Evaluating early corn stands for common insect damage
Corn plants are emerging in several fields across the Corn Belt. If yours is one of them, now is the time to make final stand counts to determine if plant populations are adequate to achieve previously set yield goals. It is also time to scout the fields for early insect damage that may have already caused a stand reduction.
There are many insects that can destroy young corn seedlings—the three insects most likely to cause corn plant loss are black cutworm, wireworm and seedcorn maggot.
Black cutworm larvae do not overwinter in the Corn Belt. Adult moths get caught in wind and storms coming up from the southern United States, typically between March to May. The moths prefer to lay eggs on grasses in no-till fields or actively growing cover crop fields.
The eggs then hatch and larvae migrate to emerging corn plants. The larvae feed in the whorls and leaves of small corn plants, giving it a “shot hole” appearance. As larvae become larger, feeding can reach the growing point of the corn seedling, which kills the plant. Cutworm larvae are only active nocturnally and hide in the soil during the day. When larvae growth reaches sixth instar (its developmental stage where larva is approximately 11⁄2 inches in length), they form a pupa under the soil surface and emerge as a moth in 21-30 days.
Based on how they reproduce, there can be three generations of black cutworms per year. You can locate cutworm larvae by carefully digging around the base of corn seedlings that show feeding or wilt. Rescue treatment is typically required if 4-6 percent of the corn plants show feeding or 1-2 percent of corn plants have been cut off. There are several other types of cutworms that feed on corn and show identical symptoms, but black cutworm is by far the most common cutworm to cause damage throughout the Midwest.
Examine the trait package of the hybrid you planted. Several traits can control black cutworm damage. Ask your area extension agronomist for the most common treatment thresholds for cutworm damage that are effective in your region.
Wireworm larvae are hard-bodied brown-to-orange-colored worms that are one-half to one-and-a-quarter inches long. The larval stage of their life cycle can last two to six years. The adult wireworm is the Click Beetle stage, and it can last one to two years.
Wireworms tend to be a problem on no-till soils and manured soils. You should inspect fields by looking for gaps in corn plant stands or wilting plants. Wireworms will typically be found in or near the seed. As soil temperatures increase, wireworms move deeper into cooler soil and damage from this pest will all but stop.
There is typically no rescue treatment for wireworm damage, instead replanting using a soil insecticide or seed treatment is an option. Because of the lengthy larval stage of wireworm, treatment may be required for several years.
First, the adult fly lays eggs on or near the soil surface. The eggs hatch in 5-10 days and the small, cream-colored larvae then feed on ungerminated seed. Once the corn seed has germinated, the chance for damage diminishes. Based on how they reproduce, there can be 3-5 generations per year. When soils are cooler, it takes a longer period of time for corn seed to germinate. That’s when seed corn maggots can really cause damage.
Final corn stands can be reduced significantly by this insect when cool soil temperatures continue following planting. By digging up a few blank areas in the row and examining individual seeds, you may discover seed corn maggot larvae in the endosperm on the rotted seed.
Unfortunately, there is no rescue treatment for this pest. If you suspect seed corn maggot damage in a field, a seed insecticide treatment or in furrow insecticide treatment is typically the best recommended treatment to address it. If damage to the plant stand is significant, replanting may be necessary.
When you are evaluating plant populations and considering replanting, consider these 10 factors:
1. Total plant population average in the field and a yield loss estimate
2. Is the cause of the stand loss still present or has the damaged ended
3. Expected plant population if replanted and the yield increase expected
4. Plant uniformity and health of existing stand
5. Date of original planting versus replanting date
6. Cost of replanting
7. Crop insurance provisions on replanting
8. Increased risk of frost damage this fall
9. Possible increase in disease and insect damage of later-maturing corn
10. Possible increase in grain drying expense