Ric Bessin, Extension Entomologist
Entomology, University of Kentucky
Planting date is one of the more important factors determining the potential for insect problems in field corn. Many may not realize it, but the time at which the seed goes into the ground determines which insect pests are more likely to be encountered. Some pests are favored by early planting dates while others are more likely to show up in damaging numbers in late planted corn.
There are two reasons for differences in pest problems due to planting date. First, for many corn pests, the plant has specific vulnerable stages. For example, with cutworms, the plant is susceptible to cutworm damage anytime between emergence and when the plant reaches 12 to 18 inches in height. Second, pest population numbers tend to run in cycles. Usually a pest reaches damaging levels only during certain periods of the season. With black cutworm, because it is a migratory pest, usually it will only reach damaging levels in the late spring. Fields with early and normal planting dates usually escape significant cutworm damage because the plants have grown out of the vulnerable stage by the time the damaging populations arrive.
Wireworms and the damage they cause on the other hand are favored by early planting. The reason for this is that the cool soil slows germination, emergence and growth. Late planting into warmer soils speeds germination, emergence and growth thusly limiting wireworm damage. With treated seed and later planting dates, wireworm is less likely.
There are two generations each of European and Southwestern corn borer each year. The first generations attack in late June and early July, and the second generations begin their assaults in early to mid August. Corn borers prefer whorl-stage corn for egg laying. Studies at the UK Research Center in Princeton Kentucky have shown that corn planted after May 10 is at much greater risk to yield loss from late season European and Southwestern corn borers. Growers using Bt corn often save this seed for later plantings to protect against corn borer attack.
Fall armyworm is another pest that migrates from southern states each spring and is favored by late planting dates. This pest also tends to prefer to lay eggs on whorl-stage corn. Corn which planted after June 1 and that is still in the whorl stage in late July and early August is more likely to be damaged by fall armyworm. Not all Bt corn for corn borer provide effective control of fall armyworm. Insecticides used for fall armyworm control in whorl stage corn need to be applied before the larva forms a frass plug in the whorl of the plant, as the larva feeds under the plug and the plug limits penetration of the spray into the whorl.
Unfortunately, we often have little control of planting date and are at the mercy of the weather. But understanding what your potential problems are should help you manage your crop more effectively.