Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Alfalfa

Potato leafhopper is active, any alfalfa fields that have been cut and are re-growing should be scouted. The cutting process destroys nymphs and forces adults to leave the field. A dynamic potato leafhopper threshold can be found here: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/pest-management/Insect_Control_in_Alfalfa_-2023.pdf. Remember, by the time the yellowing symptoms of hopper burn are visible, yield has already been compromised.

Soybean

Scout emerging soybean plants like a hawk for signs of slug damage. If a field is worked as a general management practice or as a slug management practice, the seed should have an insecticide seed treatment on it to avoid potential seedcorn maggot damage. Close your seed slots!!! As the soil surface dries, slugs will seek out dark, moist areas under residue or in open seed slots. If a seed slot is open, it concentrates slugs right on top of a germinating seedling. Waiting until plants are out of the ground to make a bait application decision is too long. Conservation tillage can be helpful to reduce slug activity in fields with high residue.

Bean leaf beetle are also beginning to become active. The threshold for bean leaf beetle defoliation is 40% with 1-2 beetles present per plant.

Field Corn

A question came in this week as to what the best way to treat for white grubs might be in field corn. White grubs will soon be cycling out of fields, so this post might be a bit on the late side for this season. Dr. Dominic Reisig has performed quite a few wireworm and white grub efficacy trials in North Carolina and found that the high rate of Poncho (1250) or a 6.4 fl oz rate of Capture LFR in the furrow provided equally good white grub control. Low rates of Poncho and Cruiser provided only marginal to fair control. Interestingly, in between, there’s quite a bit of variation. In his trials, 2 tests with the high Poncho rate plus Counter did not perform as well as the high Poncho rate alone. I think this is an artifact of just how variable insect pressure and presence in a field can be. His results were fairly similar for wireworm, with the exception that the Poncho and Cruiser 500 rates performed as well as the high rate. Capture in furrow only provided marginal to fair control and did not seem to improve on the low rate of Poncho. Based on his data, and some personal observations over the last several seasons, the 500 rates of Poncho and Cruiser should perform well enough, but if you have heavy pressure, a bifenthrin treatment may improve upon efficacy, if only slightly.

Scout for cutworm damage, black cutworms were observed cutting V3 plants this week in Georgetown. Thresholds are 10% of leaves with feeding injury and 3% cut plants or, as plants age, 5% cut plants. Also, fields late planted into a standing small grain cover crop may have stink bugs present. See the above note on sweet corn for cutworm and stink bug management.

Grasshoppers are hatching out. While Mid-Western thresholds are high, it is worth knowing where they are concentrated in case you feel that a targeted application is recommended.

Early Season Moth Flight

Many thanks to Haley Sater with UMD Cooperative Extension and Joanne Whalen, extension entomologist emeritus extraordinaire for assistance with checking traps.