Corn rootworm with resistance to Bt corn hybrids has been found in Cumberland and Mifflin counties in Pennsylvania and near Ithaca, NY. This means dairies with fields in continuous corn need to look closely at which hybrids they order this winter.
There basically are four Bt toxin strains. If you have used hybrids with the Cry3Bb1 or mCry3A toxins, Penn State Entomologist John Tooker recommends you rotate to the Cry34/35. Cross-resistance has been found between to Cry3Bb1 and mCry3A toxins.
You may have noticed fields this fall with lodged or goose-necking plants. When Tooker did root ratings in Mifflin County, he found rootworms removed 2-2.5 nodes of roots (on a 3-point scale) from many plants. “At this point it is ‘suspected resistance’,” Tooker emphasizes. However, all indicators point to an emerging situation similar to what has been observed in much of the Midwest where resistance to Bt is widely documented.
“The best cure for corn rootworm,” Tooker notes, is rotating out of corn to alfalfa or beans. However, management plans at many dairies require continuous corn. Chemicals help but their potency declines between planting and rootworm emergence (usually June…the same time lightening bugs start to fly).”
The Mifflin County fields were planted in three corn varieties: two that expressed the YieldGard rootworm trait (Cry3Bb1 toxin) and one expressing one of the Agrisure rootworm traits (mCry3A toxin). Tooker did gene checks on the plants to confirm they were producing the appropriate rootworm-targeting toxins. They were.
“Importantly, all these features are identical to those of fields in the Midwest where resistance to Bt by rootworm populations has been confirmed by university entomologists,” Tooker says.
He recommends moving to a pyramid or “stacked” hybrid that offers two Bts. There are four Bt toxin strains: Cry3Bb1, Cry34/35, mCry3A, and eCry3.1Ab. If you have used hybrids with the Cry3Bb1 or mCry3A toxins, rotate to Cry34/35. Cross-resistance has been found between Cry3Bb1 and mCry3A toxins, so rootworms resistant to one are resistant to the other. By using a Cry34/35 hybrid, no matter which of the susceptible lines you have been using, you know you have a new Bt toxin and that should work.
“In the northeastern U.S., the only reason to manage for corn rootworm is if you are growing continuous corn,” Tooker says. Talk to your hybrid vendor. Otherwise, you can look forward to fields of stunted, goose-necked corn next Fall.