Bt Battle Strategy

EPA wants advice on ways to slow insect resistance to Bt crops.

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Secondary lepidopteran pests, such as the western bean cutworm, have evolved resistance to Bt corn in recent years, Image by Frank Peairs, Colorado State University

Is Bt broken for ear-feeding pests? And, if so, can we rescue it?

Those are the questions that compelled EPA to convene a Scientific Advisory Panel in Arlington, Virginia, this July for advice on how to save the Bt technology of today and tomorrow.

“EPA has concluded that the current [integrated refuge management, IRM] approach may be inadequate to mitigate resistance risks in lepidopteran pests of Bt,” says Jeannette Martinez, a senior scientist at EPA, who spoke to the panel.

Companies react too slowly to Bt resistance in the field, and current refuge requirements weren’t designed to deal with the current size of Bt acreage, the biology of many lepidopteran pests and the behavior of growers using the traits, EPA concludes. The agency asked the panel to give expert guidance on these issues, which will shape EPA decisions on future Bt crop registrations and resistance-management strategies.

The goal is not only to extend the life span of current Bt proteins but to protect the lone remaining Bt trait with no documented insect resistance in the U.S., Vip3A, notes Pat Porter, an entomologist at Texas A&M.

“We know the other toxins are failing, and we want to keep Vip3A effective for at least eight years or so until we get something else,” he told DTN/The Progressive Farmer. “For Southern entomologists like me, we know that what is decided in this [Scientific Advisory Panel] will, to a large degree, determine how long Vip3A lasts.”

SLOW TO REACT. In the South this year, farmers sprayed Bt cotton with insecticides to control large populations of Bt-resistant bollworm streaming out of Bt corn fields, where most growers refuse to plant refuges. Farther north and west, growers are advised to scout Bt hybrids with Cry1F for western bean cutworm and treat if they find it, while still paying the technology fee that comes with that trait.

Martinez points out that EPA hasn’t heard about these Bt failures directly from the companies that manufacture, sell and steward the traits.

“EPA has become aware of resistance cases in four lepidopteran pests of Bt, but only one was reported through the regulatory process in place,” Martinez explains. “Three cases were published by academic research scientists.”

The IRM policies put in place by EPA for Bt crops were designed to make companies monitor and report resistance to Bt in a timely fashion. Instead, a combination of factors has laid the task of rapidly detecting, documenting and reporting Bt resistance at the doorstep of academic scientists, Porter notes.

Companies currently wait for reports of unexpected damage in a field to investigate and test for resistant insects. But, by the time resistant insects make up enough of a population to inflict visible damage, the fight is over--resistant genes are prominent within the insect population, Martinez noted in her presentation to the advisory panel.

To add to the problem, it can be difficult and time-consuming to successfully rear, test and formally diagnose resistance in lepidopteran insects in the laboratory. Industry and academic scientists routinely disagree on how best to execute these processes, which can lead to inconsistent results.

Only once laboratory results have confirmed resistance do companies step in and start to “mitigate” the field resistance--that is, advise the grower to rotate crops, change Bt traits or use pyramided products.

“At best, implementing mitigation would start one year after the unexpected injury report; but, for corn earworm, this would already be four to five generations past [the initial resistance report],” Martinez notes. “Lepidopteran resistance is likely widespread by that time … because we are dealing with pests with big dispersal capabilities.”

Into this large gap of time--from the moment the first resistant genes surface in a field to the time companies finally notice, test and react--step academic scientists. They often observe and test for resistant insects in their research plots much sooner and, most importantly, make their results public, Porter says.

But, researchers aren’t always guaranteed access to Bt seed to run tests, and their results and conclusions about Bt resistance are sometimes ignored or downplayed by the industry, Porter notes. Rarely do they inspire a change in resistance management from companies, where decisions are driven more by economic forces.

“Some of us now realize that our real jobs might be in helping deal with the consequences of resistance--as the [integrated refuge management] plans will be what they will be for better or worse,” Porter says.

CURRENT STRATEGY. Beyond slow reaction times, EPA presented the panel with a series of additional problems with current Bt crop management that have spurred insect resistance in ear-feeding pests:

> Most Bt traits were registered with the assumption that they were “high dose”--that is they killed 99.99% of insects exposed to them. As it turns out, with the exception of stalk borers, this is not true of most lepidopteran pests, which survive Bt exposure at higher rates than expected (low to moderate dose). For these low- to moderate-dose pests, the current Bt refuges are not adequate--hence the rise in Bt resistance in cotton bollworm (corn earworm), fall armyworm and western bean cutworm.

> Secondary pests of corn, namely western bean cutworm, have become more prominent in parts of the country, and current Bt refuge-management strategies are not designed to manage resistance in them.

> Many insects have “cross resistance” to multiple Bt traits because of the similar mode of action among some Bt proteins.

> The same, or very similar, Bt proteins are used in both corn and cotton, which gives insects repeated exposure to the same traits within a single growing season. This is particularly problematic in the South, where a pest like corn earworm is exposed to Bt in corn before moving to Bt cotton.

> “Pyramid” Bt products, which contain multiple Bt proteins, do not function as they should. Insects are often resistant to all but one of the traits within the pyramid, functionally rendering it a single-toxin product with a smaller refuge.

> Refuge compliance is very low in the Southern U.S. despite adequate education.

> The current solution to low refuge compliance--refuge-in-a-bag (RIB) corn hybrids, also known as seed blends--likely speeds Bt resistance in ear-feeding insects. Cross-pollination between refuge and Bt corn results in kernels that express a mosaic of Bt traits--single, double or triple toxins. Since very few true non-Bt refuge ears exist, insects can feed selectively on Bt and non-Bt kernels, and survive to pass on resistance genes.

PATH FORWARD. In addition to EPA’s presentation, the panel heard from Bt registrants--Syngenta, Monsanto and Corteva (formerly Dow AgroSciences and DuPont Pioneer)--as well as industry groups.

Their comments boiled down to two requests to EPA. First, all of them asked the agency not to design new resistance-management strategies based on secondary pests such as western bean cutworm but to instead focus on primary pests such as bollworm.

Second, led by Monsanto, now Bayer, a number of company and industry representatives asked the panel to consider allowing seed blend (RIB) corn hybrids with Vip3A to be marketed and sold in the Southern U.S. despite academic research showing this type of refuge can speed resistance in ear-feeding pests. The seed companies presented data from internal studies that they said show the practice is comparable to block refuges given low grower compliance in the South.

The Scientific Advisory Panel will produce a report with its recommendations on these issues to EPA, and then the agency will have to decide how to fix the current system of managing Bt resistance, Martinez says.

“The goal for this meeting is to receive science recommendations from the panel that will provide support to improve the current IRM program and extend the lifetime of Bt [plant-incorporated protectants] or future technologies coming,” she concludes.


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