Experts say it's still worth checking for corn rootworm

Corn Pest Beat: Check for underground pests at key times. 
Apr 30, 2020
My corn hybrids don’t have belowground insect trait protection, and I didn’t apply a soil insecticide. Is it worth checking for belowground insect problems? If so, how do I do it? What if I find insects?
The Indiana certified crop advisers panel answering this question includes Betsy Bower, agronomist with Ceres Solutions, Terre Haute; Jesse Grogan, regional manager at AgReliant Genetics LLC, Lafayette; and Stan Miles, agronomist with A&L Great Lakes Laboratories, Fort Wayne.
Bower: It is worth checking. Corn rootworms are your primary target in June. I would be most concerned about corn-after-corn fields in much of Indiana, and corn after soybeans in the northwest quarter of Indiana. While we haven’t seen issues with corn rootworms the past several years after use of belowground insect traits, corn rootworms haven’t gone away.
To check for corn rootworm, start around early to mid-June, depending on the time of corn rootworm hatch. Corn rootworms start hatching when you see fireflies. Randomly check at least one plant in each of at least 10 representative areas of a field. You will dig about a 7-inch cube of soil, including the plant, and place soil on a dark sheet of cloth or plastic to look for small, slender worms in soil around roots. You can also look for worms that float after submerging roots in a bucket of water. Saturating the water with salt will cause worms to float to the top more easily. Count and record the number of larvae found. Repeat the sampling procedure for all 10 plants, and average total counts.
If the average of the 10 plants reaches approximately two or more larvae per plant by hand-sorting, or eight or more per plant by washing, a soil insecticide is needed before lay-by. Follow all label directions for postemergence insecticide application.
Grogan: Decisions for belowground insect protection are best made before planting. Rescue options are very limited if an insect problem occurs. One can check for hatched corn rootworm larvae in June. The process is labor-intensive and seldom done. Insect pests such as grubs, seed corn maggot and wireworms are also found in June. Cutworms at the soil surface can be controlled with foliar insecticides when treatment threshold is reached. Beneficial insects like ground beetles are also noticed on the soil surface. There is not much one can do, except plan for next year or replant the crop with appropriate belowground insect control measures if damage is severe.
Miles: The key insect of concern would be corn rootworm larvae in unprotected fields, especially in areas with a history of rootworm feeding, corn-following-corn rotations and a high incidence of rootworm adult feeding during the previous crop. Scouting data may be most valuable when performed from around V6 through R1. While no rescue treatment is available, it’s important to document levels of damage and potential yield losses, so management changes can be considered for following years. It is recommended to dig 10 random plants and carefully separate the soil to identify the small white larvae, usually 1/16 of an inch to ⅜ of an inch in length. Pay attention to the level of damage that has occurred and the regrowth that has taken place at the point of feeding. Knowledge about in-season rootworm damage should be considered when planning harvest sequence to avoid losses from late-season root lodging when possible.