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Protect against corn rootworm

Two species of rootworm are taking hold in the Midwest; keep your corn protected.

Sarah McNaughton

January 4, 2024

3 Min Read
young corn plants in a field
BUGGING CORN: The northern and western corn rootworm are becoming more common across Midwest cornfields. Purdue Extension offers advice on protecting corn from the pest.Sarah McNaughton

Corn rootworm is the latest pest bugging cornfields across the Midwest. Both its larva and adult form can severely damage corn, reducing yield and exposing it to other stressors.

Two main species of corn rootworm can be found in the region — northern and western. Purdue University says the western variety is seen as yellow to yellowish-green in color, with a black stripe along the sides of the wing covers. The northern species is tan to pale green, changing to a deeper shade of green as it matures.

Purdue says it’s not necessary to distinguish between the two species because both inflict the same damage. The larvae feed on roots, which reduces nutrient and water uptake by the plant. The feeding sites later become pathways for pathogen infections.

Both species of rootworm have only one generation per year. Eggs are laid in the soil from summer to autumn, and hatch from May to June. The larvae then seek corn roots to feed on while passing through three stages before pupating. Adult beetles emerge in late June to early July, where they feed on pollen, green silks or leaves.

Be alert to signs

No matter the species, here are signs corn might be suffering from one of these rootworm types:

  • Root tips will appear brown with tunnels or chew marks.

  • Root damage can cause lodged or curving corn plants.

  • Silks are severely clipped during pollen shed.

  • Green plant tissue is damaged from feeding.

Sampling methods

There are two methods to find rootworms in corn:

Larvae sampling. Sampling for larvae is only needed when an insecticide or hybrid failure is suspected, or a field has a high adult population. Purdue offers these steps:

  1. Choose 10 areas of the field with one randomly selected plant in each area.

  2. Dig a 7-inch cube of soil around the base of each plant; avoid cutting the roots.

  3. Lift the plant and soil onto a dark-colored sheet or plastic, and break the soil away from the roots to inspect for white larvae.

  4. Wash the soil in a bucket of water to extract the worms, which will float to the top.

  5. Record the number of larvae found on each plant sampled and determine the average per plant.

Adult beetle sampling. Scouting should begin right after corn silking starts. Purdue recommends:

  1. Scout by quickly walking fields in the morning or late afternoon. If no beetles are observed, additional scouting is not needed for that time.

  2. Continue to monitor twice a week until the silks are pollinated, and once a week into September.

  3. If beetles are found, determine how many by randomly checking five plants in a minimum of five different areas for silk clipping. If clipping has occurred, check what length of the silk is left. Determine the average silk length by totaling the length of all plants sampled and dividing by 25.

Manage against it

Purdue says a major management strategy for corn rootworm is crop rotations. Rootworms cannot complete their larval development on crops besides corn. If eggs are laid into a cornfield in one season that rotates into a soybean field the next, the larvae are left without a suitable plant and will starve. However, with some variants in the Midwest, crop rotation alone is not fully effective.

For adult beetles, recommended pesticides include chlorpyrifos, alpha-cypermethrin, beta-cyflurthrin, bifenthrin, carbaryl or deltamethrin. For larvae, pesticides include chlorpyrifos, cypermethrin, bifenthrin or chlorantraniliprole.

Your local agronomist, Extension agent or plant protection dealer can offer recommendations for specific farms, fields and varieties. No matter what method you choose, stay alert to this pest to keep corn rootworms out of your fields.

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About the Author(s)

Sarah McNaughton

Editor, Dakota Farmer, Farm Progress

Sarah McNaughton of Bismarck, N.D., has been editor of Dakota Farmer since 2021. Before working at Farm Progress, she was an NDSU 4-H Extension agent in Cass County, N.D. Prior to that, she was a farm and ranch reporter at KFGO Radio in Fargo.

McNaughton is a graduate of North Dakota State University, with a bachelor’s degree in ag communications and a master’s in Extension education and youth development.

She is involved in agriculture in both her professional and personal life, as a member of North Dakota Agri-Women, Agriculture Communicators Network Sigma Alpha Professional Agriculture Sorority Alumni and Professional Women in Agri-business. As a life-long 4-H’er, she is a regular volunteer for North Dakota 4-H programs and events.

In her free time, she is an avid backpacker and hiker, and can be found most summer weekends at rodeos around the Midwest.

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