No. 16 Article 2/July 9, 2004

White Corn Leaves? Consider the Corn Blotch Leafminer

In issue no. 14 (June 25, 2004) of the Bulletin, Kelly Cook wrote about rootworm adults scraping green tissue from corn leaves before tasseling. However, we have been made aware of another pest that is causing "similar but different" injury, and we want to make certain that everyone understands the differences.

Corn blotch leafminers are common although often overlooked pests of corn that rarely, if ever, cause economic damage. The larvae, which are maggots, hatch from eggs laid on corn leaves by flies, and then they feed between the upper and lower leaf surfaces, creating transparent mines in the leaves. As the name of the insect implies, the mines often end in irregularly shaped blotches. However, when the larvae are numerous, many blotches seem to come together, resulting in leaves that appear white or transparent. Within the mines you can usually see the very small (1/4-inch-long) yellow-white maggots, if they are still present, or at least the excrement they leave behind in the mines (excrement appears as black flecks within the mines).

Corn blotch leafminer “mines” in a corn leaf. (Photo courtesy of Kevin Gale, AgriGold Hybrids.)

Most entomologists believe that corn blotch leafminers cause no economic damage to corn. In fact, there are no economic thresholds for this insect, and insecticide applications likely would not control the larvae inside the mines. However, you should be aware of the presence of corn blotch leafminers, if for no other reason than to impress your friends and neighbors. Seriously, being able to distinguish between injury caused by this nonpest and more serious injury caused by other leaf feeders enhances your diagnostic skills. --Kevin Steffey

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