JEFFERSON CITY, Mo. (DTN) -- During the Battle of Bunker Hill in 1775, American Revolutionary militiamen were famously ordered not to fire on British troops until "you see the whites of their eyes." Now, 250 years later, U.S. farmers are similarly being encouraged to hold back on deploying Bt corn traits until truly needed to battle corn rootworm.
A retrospective study recently published in the journal Science reveals that genetically engineered corn designed to combat rootworm pests is generally overplanted across the U.S. Corn Belt. This overuse costs farmers in two ways -- first by paying seed premiums for traits they don't necessarily need, then by eroding the effectiveness of the technology to control the pests in the future, according to the report.
"Every year you expose a pest population to the traits, you give resistance an opportunity to evolve," said Christian Krupke, an entomologist at Purdue University and lead author on the study. "That has a lot of consequences for the bottom line, not only for technology fees but for the longer-term impacts of resistance. Once you have a resistant population, you can't turn it around. The efficacy of that trait is reduced, whether you stack it with other novel traits or not. You can't put the genie back in the bottle."
The study, which involved 19 other scientists from 12 other universities, analyzed 12 years of field trial data from 10 states: Illinois, Indiana, Iowa, Michigan, Minnesota, Nebraska, North Dakota, Ohio, South Dakota and Wisconsin.
REGIONAL DISPARITIES
The first genetically engineered corn hybrid trait lethal to corn rootworms and derived from the bacterium Bacillus thuringiensis (Bt) was introduced in 2003. Since then, farmers have extensively used Bt corn hybrids targeting rootworms across the Corn Belt. Reports of Bt resistance in rootworms began in 2009, raising doubts about the long-term viability of the traits.
"When we started with these Bts for European corn borers in the late '90s, it was such a homerun and took decades to report any resistance from the field," Krupke said. "It may have given us a false sense of security about how durable subsequent Bt hybrids would be."
Beginning around 15 years ago, university entomologists began observing strong differences in corn rootworm resistance to Bt traits, he said. In states farther to the west, such as Iowa and Nebraska where corn is more often planted continuously, rootworm populations were high and showing high rates of resistance to Bt traits. To the east, in states such as Indiana and Ohio where corn is planted in rotation with other crops, rootworm pressure was low, to the point that it was difficult to find an economically damaging population.
"Even though it's the same pest, same crop, with many of the same hybrids planted in similar climates, there are big differences in how the insect is responding, how the Bt traits are holding up and how those traits should ideally be deployed," Krupke said.
When looking at data from 2014 to 2016, seed premiums for rootworm Bt hybrids cost farmers $24.10 per acre in Western states and $25.70 per acre in Eastern states. Yet yield losses attributed to rootworm damage in those years amounted to 47.5 bushels per acre (bpa) in Western states and 8.5 bpa in Eastern states, the researchers estimated.
"In the Eastern Corn Belt, there was likely way too much Bt corn for rootworms being planted," Krupke said. "The payoff was just not there to justify the use rates."
To illustrate this, the study highlighted 2015 data from Indiana showing that farmers only realized a yield benefit of $9 per acre when using Bt corn hybrids. That benefit only covered roughly one-third of the seed premium for the technology and didn't factor in future costs associated with loss of effectiveness of the trait.
"So, as a farmer, you're really getting no benefit, and you're getting the negative outcome of exposing the insects to the toxin and allowing resistance to slowly creep along," Krupke said. "There just wasn't a lot of positive on (the east) side of the Corn Belt."
THE HYBRID BUNDLE DILEMMA
While bundling might work well for insurance products or streaming television services, it creates a predicament when buying seed corn. Farmers attempt to seek out elite corn hybrid genetics that will increase their overall yield potential in a field, but seed companies often pair those genetics with other traits, such as herbicide tolerance or rootworm Bt traits, that farmers don't necessarily need.
"It's like when you buy a new vehicle. You want the upgraded audio system? Well, it's only available with the navigation package," Krupke said. "You may pay for something you don't need to get what you want. With Bt traits for rootworm, you may be paying for a stack of traits, some of which may not even work anymore or others that you don't need."
In the study, the authors suggest the development of programs to encourage seed companies to broaden trait combinations and genetic diversity in their offerings. They also proposed enhancing transparency in seed marketing through labeling for trait combinations and detailing the seed premium for each trait where possible.
BT BATTLE PLANS
Just as military leaders send out reconnaissance to assess an enemy's position and devise a strategy for engagement, farmers can similarly gather intel on the pests they will likely battle in future years by beefing up their scouting programs.
"Do you have rootworms? Are you getting pressure from seed corn maggots? If you don't know, you're likely going to opt for the most risk-averse approach that covers all the bases because the cost of making an error is potentially large," Krupke said. "When there is no knowledge, you become more cautious, but every farmer has tools to make their own informed decisions."
The entomologist noted that one of the best tools for assessing pest pressure in a field is to plant a check strip of non-treated, non-GMO corn. Yellow sticky traps are also an easy, convenient and reliable method for estimating corn rootworm populations within a cornfield.
"You'll see in that one strip what's out there. That's the barometer of what the biology is," he said. "Now, you can't just plant the strip and walk away. You have to follow up, dig up some roots, scout at certain times. That will give data from that field in that season and will let you know what's coming next year. With many of these pests, the best guide to future pest pressure is past pressure."
In an email to DTN, Shawn Conley, an agronomist at the University of Wisconsin and co-author of the study, wrote that in his state, there has been a larger discussion about the application of unnecessary inputs for risk mitigation and strategies for slowing the selection of resistant pests. He stated the end goal is to save these tools for when they are needed.
"The results of this paper fall right in line with this discussion," he wrote. "In Wisconsin, we have extended rotations and areas where these (Bt) traits are frankly not necessary. Scout and know your risk. Why bear the cost and burn through the technology? It's not a winning long-term sustainability plan."
TRAIT TABLE TACTICS
If a farmer decides that planting rootworm Bt corn is the best option for pest management in a field, the Handy Bt Trait Table -- created and updated regularly by Michigan State University entomologist and study co-author Chris DiFonzo and Texas A&M entomologist Pat Porter -- makes it easier to understand and compare options when deciphering the Bt traits and trait packages currently available.
The table provides an alphabetical list of Bt trait packages in all commercialized corn in the United States. It presents the trade names for traits, Bt event, protein(s) expressed, targeted insects and herbicide traits. The chart was most recently updated in mid-March 2025 and can be accessed here: https://www.texasinsects.org/….
A copy of the full study can be found here: https://digitalcommons.unl.edu/….
Jason Jenkins can be reached at jason.jenkins@dtn.com
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