October 25, 2004
Farmers don't need a new superstar toxin to fight bugs
WEST LAFAYETTE, Ind. - A new Michael Jordan of toxins isn't required to increase crop protection against bugs as long as the right genes are strategically placed to take their shots at destructive insects, researchers report.
Plants modified with protectant genes designed to kill resistant insects can extend the usefulness of currently used pest-control methods and delay the development of pesticide-resistant bugs, according to Purdue University scientists and their collaborators from the University of Wisconsin-Madison, Monsanto Co., the University of Illinois and the University of California, Davis. The researchers' findings will appear in the December issue of the Journal of Theoretical Biology.
"We always thought that it would take a Michael Jordan of toxins - a superstar of toxins to effectively halt insect resistance to the current generation of insecticides," said Barry Pittendrigh, a Purdue associate professor of entomology and lead author of the study. "We found that moderately effective genetically engineered protectants used in plants in the buffer zone around the main crops can play a major role in insect control, and they should be easier to identify than highly effective protectants.
"You don't find a superstar very often, but it may not be difficult to find good players, or worthwhile insect-control agents."
Farmers who use bioengineered crop protectants also use a buffer, or refuge, around the outside of fields that contains plants lacking the high-toxicity genetic modification in the main field that kills most insects. The refuge, usually about 20 percent of the acreage planted, delays development of insects resistant to the main-field, high-toxicity protectants, but some individuals in the destructive insect group have genes that allow them to survive.
Using a computer model, the scientists determined that within a refuge, one could add a moderate plant protectant, or journeyman player, that kills 30 percent to 50 percent of insects that carry a rare resistance gene.
If developed to a practical level, equipping the refuge with a moderately toxic protectant gene could dramatically delay development of new resistant insects that could attack the main crop, Pittendrigh said.
"When we first started this project, we didn't believe that you could use a genetic toxin that was effective in killing a moderate number of resistant insects, so this finding was very surprising," he said.
Over time, insects exposed to specific plant protectants undergo genetic changes so the highly effective genetic toxins no longer affect them. This latest research suggests it may be easier than previously thought to find commercially viable protectants to control these resistant insects because moderate-toxicity protectant genes are much easier to discover than high-toxicity superstars.
The specific problem the researchers attacked is that insects susceptible to the high-toxicity genetic protectant used in the main field crops can survive, breed and reproduce in the refuge. Farmers, who now use crops with high-toxicity protectant genes to fight bugs, don't use those plants in the refuge. So the crops in the border area are susceptible to insect attack.
When susceptible insects from the refuge breed with each other or with resistant insects, the high-toxicity genetically protected plants in the main fields still kill most of the bugs' offspring.
A moderately effective genetic modification inserted into crops specifically to kill resistant insects that survive in the refuge can lengthen the usefulness of the primary genetic protectant used in the main field, Pittendrigh said. These specially designed refuge-area protectants create a phenomenon called negative cross-resistance because the moderate-toxicity protectant kills the insects that are resistant to the primary protectant.
"If we could discover and use moderately effective negative cross-resistance compounds in a refuge, it would work just like an oil filter in a car," Pittendrigh said. "Like the oil filter removing impurities, the refuge with negative cross-resistance protectants could eliminate many of the genetically resistant insects that otherwise might invade the main crop.
"We used mathematical models to test this concept, and we were very surprised by the findings. Although these results are exciting, we are well aware that a number of issues must be addressed before this approach can become practically applicable."
The other researchers are Larry Murdock, Purdue entomology professor; Patrick Gaffney, formerly of the University of Wisconsin-Madison; Joseph Huesing, Monsanto Co. research entomologist; David Onstad, University of Illinois Department of Natural Resources and Environmental Sciences; and Richard Roush, University of California, Davis.
The Purdue Department of Entomology provided the funding for this research.
Writer: Susan Steeves, (765) 496-7481, firstname.lastname@example.org
Source: Barry Pittendrigh, (765) 494-7730, email@example.com
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"Active" Refuges Can Inhibit the Evolution of Resistance in Insects Towards Transgenic Insect-Resistant Plants
Negative cross-resistance (NCR) toxins that hitherto have not been thought to have practical uses may indeed be useful in the management of resistance alleles. Practical applications of NCR for pest management have been limited (i) by the scarcity of high toxicity NCR toxins among pesticides, (ii) by the lack of systematic methodologies to discover and develop such toxins, as well as (iii) by the lack of deployment tactics that would make NCR attractive. Here we present the concept that NCR toxins can improve the effectiveness of refuges in delaying the evolution of resistance by herbivorous insect pests to transgenic host plants containing insecticidal toxins. In our concept, NCR toxins are deployed in the refuge, and thus are physically separated from the transgenic plants containing the primary plant-protectant gene (PPPG) encoding an insecticidal toxin. Our models show: (i) that use of NCR toxins in the refuge dramatically delays the increase in the frequency of resistance alleles in the insect population; and (ii) that NCR toxins that are only moderately effective in killing insects resistant to the PPPG can greatly improve the durability of transgenic insecticidal toxins. Moderately-toxic NCR toxins are more effective in minimizing resistance development in the field when they are deployed in the refuge than when they are pyramided with the PPPG. We explore the potential strengths and weaknesses of deploying NCR toxins in refuges.
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