For Western Corn Rootworm, Combo of Management Methods Critical to Slow Insecticide Resistance

By Andrew Porterfield

The western corn rootworm (Diabrotica virgifera) has been a damaging corn pest since it was first observed in Colorado in 1909. It is now widespread throughout corn-producing states, from Montana in the west through Midwestern states all the way to the Atlantic coast, from New England to Georgia.

The beetle pest causes damage in its larval stage. Smaller larvae, or rootworms, cause damage by feeding on corn root hairs and smaller roots, while larger rootworms feed on primary roots. Current rootworm management consists of rotating crops, soil-applied insecticides, and transgenic corn producing toxins from the Bacillus thuringiensis (Bt) bacteria. But the corn rootworm has begun showing resistance to Bt corn, raising the question of how beneficial these treatments actually are and if alternative methods exist for controlling rootworms.

A study by researchers at Iowa State University, published in in July in the Journal of Economic Entomology, shows that combining Bt corn and soil-applied insecticide may provide short-term economic benefits. However, the study also shows that this combination treatment is not sustainable over multiple growing seasons because of increased rootworm resistance to Bt.

John McCulloch, a Ph.D. student at ISU, and Aaron Gassmann, Ph.D., professor of entomology, conducted a three-year field study (2018, 2019, and 2021) on research and demonstration farms near Ames, Iowa. The researchers used natural populations of corn rootworm in the area that were attracted to two locations. Each replication of the study had four treatments: Bt corn with soil-applied insecticide (tefluthrin), Bt corn without the insecticide, non-Bt corn with the insecticide, and non-Bt corn with no insecticide. The team erected heavy-duty mesh cages over the plots to capture emerging adults. These rootworms were already known to resist the protein produced by Bt corn that was designed to resist insect pests and reduce dependence on pesticides (to which insects can also become resistant).

McCulloch and Gassmann collected 106,792 western corn rootworms and 4,386 north corn rootworms (D. barberi). The lowest root injury and adult emergence were seen in Bt corn with the insecticide. Plots with no Bt corn and no insecticide produced the highest root injury and adult emergence. The greatest rootworm mortality rate (88 percent) was seen in plots with both Bt corn and insecticide. However, this mortality rate, while successful in the short term, is not sufficient to delay evolution of resistance to Bt, the researchers conclude.