Leafminers in home vegetable gardens
Leafminers are insects that feed within leaves, producing large blotches or meandering tunnels. The most commonly encountered leafminer species in Minnesota vegetable gardens are the spinach leafminer, Pegomya hyoscyami, and the vegetable (serpentine) leafminer, Liromyza sativae.
The first sign that leafminers are in your garden is the presence of the mines in the leaves. Vegetable leafminers create serpentine mines which wind snake-like across the leaves gradually widening, whereas spinach leaf miners are a type of blotch leafminer, creating irregularly-rounded shaped mines.
Adults of both species are small flies. Spinach leafminer flies are 1/3 inch in length and gray to brown colored. Vegetable leafminer flies are smaller than spinach leafminer flies (1/15 inch in length) and are yellow and black colored. Spinach leafminer larvae are whitish and carrot-shaped and vegetable leafminer larvae are yellowish-green colored and cylindrical shaped. Both larvae lack legs or an obvious head.
Flies of both species overwinter as pupae in the soil. In the spring, usually in late April and early May, flies of both species emerge and insert eggs into leaves. Larvae feed and develop within leaf tissue. The larvae are active for about two to three weeks, feeding within the leaf tissue before dropping to the ground and pupating in the soil. Several generations can occur during one year.
Copyright © Whitney Cranshaw, bugwood.org
Figure 4. Vegetable leafminer feeding damage on an onion leaf
The spinach leafminer feeds on spinach, Swiss chard, tomato, cucumber, and celery. The vegetable leafminer feeds on bean, eggplant, pepper, potato, squash, tomato, and watermelon, cucumber, beet, pea, lettuce and many other plants. As the larvae feed and develop, they create “mines” of dead tissue where they have fed. These mines are opaque initially and then later turn brown.
Leafminer activity has little impact on plant growth but can be quite destructive to vegetables grown for edible greens.
Regularly check young seedlings for leaf mines, especially if there is a history of leafminers attacking your garden. Most mines occur on cotyledons and the first true leaves. Normally vegetable leafminers are kept below damaging levels by natural enemies and management is usually not required.
It is also not necessary to treat spinach leafminers when they are attacking the leaves of a root crop such as beets, where the edible portion is not affected. However, if it is on spinach or a leafy green, management may be needed.
- Remove weeds, like lambsquarter, that leafminers may attack to reduce its availability as a food source.
- Remove and destroy infested leaves when mines are small.
- Till your garden after harvesting to destroy pupae and reduce the potential of migration of adult flies to adjacent plants.
You can erect a floating row cover, i.e. fine meshed netting, cheese cloth or some similar material that allows sunlight and rain in but prevents insects from getting to your plants. This should be done in areas where you have not had leafminer problems for at least one year as overwintering pupae near susceptible plants can produce adults under the barrier which can still infest plants. Row covers can be purchased at many local lawn and garden supply stores and online at suppliers like Burpee and Gardens Alive.
Applying insecticides helps prevent adults from laying eggs, but they do not kill larvae that are already feeding within plant leaves. When possible choose a low impact insecticide that is “easy” on natural enemies, such as parasitic wasps, and pollinators, such as bees. Spinosad can be an effective low impact option. It is derived from a naturally occurring soil-dwelling microorganism, provides good control, and has little impact on natural enemies.
Conventional, or broad-spectrum insecticides, are generally longer lasting but can kill a variety of insects, including natural enemies. Common examples of broad spectrum insecticides include permethrin, bifenthrin, and carbaryl.
When applying an insecticide, make sure you get good coverage on the leaves. For the best protection, make several treatments at regular intervals (check the label to determine how often you can apply a particular product). In addition, 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: Read all insecticide label directions very carefully before buying, and again before using, to ensure proper application. Be sure that the label specifies that it can be used on the specific crop you wish to treat. Be sure to follow all label directions including the storage, mixing, application and disposal of pesticides. The label is the final authority on how you may legally use any pesticide.