Bt corn trait selection determines caterpillar pest control

Writer: Kay Ledbetter, 806-677-5608, skledbetter@ag.tamu.edu
Contact: Dr. Ed Bynum, 806-677-5600, ebynum@ag.tamu.edu

AMARILLO – Newly introduced caterpillar control technology has corn producers weighing the benefits of paying more for multiple toxin Bt corn seed, said a Texas A&M AgriLife Extension Service entomologist.

The decision will depend on what pests are in the field, said Dr. Ed Bynum, AgriLife Extension entomologist in Amarillo.

“If you grow corn in the northern Panhandle and traditionally battle western bean cutworm, then the more toxins the better,” Bynum said.

Bynum and Dr. Pat Porter, AgriLife Extension entomologist in Lubbock, and Blayne Reed, AgriLife Extension integrated pest management agent for Floyd, Hale and Swisher counties, conducted field trials on multiple seed company products with added toxins in 2015. They were determining the likely benefits of Bt pollen pollinating the non-Bt refuge ears in seed blend refuge fields.

Bacillus thuringiensis, referenced as Bt, is a spore-forming soil bacterium that produces protein crystals toxic to many types of insects. The toxic protein crystals are referred to as Cry toxins, Bynum explained.

He said the Cry toxins based on Bt have the following general toxicity profile: lepidoptera – Cry1; lepidoptera and diptera – Cry2; coleopteran – Cry3, and diptera – Cry4.

Transgenic corn is inserted with a Bt gene, a promoter gene and a marker gene, Bynum said. The promoter gene allows the Bt gene to be turned on and different promoter genes may allow the Bt toxin to be expressed at different times of the year or in different parts of the plant.

Bynum said these toxins are species specific in insects, and humans and other vertebrates are not affected by the toxins.

Within the Bt technology, there is the ability to stack traits or pyramid them, he said.         Stacked traits are those that target two completely different types of organisms. An example would be a corn variety that had two very different Bt toxins for both corn rootworm control and corn borer control.

A pyramid is a suite of different toxins intended to control organisms that are similar. For example, a Bt hybrid that has three different toxins all intended to control various lepidoptera would be a pyramid hybrid.

“Producers need to understand there are differences in insertion packages and insertion events that provide a real difference in insect control in the field,” he said.

So it is important to know which toxins are effective against each of the different caterpillar pests, and whether or not to purchase corn with multiple toxins, Bynum said.

All of the toxins – Cry1Ab, Cry1A.105, Cry2Ab2, Cry1F and Vip3A – are effective against European corn borer and southwestern corn borer, he said. Corn earworm is usually not an economic pest and most toxins, except Vip3a, only suppress corn earworms.

“But fall armyworms and western bean cutworms are major economic pests,” Bynum said. “Cry1F provides limited protection against both fall armyworm and western bean cutworm, while Vip3A has provided good protection against both of these caterpillar pests.”

For these ear pests, having corn hybrids with multiple caterpillar toxins will provide better protection than hybrids with a single toxin, he said.

“If you grow corn in the northern Panhandle and traditionally battle western bean cutworm, then more toxins are better,” Bynum said. “If you grow corn in areas where fall armyworm is usually a problem, like the southern High Plains, then late-planted corn usually has the worst fall armyworm damage.

The most common scenario would be to plant cheaper Bt corn early when less fall armyworm pressure is expected at pollination and grain fill, but use the more expensive Bt corn with Vip3a as one of the toxins if corn is planted late.”

He said there are effective insecticides for fall armyworms, but timing the applications is difficult, so Bt corn is the first line of defense.

“If you grow non-Bt corn, then insecticides will be helpful if they are needed, but planting toward the early side of the window would be a good idea,” Bynum said. “Late planting of non-Bt corn, especially in organic production where there is only one moderately effective organic insecticide available is dangerous.”

Another consideration is the requirements based on the refuge zones in the Texas Panhandle.

As part of the U.S. Environmental Protection Agency’s resistance management program, Bt hybrids are required to have a certain percentage of non-Bt corn plants to serve as a refuge to keep pests from developing resistance to the Bt toxins, he said.

Most multiple toxin hybrids have a 5 percent refuge in the “corn zone,” which is basically the two tiers of counties at the top of the Panhandle, and many are available in this zone as seed blend refuges, Bynum said.

However, seed blends are not supposed to be sold in the “cotton zone,” he said. Even though seed blends are not supposed to be sold in the cotton zone, they can be planted in the cotton zone, but 20 percent or 50 percent structured refuges of block plantings or strips no less than four rows wide are still required.

Individuals can go online at http://ncga.com/irm-calculator to determine the refuge requirements for his corn hybids.