Northern and western corn rootworms are common and economic insect pests of corn in North Dakota and other corn producing areas of the United States. Corn rootworms cost U.S. corn producers about $1 billion annually in yield losses and input costs to control them. In North Dakota, corn rootworms are most problematic in the southeastern part of the state, where most of the corn acreage is grown.

The larvae (immature worm life stage) of corn rootworm cause most of the damage to corn by feeding on corn roots in the soil (Figure 1). Severe larval root feeding injury can cause up to 50 percent yield losses. Severe corn rootworm feeding injury to roots also can cause plants to become weak and exhibit plant lodging or goose-necking (Figure 2). In addition, the corn rootworm beetle (adult life stage) feeds on leaves, pollen, silks, and immature kernels. However, beetle-feeding injury usually does not cause yield losses in North Dakota.

Corn producers use crop rotation, soil insecticides, and corn hybrids expressing Bacillus thuringiensis Berliner (Bt) endotoxins to manage corn rootworm populations. Rootworm Bt-corn hybrids are specifically toxic to the larval stage. However, pest managers have increasing concerns that corn rootworms are developing resistance to the Bt endotoxins. Recently, it was reported that the western corn rootworms have developed resistance to the Bt corn hybrids expressing the Cry3Bb1 toxin (such as Yieldgard) in Iowa, Minnesota, Illinois, Nebraska, and Indiana. In addition, incomplete or complete cross-resistance also has been documented between Cry3Bb1 and both mCry3A and eCry3.1Ab toxins in populations of western corn rootworms in Iowa and Minnesota. In North Dakota, increasing populations of northern and western corn rootworms were observed in 2013-2015, in spite of the use of Rootworm Bt corn technology.

Corn producers depend on using and planting Rootworm Bt corn hybrids to reduce insecticide inputs for managing this devastating pest. Thus, our objective was to determine if North Dakota populations of northern and western corn rootworms have developed resistance to the following Bt corn endotoxins:  Cry3Bb1, Cry34/35Ab1, and pyramided Cry3Bb1 + Cry34/35Ab1.

Many cases of field evolved-resistance to insecticidal endotoxins expressed by Bt corn hybrids have been documented, but only for populations of western corn rootworms in other states. In 2016, corn rootworm beetles were collected from five North Dakota corn fields and reared in a growth chamber. In 2017-2018, larvae reared from field-collected populations were subjected to single-plant bioassays to screen these North Dakota populations of northern and western corn rootworms for potential resistance to three major Bt corn endotoxins:  Cry3Bb1, Cry34/35Ab1, and pyramided Cry3Bb1 + Cry34/35Ab1.

Results demonstrated the first documented cases of field-evolved incomplete resistance in northern corn rootworm to Cry3Bb1 and Cry34/35Ab1 in selected populations. Complete resistance to Cry3Bb1 also was observed in the one population of western corn rootworm. Increased larval survival on the pyramided Cry3Bb1 + Cry34/35Ab1 corn hybrid was observed in both corn rootworms. However, cross-resistance between Cry3Bb1 and Cry34/35Ab1 was not evident for either corn rootworm.

These findings indicate that field-evolved resistance to Bt toxins is occurring in North Dakota populations of northern and western corn rootworms, and strongly underscore the importance of producer adoption of and adherence to Bt resistance management strategies. The following strategies are recommended to reduce the risk of rootworm populations developing resistance to Bt endotoxins:

  • Crop rotation – Rotate fields annually between corn and non-host crops (i.e., wheat, canola, sunflower). If practical, use a two to four years corn rotation.
  • Plant a conventional (non-Bt) corn hybrid and use a soil insecticide instead of a Bt hybrid, especially when corn rootworm pressure is expected to be low.
  • Always plant the non-Bt corn refuge according to guidelines on the Bt corn seed bag tag.
  • Rotate fields with Bt corn hybrids that have different modes of action each year. For example, use a ‘Cry3’ Bt corn hybrid protein (for example, Cry3Bb1, mCry3A, or eCry3.1ab) in the first year, then switch to a Bt corn hybrid expressing a different mode of action (for example, Cry34/35Ab1) in the subsequent year.
  • Be vigilant in scouting fields for corn rootworm adults and assessing larval root-feeding injury.