Northern rootworm is an unwelcome arrival

MFA’s Crop-Trak consultants have been finding an alarming number of northern corn rootworm (NCR) beetles in northwest Missouri and southwest Iowa. Kevin Moore with MFA’s Crop-Trak first noticed high numbers of adult northern corn rootworm in a cornfield rotated from soybean near Fairfax, Mo., in July 2016. In the weeks after that discovery, large populations of NCR beetles were discovered in multiple fields in Atchison, Nodaway, Holt, and Worth counties in Missouri as well as Page county, Iowa. The presence of a few rootworm beetles moving from corn-on-corn fields into later planted rotated cornfields is not uncommon because rootworm beetles feed on corn pollen and will seek out a food source. The alarm comes from the fact that such a high number of beetles are being found in areas where acres are dominated by corn/soybean rotations.

Rootworm species of economic importance include the northern corn rootworm first discovered in 1824 in the north central U.S. and the western corn rootworm (WCR) first discovered in western Kansas in 1868. In North America, corn rootworms are the most devastating insect pest to corn. Agronomists estimate that before the development and release of Bt corn traits to control corn rootworm larvae in 2003, 50 million acres were infested, accounting for over $1 billion in lost revenue. Before the adoption of Bt corn traits, the USDA estimated growers spent $200 million in control measures and suffered $800 million in lost yield. Yield loss and standability issues are a result of diminished root systems when rootworm larvae feeding is severe enough.

Most of Missouri’s corn is grown in a corn/soybean or corn/soybean/wheat rotation. While Missouri has populations of NCR and WCR, both species typically lay eggs in cornfields with the eggs hatching the following season. This makes corn-on-corn fields vulnerable to larvae feeding on corn roots. However, when fields are rotated to soybean, the newly hatched larvae starve without a host such as corn. Most Missouri growers have seen fewer rootworm problems than regions with more continuous corn or regions where rootworms have evolved new strains to survive.

In some areas of the Midwest, rootworm populations have adapted to crop rotation control measures. A “soybean variant” of WCR was discovered in Illinois in 1995. This strain will lay eggs in soybean in addition to corn, which makes crop rotation an ineffective method of control. Northern corn rootworm populations have also adapted to crop rotation by developing populations with an extended diapause. Extended-diapause rootworm beetles still lay eggs in cornfields, but the eggs can wait two seasons or more to hatch—allowing the larval pest to find corn roots to feed upon in a corn/soybean/corn rotation.

With high levels of northern corn rootworm beetles being documented in areas of northwest Missouri and southwest Iowa where corn on corn rotations are somewhat rare, the likelihood of extended diapause northern corn rootworm finally reaching Missouri is high. To confirm that beetles are emerging from rotated cornfields and not migrating into the fields, Crop-Trak consultants have searched for and found evidence of larval root feeding in rotated corn. Larvae and adults were also discovered by the float method in which corn roots are submerged in water causing underground larvae, pupae, or adults to float to the surface. Further confirmation of extended-diapause rootworm in Missouri is still needed. MFA’s Crop-Trak is working with the USDA, Kansas State University and the University of Minnesota for the necessary laboratory and field-testing needed to verify the cause of increased northern corn rootworm populations.

The possibility of extended-diapause northern corn rootworms in Missouri will greatly impact growers in Missouri. The two most effective methods of rootworm control, if crop rotation is not effective, is the use of Bt-rootworm hybrid traits such as SmartStax from Monsanto or Syngenta’s Duracade; or soil applied granular insecticides such as Aztec or Force. Discuss this issue with your MFA location or Crop-Trak consultant to assess rootworm threat levels and management strategies.