If you’re scouting fields this week, be on the lookout for corn rootworm damage. Western and northern corn rootworm are more prevalent in the Midwest as their eggs can overwinter in the soil. If left untreated, larvae and adult rootworm beetles can significantly damage a crop as they grow in numbers and feed on the plant — the roots as larvae and the leaves and silks as adults. As adults, they can greatly impact pollination during silking.
What to Look For
By now, most corn rootworm larvae should have hatched, so what you want to look for is damage to the roots. To scout for damage caused by rootworm larvae, experts from Iowa State University suggest digging an eight-inch block of soil centered around the corn plant. They recommend sampling several plants per field, and it’s best practice to take these samples from different locations. To detect the presence of larvae, you need to carefully examine the plant roots and surrounding soil. One method experts recommend is performing the “float test.” This includes using a five-gallon bucket and filling it with water. Place the soil and roots into the water. The soil should break apart and the larvae should float to the top. Root damage, which can cause significant interference in the nutrient uptake of the plant, should also be easier to notice with the soil cleared from the roots. Iowa State University has an interactive node injury scale that can help growers determine the damage caused by corn rootworm larvae.
In fields that have begun silking, you’re more likely to see adult rootworm beetles. Beetles can be found on the upper portion of the plants as they move up to feed on the silks and green tissue. Beetles will look different based on the kind of rootworm. Northern corn rootworm beetles are typically a light green or brown in color with no marks. Western corn rootworm beetles are yellow or green and typically have black striping on their wings. Southern corn rootworm, which aren’t as common in the Midwest, are also yellow and green in color and have distinctive black spots on their backs. In most instances, the beetles are easier to detect as they can be found eating on the silks and leaves of the plant. However, experts from Purdue University note that beetles are more active in the morning or late afternoon, so it’s best to scout the upper portions of the plants during those times. Some experts even recommend using sticky traps to determine the amount of infestation.
Prevention and Management
If you detect rootworm larvae or beetles, determine the amount of infestation before considering inputs. Then, discuss your infestation threshold with a university extension expert or your local Stine agronomist to determine what steps to take for mitigation or future prevention of rootworm in your fields.
During the growing season, insecticides may be your best and only option to kill off rootworm. For next year, you’ll want to consider crop rotation or investing in Bt-traited corn. While there is some known rootworm resistance to Bt traits, there are still hybrid options that have proven effective towards managing infestations. Stine Agrisure Duracade® brand corn, for example, combines Stine’s high-yielding genetics with the most powerful tool for corn rootworm pressure.
Stine Agrisure Duracade brand corn features both above- and below-ground control of more than 16 yield-robbing insects, including corn rootworm and many lepidopteran insects. The Agrisure Duracade trait is designed to control corn rootworm as soon as newly hatched larvae begin feeding on corn roots. How? The trait expresses a unique protein that binds differently in the guts of corn rootworm, providing effective control of western, northern and Mexican corn rootworm. That said, the cutworm MUST ingest Duracade-treated corn to die, so it’s more of a reactive treatment vs. suppressive.
To learn more about corn rootworm detection and management or Stine Agrisure brand corn, please contact your local Stine sales rep.