By the time you see corn plants falling over in late summer, it’s too late to change the outcome if it’s the result of below-ground feeding by western corn rootworm (CRW) larvae.
CRW larvae target developing corn nodal roots, pruning them as they grow. In severe cases, they can destroy entire nodes, reports Aaron Gassmann, Iowa State University entomologist and professor.
“They’ll feed on the actively growing node and prune it, or basically stop those roots from growing,” says Gassmann. “Sometimes they’ll actually prune it all the way back to the base of the plant. There can be complete nodes that are simply absent.”
The impact on corn yield can be severe. Research from the University of Illinois and the University of Wisconsin indicates that each missing node can reduce yield by 15% to 17%.
Moving Beyond ‘Plant It and Forget It’
While Bt (Bacillus thuringiensis) technology has historically helped farmers control CRW, the pest has evolved. After over a decade of research, Gassmann warns that the “plant Bt and forget it” era is over. Effective management today requires a diversified strategy, particularly in continuous corn fields where the problem is most common.
Gassmann recommends using rotation, varied traits, and in-season scouting to break the cycle. His advice depends heavily on whether Bt resistance is already present in your fields. Here are the two scenarios he recommends:
Scenario A: You Likely Do Not Have Resistance Present
- Don’t rely on Bt alone: Keep it in the toolbox, but diversify.
- Rotate to soybeans: This remains the single best method to address the pest.
- Stay vigilant: Follow refuge requirements and scout routinely.
- Switch it up: Avoid long-term continuous corn with the same trait package.
Scenario B: You Suspect (or Have Confirmed) Resistance
- Rotate immediately: Rotating out of corn is the necessary first step.
- Change the plan for future corn: Consider non-Bt hybrids paired with insecticide, or carefully selected new traits (e.g., RNAi).
- Be selective with sprays: Use adult beetle sprays only when scouting confirms they are well-timed and justified.
- Don’t repeat mistakes: Assume that planting the same Bt hybrid again will only worsen the problem.
Why Bt Alone Is Struggling
Farmers often ask Gassmann why Bt has worked so well against European corn borer but has not been as effective with CRW. The difference lies in the “dose” of toxin, he says.
Bt traits provide a “high dose” of toxin for corn borers. However, Bt traits generally provide a lower dose of toxin for CRW. This allows CRW insects with even a single copy of a resistance gene to survive, mate with other survivors in the same field, and pass that resistance on to the next generation.
Gassmann and his colleagues have documented field-evolved resistance to all four Bt toxins used against rootworm (the three Cry3-based traits and the Gpp34/Tpp35 trait).
A Note on Soil-Applied Insecticides
While combining soil insecticides with Bt can improve standability and protect corn roots in the short term, Gassmann notes it is not a perfect cure:
- It rarely reduces adult emergence significantly.
- It continues to select for resistance (survivors are still Bt-resistant).
- It adds input costs that may not pencil out for farmers’ bottom line.
In many cases, if Bt is clearly failing in your fields, it may make more sense to switch to non‑Bt hybrids plus insecticide, or better yet, rotate out of corn, rather than stack more cost on a compromised trait, he adds.
The complete podcast featuring Aaron Gassmann is available via the Crop Protection Network here.
To help farmers make informed decisions about their seed choices, Chris DiFonzo, professor & field crops entomologist at Michigan State University, provides the Handy Bt Trait Table for U.S. Corn Production, a valuable resource that outlines available Bt traits, their targets, and other key information.
