May 7, 1999 MANHATTAN -- Examining if and how corn borer resistance to Bacillus thuringiensis (Bt) toxins develop so farmer-friendly resistance management plans can be identified, is the focus of a genetic study done by a team of entomology researchers at Kansas State University. Their findings appear in the May 7 issue of Science, the weekly journal of the American Association for the Advancement of Science.
In the past, pests such as the European and Southwestern corn borers have inflicted hundreds of millions of dollars worth of damage to crops. Farmers were forced to either chemically spray the pests with insecticides or suffer the losses. A current method for combating pests like these corn borer is to genetically engineer maize and other crops to produce a toxin inside the plant that will kill them.
The method is safe, with no harm to the environment and the economic return can be enormous in years when these pest populations are high, provided the pests do not develop resistance to the toxin.
Randall Higgins, a research entomologist with K-State research and extension; Fangneng Huang, a research associate and post-doctoral student; Larry Buschman, a research entomologist; and William McGaughey, a professor emeritus of entomology, say the strategy for this resistance plan hinges on several assumptions, one being that the genes that make the borer resistant to the toxin are recessive. When these pests reproduce, their offspring must inherit the resistant gene from both parents to be able to survive exposure to the toxin.
In lab studies where the researchers mixed certain forms of the Bt toxin into the pests' diet, the European corn borers' resistance appears to be inherited as an incompletely dominant trait. Higgins said, though somewhat surprising, this does not mean this borer will exhibit resistance dominance to toxin genes inserted into transgenic corn in the field.
"The Bt toxins that we used are not identical to the material expressed in intact transgenic plants," Higgins said. "Because they are in a different form, it is also not possible to compare the level of toxin in a transgenic corn plant with that used in our laboratory study."
The researchers said it is important to realize these insects must survive to adulthood before they can pass their genes to the next generation. Even if some of these insects are able to cause more damage to plants than similar non-resistant insects, the resistance will not be passed on to future generations if they are killed before they mature and mate. They indicated the pests would "probably be killed" by high dose Bt corn plants, but are conducting further study.
"One of the key factors is that you have these individuals that you don't know what side of the line they are going to fall," McGaughey said. "These European corn borers appear to be closer to the dominant side but one or more of the Bt resistance genes of some type is out there."
McGaughey said the study is a real "hot button" issue because of the environmental impact of the resistance management plan.
"It's awfully important that we get these resistance management plans just right," McGaughey said. "If we're too cautious we may deprive ourselves the benefits of this technology. On the other hand, if we're not cautious enough, we may have to go back to using chemicals."
Higgins emphasizes that while no resistance has yet been detected under field conditions, there is strong concern that it could develop.
"Probably one of the key outcomes of this study is that resistance management plans are important," Huang said. "It's very clear that since the borers have the opportunity to develop resistance to some of these toxins, it is not a question of should you have a plan or not. Developing a workable plan that farmers will use represents an important investment."
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