Nick Seiter, University of Illinois field crop entomologist, says Illinois farmers should keep corn rootworm pressure and Bt trait resistance front of mind this growing season.

Overall, rootworm populations in Illinois are low. However, populations of both western and northern corn rootworm have resistance to all available Bt traits, particularly in corn-on-corn areas like northern Illinois.

Rotation-resistant western corn rootworm pressure is almost nonexistent. Populations of northern corn rootworm have increased in recent years, but not in 2023.

“Western corn rootworm resistance to Bt traits is increasing in Illinois,” Seiter says. “Western corn rootworm larval survival on single-trait Bt hybrids is increasing at a whopping 7.4% per year.”

Traditionally, there were four Bt proteins and two modes of action, with many combinations on the market, such as:

Seiter says Cry3Bb1, mCry3A and eCry3.1Ab all have similar modes of action against corn rootworm. Cry34/35Ab1 is a distinct mode of action against pest pressure. Cross resistance has been found in western corn rootworm among Cry3Bb1, mCry3A and eCry3.1Ab.

“There’s a dramatic difference between continuous corn and heavy rotation regions in Illinois,” he says. “We see high rates of resistance, failures on pyramided traits and high northern corn rootworm populations north of Interstate 80. But there’s low to no pressure in much of east-central Illinois.”

Fortunately, there’s a new mode of action in the toolbox against Bt trait resistance: RNA-interference, or RNAi. How does it differ from existing Bt proteins on the market?

Bt proteins are available for corn rootworm and caterpillars. Bt proteins cause a pore formation in the gut of the insect and work quickly. The insect stops feeding and dies shortly after. However, resistance is widespread across the state of Illinois.

RNAi, on the other hand, is now available for corn rootworm. RNAi interferes with growth and development in pests and works more slowly. It takes about five days for the insect to stop feeding and die. Lab studies have found the potential for resistance, but none has been found in the field yet.

Rootworm trait packages

Bayer released SmartStax Pro for commercial use in 2022, Corteva had a limited release of Vorceed Enlist in 2023, and Bayer is estimated to commercially release VT4Pro in 2024. Seiter says each product contains a rootworm trait package containing the RNAi mode of action.

Bayer’s SmartStax Pro:

Corteva’s Vorceed Enlist:

Bayer’s VT4Pro:

Seiter says to expect pruning if RNAi traits are challenged with a high Bt-resistant corn rootworm population, and to be aware of northern corn rootworm populations.

“Where unexpected damage is observed and Bt resistance is expected, your best option is to rotate to soybeans to kill the larvae,” Seiter says. “The next best option is to rotate to a soil insecticide or RNAi trait. The absolute worst option for corn rootworm management is to plant continuous corn, using the same trait package.”