We have studied two insects that can cause this type of damage - 1) the corn flea beetle (adult only) and 2) adults and larvae of the cereal leaf beetle. Slugs can cause a somewhat similar type of damage but they would leave slime trails on the leaves.
Corn flea beetles are very small black beetles with large hind legs that allow them to jump like fleas. They overwinter as adults and feed on the leaves of corn as soon as plants are present in the field. The potential for damage is greatest when the sum of the average monthly temperatures for December, January, and February is greater than 100. Little injury ocurs when this sum is < 90; damage is usually moderate if the sum is between 90 and 100.
The digestive tract of this insect serves as the overwintering site for the bacteria that causes Stewart's disease of corn. The symptom is a wilting of the plant. The bacteria is introduced into the plant as the beetle feeds on the leaves. Most field corn and popcorn varities are resistant to this disease but sweet corn and seed corn is generally very suseptible.
Corn flea beetles lay their eggs in the soil near the base of corn plants. The larvae look like corn rootworms and feed on corn roots. They are so small that they do not cause any significant damage.
Cereal leaf beetles cause similar narrow feeding scars. These beetles would feed on wheat early in the spring and lay eggs on the leaves. There must be an average of at least one adult or larvae per head-bearing stem before control is required, and this is a very conservative threshold. It takes an awful lot of these insects to have an economic impact. Adults that develop from these eggs may feed on small grains or corn before going to hide for the summer. Their injury would appear on corn several weeks into the season, not on seedlings.
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