Still some debate over natural refuges for 2-gene Bt cotton

Most producers applauded EPA last spring when the agency approved a natural refuge option for Bollgard II and Widestrike cotton.

For those who plant Bollgard II and Widestrike cotton, from Monsanto and Dow AgroSciences, respectively, there will be no more dealing with complicated, onerous requirements for a non-Bt cotton refuge, which not only limited options on insect control, but often limited yield as well. EPA's ruling does not affect single-gene Bollgard cotton, which is registered for commercial use through the 2009 season.

However, not all entomologists are completely on board with the natural refuge, noting that there is just as much uncertainty about insect genetics as there is good science.

Originally, structured refuge requirements were imposed by EPA to provide susceptible moths for mating with potentially Bt-resistant moths coming out of Bt cotton. The idea was to hopefully extend the onset of resistance in the early, single-gene Bt product — original Bollgard.

When EPA eliminated the structured refuge requirements for Bollgard II and Widestrike cotton, it accepted Monsanto's premise that two genes were significantly better than one for managing resistance. “It was broadly believed that if you put two genes in the plant that worked differently on the insect, this would reduce the probability of resistant insects surviving both genes and significantly decreasing the time to resistance,” said Walt Mullins, Bollgard technical development lead for Monsanto.

Roger Leonard, research entomologist at the LSU AgCenter, in Winnsboro, La., agrees with the concept that two genes are better than one. “But a caveat is that for biology and for science, it would have been better to have two genes out there before you had one. With one, you already reduce the effectiveness of that one gene. So it's not like one plus one is two. It's like one plus one is 1.5. You've already used up some of the life of that first technology.”

Another debate is how a two-gene Bt affects the need for a structured refuge. EPA acknowledged research showing that the required percentage of refuge needed to produce susceptible insects was significantly less in two-gene products than in single-gene products.

This is the basis for the concept of a natural refuge for two-gene Bt in which natural host plants, including weeds and other crops, can produce an ample supply of susceptible moths.

Research on this subject began at North Carolina State University where scientists had long felt the cotton bollworm, also known as the corn earworm, a major pest of corn, had numerous alternate hosts in the Carolinas and in other parts of the Cotton Belt.

According to North Carolina State research, analysis of Helicoverpa zea (bollworm) wings indicated that so-called C4 host plants, such as corn, are the major larval hosts in the early to mid-season in Louisiana and Texas; that is, 90 to 100 percent of tested moths developed as larvae on a C4 host.

“Later in the season when cotton is an attractive host, less than 50 percent of bollworm moths were produced from C3 hosts — cotton, soybean and other non-grass hosts,” North Carolina State scientists said in a paper presented at the Beltwide Cotton Conferences in 2006.

“Since nearly 100 percent of the early season moths were produced from corn, non-Bt corn appears to be the most useful early season crop host refuge,” the scientists said. “Soybeans serve as a major late season non-Bt host and may play a more pivotal role than a 20 percent sprayed or 5 percent unsprayed non-Bt cotton refuge in delaying resistance evolution to Bt in eastern North Carolina.

“This includes only production on the major crop hosts and ignores production in the numerous H. zea minor crop hosts, wild hosts and immigration of moths from other areas.”

In Heliothis virescens or tobacco budworm, the range of alternate host plants is considered much narrower than that of the bollworm. In the Mid-South, weedy plant species serve as hosts for the first generation of budworms, while subsequent generations are more likely to be associated with cotton.

The same situation occurs with first generation budworms in the Southeast. But later generations of the insect can be found on a number of weedy hosts and on tobacco, cotton, peanuts and, to a lesser extent, on soybeans in North Carolina, according to North Carolina State researchers.

To try to better assess the situation, Monsanto expanded the alternate host studies from North Carolina to five states across the Southeast and Mid-South in 2004 and 2005. In 2004, tobacco budworm populations were so low they could not capture enough moths to make the study worthwhile, said Mullins.

In 2006, the study was expanded to seven states, including Tennessee and southeast Texas. Researchers found that along the East Coast states, 80 percent to 100 percent of the tobacco budworm moths are derived from sources other than cotton.

“In North Carolina and Georgia, we found significant numbers of tobacco budworm moths, but they were largely non-cotton moths all season long,” said Mullins. “We anticipated a certain degree of that based on tobacco as a host. But peanuts are also a fairly good host for tobacco budworms as are soybeans to an extent and other, weedy hosts for the area.

In the Mid-South states of Arkansas, Louisiana, Mississippi and Tennessee, the profile was different because of the larger fields and absence of larger blocks of weedy hosts that are often found in the Southeast.

“But, in the very worst case scenario we ran into, we saw no less than 10 percent of the moths that were non-cotton moths all season long in any location,” said Mullins. “That means if you have 100 moths at least 10 of those moths were non-cotton derived moths. And more typically in Mississippi and northeast Arkansas, it would be closer to 40 to 50 percent.”

Based on that 10 percent level and computer modeling, Monsanto representatives believe the effective natural refuge that would be required based on the strength of a two-gene system would be 0.1 percent.

“So what we said in our proposal to EPA was that when we look at the amount of refuge necessary, that even in the worst case scenario given the lower end of the statistical significance of the number, we still have significantly more refuge than would be required to maintain efficacy for an extended period of time,” said Mullins.

For Roger Leonard, there's just not enough information available to convince him to support the natural refuge concept. “As for the overriding implications of how many insects have to be produced to keep susceptibility out there, there are still some questions in my mind. All that is based on a single model produced by the registrant, Monsanto.”

University of Arkansas entomologist Randy Luttrell says he's comfortable with the natural refuge concept, “because some of the recent research indicates that with bollworm, there are lots of insects coming from lots of places. There are scientific colleagues who have concluded that resistance has evolved, but I don't agree that resistance has evolved.

“The thing that concerns me is what I don't know. Is this refuge of susceptible genes actually sustaining this technology that we've had in place now for more than a decade. I can't see where that's the case, but I wonder. I think American cotton farmers should not assume resistance should not develop. We need to be vigilant.

“As for tobacco budworm, the expression of toxin in those cottons is so high that with a little bit of refuge, resistance management could really work. And we haven't seen any signs at all of there being a problem. With tobacco budworm, I would be more concerned about eliminating the refuge than I would with bollworm, because its populations are probably strictly tied to that crop solely.”

Some scientists and crop consultants note that Mid-South cropping systems can change from year to year, which can impact a natural refuge. Weeds can die from drought, prices can force farmers out of some crops and into others.

And the rules of the game can change, too. For example, For YieldGard VT Pro, which contains two Bt toxins, Monsanto is petitioning EPA to decrease non-Bt refuge requirements in corn, proposing a minimum 20 percent refuge in the Cotton Belt and a 5 percent refuge in the Corn Belt. Current non-Bt corn refuge requirements are 50 percent and 20 percent, respectively.

According to a paper presented by entomologist and IPM specialist Scott Stewart at the 2008 Beltwide Cotton Conferences in Nashville, “In areas where corn borers typically cause yield loss, corn growers could benefit from these relaxed refuge requirements. However, corn is an important host of bollworm. Because new, stacked-Bt technologies have greater efficacy on bollworm, and because similar Bt traits are present in cotton, the impact on Bt resistance management for bollworm should be considered.”

History is also a factor. The despair wrought by tobacco budworm in the mid-1990s is still fresh on the minds of many growers, entomologists and consultants, says Luttrell. “A lot of the people in Louisiana remember what the problems were with pyrethroids and tobacco budworm, and they know what could happen if it completely falls apart.

“The risk of resistance development is big. We don't want to go back to doing what we used to do to control pests.”

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