Bollgard II cotton varieties could be the closest thing to worm-proof cotton that growers have ever seen, but they may not have as much economic benefit to producers in the northern reaches of the Cotton Belt, according to a University of Tennessee entomologist.
Cotton varieties with Bollgard II are expected to be available in 2003, although EPA has not granted final approval to Monsanto for the new technology. Bollgard II expresses two Bt proteins — Cry 1 Ac and Cry 2 Ab — which are toxic to caterpillar pests. Original Bollgard, on the market for seven years, expresses only the Cry 1 Ac protein.
According to University of Tennessee cotton insect specialist Scott Stewart, Cry 2 Ab is not necessarily better than the original protein. “It's a little better on armyworms and loopers, but it is not as good as Cry 1 Ac on bollworm and budworm.”
Stewart spoke on Bt cotton at the recent Milan No-Till Field Day.
On the other hand, two proteins are better than one for broadening control of caterpillar pests and for managing resistance to Bt cotton. In addition, Stewart noted, the second protein is in higher concentrations in the plant.
“Cry 1 Ac levels are usually expressed from 1 to 3 parts per million while Cry 2 Ab is expressed from 7 to 19 parts per million. So it's really a dosage effect.”
The take-home message, according to Stewart, “is that Bollgard II is going to be better than Bollgard. In my opinion, it's going to give essentially complete caterpillar control.”
Stewart says the value of Bollgard II will depend largely on where you farm. “The benefits to west Tennessee growers is not going to be as much as it is for growers in south Georgia and south Mississippi. That's because we don't have loopers and fall armyworms on a consistent basis in Tennessee.”
Stewart noted that both original Bollgard and Bollgard II will be available for several years after the release of Bollgard II, but eventually, Bollgard will be phased out to comply with EPA resistance management requirements.
WALT MULLINS, Bollgard technical manager for Monsanto, hopes that EPA is mindful of the problems that could crop up from too quick a phase out.
The industry would like to avoid a situation similar to what happened when transgenics were first introduced in the mid- to late-1990s, “when growers rushed to buy the technology without considering that the variety aspect was very important,” Mullins said in a recent interview.
When you add the technology to a variety “you can't always assume that the only change will be the technology,” Mullins said. “It will take some time to make sure we have good varieties with Bollgard II, ones that will fit all the areas where we have Bollgard penetration.”
On the other side of the issue is EPA's concern that “every year you have Bollgard out there, it puts the Bollgard II product at risk,” Mullins said. “If we get resistance to Cry 1 Ac in Bollgard, EPA can say that you only have a single-gene product in your Bollgard II. That's what we're trying to balance.”
EPA has not imposed a phase-out schedule for original Bollgard, but could if resistance to Bollgard begins to occur, Mullins added.
Tests on original Bollgard in Mississippi show that it's doing its job, according to Stewart, a former Extension entomologist in Mississippi.
“In replicated large-plot field experiments over five years in Mississippi, Bollgard cotton reduced Heliothine larvae 56 percent, compared to non-Bt cotton. That percentage probably underestimates the extent to which Bollgard was controlling the caterpillars because a lot of times, we would catch larvae that had just hatched out and hadn't had the chance to feed and die.
“We did see a 77 percent reduction in Heliothine damage in squares and that's more reflective of what Bollgard can do to tobacco budworm and cotton bollworm populations. We didn't see 100 percent reduction and the point here is that bollworms can do damage on Bollgard cotton.”
Stewart noted that a six-year study by Mississippi State University Extension entomologist Blake Layton showed that in non-Bt fields, growers sprayed for tobacco budworm and cotton bollworm an average of 3.1 times per acre per season. In Bt fields, they sprayed 0.7 time per acre.
“That's a significant reduction, but not a 100 percent reduction because it was necessary to spray over the top of Bt cotton to control bollworms and occasionally some other caterpillars.”
The study also found a significant reduction in boll damage in the Bt cotton compared to the non-Bt cotton. “We did see some boll damage and some of that could have been from fall armyworms. But the technology is doing a good job of controlling budworms and bollworms.”
Stewart noted that while Bollgard cotton is sprayed less than non-Bt cotton, “we are losing a little bit of that benefit because when we remove sprays targeted toward budworm and bollworm, we let pests like stinkbugs, plant bugs and weevils sneak up on us and we end up having to spray cotton a little more for those pests.”
Studies indicate that Bollgard II could create a similar situation by eliminating oversprays for cotton bollworm, which growers often had to make in cotton varieties with original Bollgard.
Stewart noted a study by John Adamczyk, USDA scientist in Stoneville, Miss., which showed that Bollgard II significantly improved control of cotton bollworms, compared to original Bollgard. It also provided excellent control of loopers, fall armyworms and beet armyworms.
“We're getting 95 to 97 percent reduction in caterpillar populations with Bollgard II,” Stewart said.
Stewart also pointed to yield data collected by UT Extension entomologist Gary Lentz in 2000-01 which reinforces the need for a carefully thought out transition from original Bollgard to Bollgard II.
“In some tests, Bollgard II plots yielded less than Bollgard and non-Bt plots. That may concern you a little bit,” Stewart said. “This is not the result of insect control. What is happening is that the experimental lines of Bollgard II are under development and may not be quite to the commercial stage yet.”