The Purdue University Corn & Soybean Field Guide carries a crop scouting calendar, a one-page chart showing when certain pests should be scouted for in corn. Your state may publish its own calendar, and the dates and key pests may vary somewhat state to state. But to some degree, corn is corn, and subject to certain pests at certain times of the year, no matter where you grow it.
The May calendar slots are filled with a host of diseases and insects to watch for. Most may not cause major damage overall, but any one, if very severe in one particular field, could pose a threat that could dampen yield potential. Among the first insects to scout for in young corn are corn flea beetle, seedcorn maggot, which would work on the seed before plants emerge, white grubs, wireworms, and maize billbugs.
There are no remedies for some of these, such as wireworms. The cure is hot weather which will drive them deep into the soil and stop the insect form cutting corn plants. Mark the location- this pest can stay within the soil 4 to 7 years. Seed-coated insecticides of moderate to strong strength will often control wireworm.
Included in the next group of pests to look for, beginning in late May, is armyworm and stalk borer. Armyworms were reported to be active this spring in southern geographies. They can infest a field quickly, and eat lots of foliage in a hurry. However, they typically don't attack large acreages at one time.
Be on the lookout for corn rootworm, depending upon where you live, from the last week or so of May until the first 10 days of June. Their development corresponds almost exactly with the heat units needed for fireflies to develop. So if you see the first firefly, you can assume rootworm larvae are likely hatched as well. Scouting for them, especially when they're small, can be difficult at best.
In terms of early diseases on corn, look for evidence of seedling blights and root rots anytime after planting. Expect these conditions to be worse if the seed sits in the ground for long periods, or if cool, wet weather develops after panting. Anthracnose leaf blight can begin to show itself in May, as can Goss' bacterial wilt and the seedling phase of Stewart's wilt. The latter disease, Stewart's wilt, is tied into transmission by corn flea beetles.