Description of the Pest
The corn leafminer is the larva of a small black fly. The adult fly is seldom seen because of its small size and nondescript nature. Female flies lay eggs on the leaf surface. The eggs hatch into larvae that are tiny legless maggots, pale green to yellowish in color with a dark mouth hooks. The larva can be seen at the leading edge of the transparent "window" of the leaf mine. Leafminers are generally restricted to the lower leaves, up to the sixth leaf, on the plant. The upper leaves, those above leaf 7, have a much thicker cuticle and are less prone to injury.
At maturity, larvae drop from the leaves and pupate in the soil. The number of generations per year in California is not known, but there are probably at least five each season. The corn leafminer overwinters in the pupal stage in the soil.
Currently, corn leafminer appears to be restricted to the southern San Joaquin Valley. It is not present in either the high or low desert and there are no reports of it from the Sacramento Valley.
Damage
In California, like most of the U. S., leafminers cause little or no economic damage in corn. After hatching, the larvae burrow into the leaves where they tunnel between the upper and lower leaf surface. Larvae feed on the mesophyll, leaving behind transparent tunnels or mines. As the maggots grow the mines increase in size, and the larvae form blotchy mines rather than the more familiar serpentine mines commonly observed on vegetables. (The parallel venation of the corn leaves prevents the development of serpentine mines.)
Management
The corn leafminer is not considered a pest, and no management practices are recommended for its control. One consideration to keep in mind, however, is the increasing practice of planting corn after corn, which could result in greater problems with this insect. Because the corn leafminer overwinters in the soil, the practice of planting corn after corn can lead to an increase in corn leafminer populations and attacks earlier in the season. Insecticides are not effective for leafminer maggots because the maggots are protected inside the leaf.