There are dozens of different pests that affect the annual cycle of corn production from April through September.(1) The most injurious are wireworms, the European corn borer, the Southwestern corn borer, corn rootworms, grasshoppers, fall armyworms, and corn earworms.
Wireworms grow from small insect larvae to small or medium-sized beetles. They feed primarily on the germinating seeds of corn, from April through early June, and can spend astonishingly up to six years in the soil of a cornfield.
Another problem with wireworms is that there is no post-emergence treatment; safeguarding against these pests must occur at the planting stage or earlier. They can bore into the corn plant base, below ground and the drill upwards, like termites in wood. Or they can simply feed on the tender young roots of a corn plant. It is recommended that two to three weeks before corn planting, that bait traps be laid for wireworms, to the tune of five to ten bait traps per corn field.
Late Harvest Cycle Pests
The rest of the most damaging types of corn pests all come much later on, at the height of summer. Fall armyworm moths migrate from the Gulf Coast, with the first wave arriving in the Midwest around the middle of June. The are active in the early morning and early afternoon, and can bore through the corn husk to get at the sweet kernels.
Corn borers, of both the southwestern and European variety, have a different life cycle. In the former case, for example, they live during the winter in the lower part of a corn stalk. Then, in late May, they lay eggs that hatch as larvae and begin to do damage ahead of a second wave of infestation, which occurs when these larvae grow into moths and lay eggs anew.
The corn earworm is the most feared by sweet corn farmers because it directly attacks the end-market, cob product.(2) Farmers must stop these pests from becoming established within the ear, because once they have embedded themselves, they are virtually impossible to eradicate by means that do not involve also destroying the ripe corn crop.
(1) University of Illinois – Corn Insect Pests: A Diagnostics Guide, Retrieved July 17, 2011 from http://ipm.illinois.edu/pubs/cip.pdf
(2) University of Kentucky – Sweet Corn Pests, Retrieved July 17, 2011 from http://www.ca.uky.edu/entomology/entfacts/ef302.asp