Since 1996, corn and cotton farmers have rapidly adopted the use of Bt crops. They could plant and rest, knowing the Bacillus thuringiensis (Bt) protein would protect their crop from hungry insects. However, Bt is losing its power.
“We never expected these Bt crops to last forever — we knew insects would gain resistance,” says Pat Porter, Texas A&M Extension entomologist. “We use refuge to slow it, and we made it to our 20-year goal for most pests. Now we’re seeing more and more resistance.”
Mother Nature Always Finds A Way
When it comes to insect resistance it’s not “if” it happens but when. Matthew Carrol, Bayer head of corn insect resistance management, recommends these practices to help slow insects‘ evolution:
- Use pyramided traits for a multi-pronged defense.
- Make refuge a priority so susceptible pests breed with resistant pests.
- Consider options specific to the pest. For example, rotate crops if dealing with corn rootworm.
It’s important to be careful when using insecticides to treat pests as they can develop resistance, too. Carroll says your best bet is to apply an over-the-top insecticide evenly across the field and to monitor it for escapes.
Something is Better Than Nothing
“There have been multiple studies based on computer models that show seed blend refuge can slow the development of resistance when compared to no refuge at all,” Porter explains. “However, the same studies also show that block refuges are superior to seed blend refuges in slowing the development of resistance.
“But, we know the compliance with structured block refuge is very low, so the argument is if growers aren’t going to plant the best kind of refuge we’re better off if they use the lesser refuge than nothing at all,” he adds.
Who’s on the hook for non-compliance? Seed companies. EPA has an agreement with seed companies, who are responsible for enforcing refuge requirements with their growers. There is no financial penalty.
Certain experts suggest seed blend refuge has more benefits than just grower compliance.
”Seed blends distribute refuge plants randomly throughout the field, which improves the mixing of adult susceptible insects coming off non-Bt plants and those rare resistant insects coming off Bt plants,“ says Tim O'Brien, Syngenta commercial traits manager. ”Secondly, because refuge plants are interspersed randomly throughout the field, growers are forced to manage the field as a whole unit, and any insecticide application is equally applied to Bt and non-Bt plants.“
Resistance is Spreading
For corn earworm/cotton bollworm, there‘s only one remaining effective in-plant mode of action, Vip3A, to control the pest. However, the Vip3A trait has confirmed resistance in fall armyworm.
While there is no known resistance to Vip3A in field populations of corn earworm, individual insects collected from the field and tested in the lab have been found to have some level of resistance. So, the genes for resistance are in the wild populations. (Turn to page 16 to learn more about corn earworm/cotton bollworm resistance.)
Blended Refuge Increases Risk
Porter fought against seed blend refuge in 1996, and today he still sees hazards.
Partially resistant insects ”can move from plant to plant and live to reproduce, potentially spreading resistance,” he says. “The risk we see with seed blend refuge is this big question: How dangerous are the single toxins expressed in non-Bt refuge ears that have been cross pollinated by Bt pollen? We know the toxins are now expressed at lower doses in these ears and might be accelerating resistance. That’s what scares us all.”
”[Seed blend refuge] provides built-in refuge compliance,” EPA explains. ”However, block refuge, when followed, is more effective for ear-feeding pests, such as corn earworm.”
Bt Traits Aren’t All Equal
For certain insects, Bt proteins are high dose, which means they kill almost all of the pests. The only survivors would be those with two resistance alleles, says Pat Porter, Texas A&M Extension entomologist.
Scientists estimate there should only be one in 1,000 or even 10,000 insects with the resistance allele. However, their offspring would be unlikely to survive because they would likely be heterozygous, meaning they have one resistant and one susceptible allele and would be killed by the high dose.
“Most older Bt toxins are high dose for European corn borer, and they have held up for 20 years,” Porter says.
However, for many pests the Bt toxin isn’t a high dose, which means even a single resistant allele might allow the insect to survive. For example, armyworm has resistance to multiple traits on the market that once controlled the caterpillar.
“If you grow Bt crops and see unacceptable pest populations you need to spray them with insecticide. What happened recently with western bean cutworm is a great example,” Porter says. “Farmers weren’t checking their fields because they didn’t think or know they needed to. It was disastrous, and now farmers are scouting in areas with resistance to the Cry1F and Vip3A traits.”