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Generally, the first generation of flies causes the greatest amount of injury to corn. As corn grows larger and there is a greater total leaf area, the proportion of damage is less. Also, the older leaves are thicker, and often the leaf mines are restricted to only the upper or lower leaf surface.
The flies emerge over several weeks. Some people are attempting to control the adult flies to prevent egg-laying. They have reported seeing dead flies after treatment with insecticides.
In general leafminers are parasitized in the larval stage by a wide variety of parasitic wasps. Reports indicate that more than a dozen species of wasps attack corn blotch leafminers in the United States. So far this summer I have reared two wasp species from corn blotch leafminers at Clay Center. Foliar sprays with insecticides may kill some leafminer flies, but they also will kill any parasitic wasps present, allowing greater survival of future generations of leafminers. Also, given the hot, dry weather, application of broad spectrum insecticides has the potential to encourage growth of spider mites in corn.
Bob Wright
Extension Entomologist, Lincoln
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