COLUMBIA — Insect resistance to a genetically modified variety of corn that's been discovered in several Midwestern states hasn't been found in Missouri, local plant scientists say.
Monsanto, the agricultural biotechnology firm based in St. Louis, developed the variety in 2003 to combat the corn rootworm that's long been a problem for farmers in the Midwest.
The corn is equipped with an endotoxin that kills rootworms when they eat the corn. This endotoxin is the product of a soil bacterium called Bacillus thuringiensis, or Bt, said Bruce Hibbard, an entomologist specializing in the corn insects with the U.S. Department of Agriculture's Agricultural Research Service.
A recent study by Aaron Gassman, an entomology professor at Iowa State University, found that the Western corn rootworm was evolving resistance to Bt corn in some rootworm populations in Iowa.
"This is the first case of the Western corn rootworm, or any species of beetle, evolving resistance to a Bt toxin in the field," Gassman's study concluded.
While Monsanto has acknowledged the results of Gassman's study, the company said in a news release that it was "too early to tell whether there are implications for growers in the field."
Jim Jarman, an agronomy specialist for MU Extension, said the technique of planting corn continuously in the same field, a common practice in states like Iowa and Illinois, makes it easier for corn rootworms to develop Bt resistance.
Missouri farmers are more likely to rotate their fields between soybeans and corn every other year, a practice that has kept the rootworm populations in the state from developing this resistance, Jarman said.
"There are yield advantages for both crops in a soybean and corn rotation system in Missouri," Jarman said.
The deciding factor in crop decision is soil quality. In Iowa and Illinois, Jarman said, higher quality soil allows farmers to grow higher yields of corn. As a result, farmers in these states often grow corn in the same field every year.
The rootworm reproduces by laying its eggs into the soil, where they remain dormant until the next planting season. When the farmer plants corn again the next year, the rootworm has a fresh crop to attack. If the farmer plants soybeans instead, the rootworm larvae die, with nothing to feed on, Jarman said.
This practice has served Missouri farmers well, said Wayne Bailey, MU associate professor of plant sciences. Continually rotating fields between soy and corn keeps the rootworm population from becoming highly concentrated. And it’s the fields with high populations of rootworms that become breeding grounds for Bt-resistant species.
"It’s a pressure factor," Bailey said. "With a high population it just takes a few survivors to start breeding."
Once those survivors mate and combine their Bt-resistant genes, a new, Bt-resistant species begins to gain numbers.
Hibbard said that though crop rotation has kept Missouri fields free from Bt-resistant rootworms, there are other rootworm threats..
A northern strain of the rootworm has developed the ability to remain dormant for more than one planting season, a phenomenon known as “extended diapause.” This allows the rootworm to survive in a field that is rotated into soybeans until corn is replanted the next year, Hibbard said.
Likewise, a Western strain of rootworms have developed the habit of flying away from the corn field and laying eggs in nearby fields, some of which will be planted in corn the next year. Some of these rootworm larvae survive and reproduce, and a strain adapted to survive crop rotation emerges.
These crop rotation-resistant rootworms are well documented in Illinois, Indiana and Ohio, but so far, there is no evidence of their existence in Missouri fields, Hibbard said; if the strain does migrate to Missouri, farmers on the state's eastern border, north of St. Louis would probably be the first to see the effects.