March 31, 2013
Pyramid Strategy For Biotech Crops May Not Be As Robust As Once Believed
Brett Smith for redOrbit.com - Your Universe Online
Genetically engineered crops that are designed to be pest resistant may not be as effective as previously thought, according to a new study in the Proceedings of the National Academy of Sciences (PNAS).First grown in 1996, the pest-resistant plants produce toxins from the bacterium Bacillus thuringiensis, or Bt for short. Deemed safe for human consumption, Bt crops increase yields and reduce the need for pesticides that can damage the environment and seep into groundwater.
Some pests had begun to show resistance to Bt crops as early as the late nineties. To combat the potential evolution of pest resistance to Bt crops, the plants were reengineered to produce two or more toxins that kill the same pest, as part of a plan referred to as the “pyramid strategy.”
Although the pyramid strategy has been widely adopted, the new PNAS study indicates that it may not be a robust as farmers think it is.
"The pyramid strategy has been touted mostly on the basis of simulation models," said co-author Yves CarriÃ¨re, a professor of entomology in the University of Arizona. "We tested the underlying assumptions of the models in lab experiments with a major pest of corn and cotton. The results provide empirical data that can help to improve the models and make the crops more durable."
"We obviously can't release resistant insects into the field, so we breed them in the lab and bring in the crop plants to do feeding experiments," he added.
For their experiments, the group collected a major crop pest, the cotton bollworm, and bred it for resistance against one of the Bt toxins, Cry1Ac.
One assumption of the pyramid strategy is the use of two toxins that act in different ways to kill the same pest, referred to as redundant killing. However, resistant caterpillars survived significantly better on two-toxin plants than caterpillars from a susceptible strain.
The team also performed a research review that covered eight species of pests. They found evidence in those studies data that shows some degree of cross-resistance between Cry1 and Cry2 toxins occurred in 19 of 21 experiments. This cross-resistance means that selection with one toxin increases resistance to the other toxin.
The team also tested another facet of the pyramid strategy: the use of refuges to delay resistance. Refuges are plots of crops that do not make Bt toxins, allowing for the survival of susceptible pests. These pests are then able to breed with resistant pests, which prevent the selection of resistant genes.
According to the scientists, assumptions by the EPA have led to insufficient requirements for planting refuges to impede the evolution of pest resistance to two-toxin Bt crops.
"Our simulations tell us that with 10 percent of acreage set aside for refuges, resistance evolves quite fast, but if you put 30 or 40 percent aside, you can substantially delay it,” CarriÃ¨re said.
"Our main message is to be more cautious, especially with a pest like the cotton bollworm," he added. "We need more empirical data to refine our simulation models, optimize our strategies and really know how much refuge area is required. Meanwhile, let's not assume that the pyramid strategy is a silver bullet."