A spinach leafminer, Pegomya hyoscyami, is a small anthomyiid fly whose larvae attack the leaves of spinach, beets, chard, lambsquarter, and other plants. This fly overwinters as pupae and the adults emerge the following April and May. The adult is hairy, about 1/4 inch long, and grayish or brownish. It lays eggs on the underside of older leaves. A spinach leafminer larva hatches into a carrot-shaped, whitish maggot that lacks legs or an obvious head.
The larvae tunnel into leaves, between the two leaf surfaces. The mines are long and narrow at first, but eventually become an irregularly shaped blotch area. These mines are opaque initially and then later turn brown. The larvae are active for about two to three weeks before dropping to the ground and pupating. Several generations can occur during one year. This activity has little impact on plant growth but can be quite destructive to vegetables grown for edible greens.
Home gardeners can ignore spinach leafminer damage when it is on beets as the feeding injury does not really impact the root crop. However, when spinach leafminer attacks spinach or other leafy green crops, you should take some action to protect them. Remove weeds, like lambsquarter, that spinach leafminers may attack, to reduce their available food source. 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 plots where you have not had spinach leafminer problems for at least one year as overwintering pupae near susceptible plants can produce adults under the barrier which can still infest plants. Remove and destroy infested leafs when mines are small.
You can apply a garden insecticide, such as permethrin, in the spring when adult flies are first active. Double check that the insecticide 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. For the best protection, make several treatments at regular intervals (check the label to determine how often you can apply the product you are using). Be sure you observe the number of days from the last treatment to when you can safely harvest your crop.
Published in Yard & Garden Line News, July 1, 2009