Seiter: Risk factors for early-season pests in corn and soybeans

By Nick Seiter

While we have a variety of pests that can reduce stand or otherwise impact yields in Illinois, they are sporadic, and economic damage is relatively uncommon. That is no consolation if you are one of the unlucky few who has to replant part of a field due to slugs, wireworms, or another pest in a given year. Predicting these events can be difficult – start with predicting the weather, then add layers of variability due to soil type, surrounding habitat, natural enemies, and movement of the pests themselves. However, there are certain risk factors that make damage more likely, and increase the likelihood that control will be necessary.

Weather and environmental conditions

Temperature and moisture early in the season have major implications for both pests and the crops themselves. Warm temperatures around planting and during the early vegetative stages will go a long way toward mitigating the risks of early-season stand reducers, as they allow plants to “outrun” pests such as cutworms, armyworms, slugs, and wireworms. Once the crop develops a few true leaves (especially by about V5 in corn), the risk of stand loss from these pests declines rapidly. Cooler temperatures slow plant growth, allowing these pests more time to “tee off” on the developing plants while they remain in a vulnerable stage. Moisture plays a major role as well: slugs, for instance, thrive in a moist environment, while spider mites can become an early season pest if drought conditions prevail shortly after planting.

Cultural practices

Not surprisingly, cultural practices influence the likelihood of early-season pest damage. Slugs are almost exclusively a pest in low- or no-till systems, while stand loss from grape colaspis is only a risk where the previous crop was a legume. (This is why we usually see them in corn in Illinois, but they can affect soybean following soybean as well; corn following corn is not at risk from grape colaspis, but faces increased risk of damage from corn rootworm among other pests). Seedcorn maggot is most problematic where manure (whether from livestock or green manure) is incorporated shortly before planting, but can be attracted by recent tillage in general. Early-season weed control is also critical: black cutworm (winter annuals and other weeds) and armyworm (grasses) often become an issue in fields where early-season weed control was either poor or late. These insects will gladly move over to corn if it is emerging while their initial host weeds are dying.

Field history

Certain fields tend to have a history with certain pests. Wireworms, which have a multi-year life cycle, often remain a problem for several years in an individual field once they become established. Grape colaspis are more problematic in some lighter soil types and well-drained areas, meaning certain fields (and even certain locations within fields) are more likely to experience stand loss. Fields converted from sod or pasture to cropland will have an increased risk of white grub and wireworm damage.

The good news about these early-season pests is that they affect a relatively low acreage of corn and soybean each year in Illinois. Remember that both crops are resilient to early-season defoliation – if we maintain stand and avoid stunted growth, a little feeding is unlikely to impact yields. Preserving stand (especially in corn) should be the priority, rather than avoiding insect defoliation. Understanding the risk factors that make early season stand losses more likely can lead to more effective (and profitable) pest management decision-making.

Nick Seiter is a research assistant professor, field crop entomology, Department of Crop Sciences, University of Illinois.