David Owens, Extension Entomologist, owensd@udel.edu
Corn
Continue scouting for cutworm, stink bug, slug, and the soil pest complex. Cutworm thresholds are 5% cut plants at V2-V4 or 10% of plants with signs of fresh leaf feeding. Dig around plants to determine if cutworm larvae are actively present. We visited a field last week that had cutworm feeding and cut plants, but many of the larvae we found were dead or dying as a result of an in-furrow insecticide application. Cutworms tend to be more common in late cover crop terminated, late planted corn fields. Another pest that can be present in such fields are brown stink bugs. They are attracted to small grains to reproduce, and if corn is planted into a small grain cover crop, they may feed on the seedlings. This can result in plants with rows of ragged holes, often with yellowing streaks above the hole. Severely impacted plants may tiller multiple times if the growing point has been fed upon. Early thresholds in corn are 13 or more bugs per 100 plants.
There are no rescue treatments for soil pests. Damage will appear as wilting whorl leaves or leaves with bright yellow streaks along the leaf margins.
Soybean
Continue scouting for slugs. Fields that have not yet been planted or beans that have not yet emerged, planted in no-till, cover crop, and/or fields with prior history of slug infestation should be carefully monitored. Slug baits are most effective if applied before soybeans have begun to crack the ground. Once soybean emergence begins, slug bait rapidly loses its best window of opportunity to protect a stand. Do not apply bait once stand emergence issues are evident unless your goal is to protect a replant. Remember to close seed slots, and if a field is a heavily infested, it might be more beneficial to lightly work the ground. I do not think tillage needs to be aggressive. We want to remove residue from the furrow and work the ground around the furrow. Anything that stimulates rapid emergence and unifoliate leaf production will aid in slug management.
Alfalfa
Begin monitoring for potato leafhopper, especially on any fields that have just been cut. This is about when potato leafhopper begins to appear in our area. PLH saliva is phytotoxic to the plant, resulting in yellowing leaf margins. Once you see this characteristic ‘hopper burn’, yield potential is reduced. If you begin to see significant PLH numbers and the plants are large enough to cut, consider an early harvest. Harvesting alfalfa destroys nymphs and forces adults to leave the field.