Rootworm Realities

LAWRENCE, Kan. (DTN) -- Along with the flowers, budding trees and April showers, spring brings a less welcome development for Corn Belt farmers: Continued spread of Bt-resistant western corn rootworm.

Three more Illinois counties --- Sangamon, Mercer and McDonough -- have confirmed western corn rootworm populations resistant to Bt-hybrids that express the Cry3Bb1 protein. University of Illinois entomologist Michael Gray announced the latest confirmations in a university pest bulletin on April 3. Since the first documented case of Bt-resistant rootworm in 2009 in Iowa, confirmed and suspected cases of it have surfaced throughout the Corn Belt.

In his announcement, Gray urged growers to "aggressively" diversify their rootworm management programs to slow the development of Bt-resistance.

However, many entomologists' recommendations -- to rotate crops, move to hybrids expressing more than one Bt-protein for rootworm, rotate the types of Bt-proteins and use non-Bt hybrids with a soil insecticide -- aren't making it very far into the field. While these steps are supported by peer-reviewed science and data, their implementation often clashes with the preferred practices and realities of farming, some Corn Belt growers told DTN.

REVISITING SOIL INSECTICIDES

Macon County, Ill., farmer David Brown is watching rootworm resistance chip away at Bt-technology in real time. He estimates that rootworms cost him 60 to 75 bushels per acre in yield last year in a first-year cornfield planted to a Bt-hybrid that expresses Cry3Bb1.

Brown's field may represent an even more dangerous form of resistance -- Bt-resistant rootworms that appear able to survive crop rotation. Gray found a potential case of this type of resistance in the Illinois counties of Kankakee and Livingston last summer. Bioassays are underway to confirm it.

With failures like these, many farmers are reaching for older rootworm tools, namely planting-time soil insecticides.

Ben Riensche, a corn farmer near Waterloo, Iowa, is considering adding soil insecticides to his Bt-hybrids as an insurance policy. "We think there will be financial benefit to adding insecticide to the program," he told DTN. "We can see that time is running out on the technology clock, and that we're going to have to go back to some older methods."

Gray and other scientists said they do not recommend adding soil insecticides to Bt-hybrids. The concern stems from studies showing that adding soil insecticides to Bt-hybrids can actually encourage or speed up Bt-resistance in that field. Using a soil insecticide along with a Cry3Bb1 Bt-trait hybrid in problem fields might be necessary, but pyramided, or multi-rootworm-trait Bt-products, should provide enough protection, Gray noted.

Bt-corn has been shown to delay the emergence of rootworm beetles, and adding a soil insecticide boosts that trend, Gray explained. "One of assumptions when we use Bt-hybrids along with refuges is that you get a random mating of individuals -- resistant and susceptible," he said. "By pushing back or delaying the time in which the adults are emerging from fields, you end up with more non-random mating, so the likelihood of resistant individuals coming together tends to increase. Where soil insecticides are applied to Bt-hybrids, it even further skews that emergence later into the season."

Gray and other entomologists instead recommend using a non-Bt corn hybrid with a planting time soil insecticide, a tactic many farmers like Riensche find infeasible.

"The inherent problem is most of the seed companies have shifted their elite, newest, best genetics into their traited corn," Riensche said. "So if you don't plant the (Bt) trait, you're really giving up genetic potential."

Using an older germplasm doesn't line up with his farming needs, Riensche concluded. "It doesn't fit my budget," he told DTN. "I have farm payments to make. Big farm payments."

Brown was surprised to hear that scientists are worried about farmers adding soil insecticides to Bt-hybrids. He said the issue exemplified his overall frustration with the difficulty of getting reliable, clear information about the latest products and tactics to fight rootworms.

"Where does my due diligence start and where does it stop?" Brown said. "You buy these products with the confidence that you're going to have protection. I feel like I'm pretty well informed, but I was not hearing this."

THE WEAK LINK IN PYRAMIDED TRAITS

Because resistance has been officially documented in only two Bt-proteins, mCry3A and Cry3Bb1, many farmers are under the impression that pyramided Bt-hybrids that hold two or more rootworm-specific Bt-proteins are much less vulnerable.

Yet, entomologists say this perception is misleading. "It's not a long-term solution to resistance management because for many of these pyramided products -- especially in areas where resistance has been confirmed to one of the proteins -- in effect you only have one protein that's doing the bulk of the rootworm protection in a lot of fields," Gray explained.

The pyramided products have made refuge compliance easier by mixing non-Bt seeds into the bag, but scientists warn that these products are not fail proof. "Keep in mind too that these pyramids have been registered with refuge requirements that have been reduced from the historical 20% structured refuges to 5% seed blends in refuges in a bag," Gray warned. "So you're not only putting a lot of selection pressure on the one protein that may be still working in fields where resistance has been confirmed, but the refuge has shrunk."

The longer-term solution needs to involve alternating these pyramided products within a more diverse crop rotation, entomologists have noted.

Brown said while commodity prices drove lots of farmers in his area to continuous corn, that trend might be slowing. "I'm seeing a lot more soybeans," he said. "The guys that did corn-on-corn through the years at some point in time or another, they got burnt. It was economically driven, but it was bad business."

But over in northeast Iowa, simple economics still rule for Riensche. "Right now, my usual crop mix is two-thirds corn and one-third soybeans, because corn is generally more profitable," he said. Only a rise in the profitability of soybeans would encourage him to add more beans to his rotation, he added.

In the meantime, he'll stick to rotating corn varieties and using beans in problem fields. "We do try to rotate brands of corn," he said. "We don't put same traits same place every year. And where we have severe problems, we rotate to soybeans."

Overall, Riensche said the demands of running a large farming operation don't give him the leisure to think through every rootworm development and problem. "I've got 1,000 problems, and worrying about corn rootworm resistance is number 993," he said.

"We'll just use this until the horse is dead, and then they'll come out with something new," he concluded.

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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