Bt technology in corn helps in mitigating corn borers as well as a few other pests, but it is important to use strategies to prevent or delay resistance.

Scott Stewart, with the University of Tennessee Department of Entomology and Plant Pathology and UT Extension, discussed the basics of Bt or GMO technologies in corn. During the virtual Milan No-Till Field Day presentation, Stewart answered six questions on Bt technology, how it is used in corn, and how to prevent resistance.

What pests are affected by Bt technology?

Most of the Bt technologies used in corn affect two groups of insects. In Tennessee, the ones we're most interested in controlling are the caterpillar pests. In the Midwest, where beetle pests are significant, different Bt traits are used to control corn rootworms. Bt corn grown in Tennessee contains toxins called Cry proteins. They may also produce a toxin called a Vip protein, and there are different kinds of Cry proteins such as Cry1, Cry2, Cry2Ab, etc. This class of Bt toxins has different spectrums of activity, and some are better at controlling corn earworms than others.

Now, Bt corn can express two, three, or more different Bt traits, and this expands its control over a wider group of insects. One insect we're concerned about in west Tennessee is the southwestern corn borer. There's another one called the European corn borer, which is also a significant pest of corn in the Midwest.

Related:How to identify and manage for corn ear rots

What other pests are controlled by Bt technology and how well are they controlled?

The newer Bt technologies can control corn earworm very well or very poorly, depending on the group of toxins. Fall armyworms are another one we occasionally see in west Tennessee, and how well Bt corn works on fall armyworm depends on the type you get.

When Bt corn was first introduced, the primary target pest was the corn borer species, either southwestern or European. In my opinion, this is still the primary pest the technology controls. The good news is all the Bt corn technologies provide excellent control of southwestern and European corn borer. When we talk about corn earworm, though, I have a hard time proving to anybody, let alone myself, that the pest is causing much economic damage in corn, so corn earworm doesn't concern me much.

You did some research on the economics of Bt technology. What did you find?

In my test plots over the last several years in Milan, we've seen anywhere from a 20-, 40-, and 50-bushel yield responses from growing Bt corn compared to the same variety without Bt technology. It is more consistent in your later planted crops, which makes sense because later plantings tend to face higher populations, and they tend to be more susceptible when they get infested. It is hard in today's environment to know what value Bt has because so many people are growing it. If we weren't growing so much of it, though, we'd have much worse corn borer problems.

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What are some examples of different Bt traits?

I mentioned the Cry proteins, which help to control certain beetles. They have activity specifically when dealing with some of the corn rootworm species. This technology works best on the western corn rootworm. In the South, we rarely see problems with that pest, so we don't traditionally grow the technologies that have rootworm activity. You must study corn varieties because the variety name doesn't necessarily give away what Bt technology is used.

The original Bt corn was called YieldGard, which worked well on corn borer. Now, we have hybrids with multiple Bt gene for control of various caterpillars and some have Bt traits for rootworm control. We have corn which has a Bt Vip or Viptera trait, which improves control of corn earworm. It’s complicated.

For more information, go to https://guide.utcrops.com/, which has disease and insect field guides with management recommendations listed under the pests.

You mentioned that Bt corn is widely used in Tennessee. How do farmers manage for resistance?

Because it is widely used and effective on some of the primary target pests, resistance is a concern. The primary method to mitigate resistance is by making sure we grow a refuge of non-Bt corn. In corn, there's a mandatory refuge requirement. For example, in designated 'cotton-growing counties' in Tennessee, you're supposed to have, at least, 20% non-Bt corn planted. Another method hoped to prevent resistance in corn borer populations is to plant hybrids with multiple Bt traits.

The purpose of this non-Bt corn refuge is to produce insects that haven't been selected for resistance to Bt technologies. The idea is that, if you select for resistance in the rest of your Bt crop, a few rare individuals survive and resists the technology. These resistant insects would hopefully mate with the insects coming out of the refuge of the non-Bt crop. When the genes blend, the offspring are presumed to be susceptible. They will likely die if they feed on Bt corn, removing the resistance genes from the population.

What is the take-home message on Bt corn?

I'm a big fan of Bt corn to control corn borers. You can manage for corn borers with insecticides, but it's tricky and requires expertise. I think we need to use Bt corn to its fullest capacity allowed. If we start growing a lot of non-Bt corn, those corn borers will sneak back in and start causing yield loss. It is important to follow the refuge guidelines to make sure the technology remains durable for a long time.