Jerry Brust, IPM Vegetable Specialist, University of Maryland; firstname.lastname@example.org
In high tunnels and in the field, I have been seeing spinach or beet leaf miner Pegomya hyoscyami and P. betae respectively in swiss chard and spinach. These leafminers are a type of blotch leafminer, creating irregularly shaped mines (Fig.1). Adults are small flies about 1/3 inch in length and gray to brown. Larvae are whitish and cone-shaped. Flies of both species overwinter as pupae in the soil. In April and early May (although this can occur in March if in a high tunnel), flies emerge and lay white eggs in groups of 4-8 on the underside of leaves. Eggs hatch and larvae feed within the leaf tissue. As the larvae feed and develop, they create areas of dead tissue where they have fed. These areas are opaque at first and then later turn brown (Fig. 2). Once inside the leaf tissue larvae are difficult if not impossible to control. The larvae are active for about two to three weeks, before dropping to the ground and pupating in the soil. The entire life cycle is 30-40 days. There are three to four generations per season. Once the summer is over, leafminers will overwinter as a puparium in the soil emerging in early spring the next year to start the cycle again.
Both leafminers feed on spinach, Swiss chard, beets and weeds such as pigweed and lambsquarter. Leafminer activity has little impact on overall plant growth but can be quite damaging to vegetables grown for edible greens. So, a crop such as chard and spinach that you are trying to sell the leaves of are greatly impacted while something such as turnips or beets that you are selling the bulbs of are less impacted (unless you are selling the tops too).
The damage to Swiss chard and spinach I saw probably could have been less if the first infested leaves with leafminers had been removed and destroyed. However, once the population was in its second generation the damage was too extensive. Any additional plantings of spinach or chard this season (or next year) should be planted in a different area of the field because of the pupae still in the soil.
Once the spinach or chard is planted in a new area a row cover could be used to cover the plants and keep the leafminer flies out that eventually will emerge from previously infested areas. Applying insecticides helps prevent adults from laying eggs, but they do not kill larvae that are already feeding within plant leaves. Spinosad (organic) can provide good control and has only a minor impact on natural enemies. Neem oil also can be used to prevent adult egg laying but is not as effective as spinosad. As always thorough coverage is necessary for good control which includes getting the material to the underside of the leaf.
Figure 1. Leafminer in Swiss chard
Figure 2. Leaf mine turning brown