Leafminers do not affect plant growth but destroy the edible leaves of vegetables.
How to identify leafminers
If you see patches or winding mines in the leaves, this means there might be leafminers in your garden.
Spinach leaf miners are a type of blotch leafminer, creating irregular round shaped mines. The mines are long and narrow at first, but eventually become an irregularly shaped patch.
- Spinach leafminer flies are 1/3 inch in length and gray to brown colored.
- Spinach leafminer larvae do not have legs and head. The larvae are whitish and carrot-shaped.
- The larvae tunnel into leaves between the two leaf surfaces.
- The adult fly is hairy, about 1/4 inch long, and grayish or brownish. It lays eggs on the underside of older leaves.
- Vegetable leafminers wind snake-like across the leaves and create winding mines.
- Vegetable leafminer flies are smaller than spinach leafminer flies (1/15 inch in length) and are yellow and black colored.
- The larvae do not have legs or a head. They are yellowish-green colored and cylindrical shaped.
- The adult of this species is a small fly.
Life cycle of leafminers
The pupae of both species survive through the winter and the adult flies emerge the following April and May. The flies insert eggs into leaves. Larvae feed and develop within leaf tissue and are active for about two to three weeks. Then, they drop to the ground to transform into pupae. Several generations can occur during one year.
How to protect your plants from leafminers
Check your plants regularly
Regularly check young seedlings for leaf mines, especially if leafminers have attacked your garden in the past. Most mines occur on the first true leaves.
Generally, vegetable leafminers are kept in control by their natural enemies and management is not required.
It is also not necessary to treat spinach leafminers when they are attacking the leaves of a root crop such as beets. But, if it is on spinach or a leafy green, management may be needed.
Keep your garden clean
- Remove weeds, like lambsquarter, to reduce its availability as a food source, for leaf miners.
- Remove and destroy leaves when the mines are small.
- Till your garden after harvesting to destroy pupae and reduce the chances of adult flies moving to neighboring plants.
Use a physical barrier
You can install a fine meshed netting, to protect the plants from insects. A material that does not prevent sunlight and rain from reaching the plants should be used (e.g. cheese cloth) . This should be done in areas where leafminer problems have not been seen for at least one year.
If leafminer issues have been observed within the last one year, this netting will not help. This is because pupae that survived the winter can produce adults under the barrier and can still infest plants.
Use of pesticides prevents adults from laying eggs, but does not kill larvae that are already feeding within plant leaves. When possible choose a low impact pesticide that does not target natural enemies (such as parasitic wasps) and pollinators (such as bees).
- Spinosad can be an effective option. It is derived from a naturally occurring soil-dwelling microorganism, provides good control, and has little impact on natural enemies.
- Broad spectrum pesticides (like permethrin, bifenthrin, and carbaryl) are generally longer lasting but can kill natural enemies too.
- You can apply pesticides in the spring when adult flies are first active.
- Double check that the pesticide you want to use has the vegetable, e.g. spinach, you intend to treat on the label.
- Make sure you get good coverage on the leaves.
- Make several treatments at regular intervals (check the label to determine how often you can apply a particular product).
- Be sure you observe the number of days from the last treatment until you can safely harvest your crop (known as a pre-harvest interval).
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Be sure that the vegetable you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018