(Beyond Pesticides, April 8, 2013) Given the rise of targeted plant and animal pests that are resistant to the tactics of the biotechnology industry, companies that produce genetically engineered (GE) crops have begun producing plants with “stacked” traits. For herbicide resistant crops, this means adding traits that incorporate crop resistance to increasingly dangerous and toxic chemicals, such as 2,4-D, a major component in the Vietnam-era herbicide Agent Orange. For crops engineered to produce their own natural insecticide, namely the toxin Bacillus thuringiensis, this means adding new formulations of the bacterium. Although this practice is widely considered acceptable and effective by the biotechnology industry, a new study from the University of Arizona College of Agriculture and Life Sciences, published in the journal PNAS, casts doubt on this assumption.
Most scientists assume that two-toxin plants will be more durable than one-toxin plants. The extent of the advantage of the pyramid strategy, however, rests on assumptions that are not always met, the study reports. “The pyramid strategy has been touted mostly on the basis of simulation models,” said Yves Carrière, PhD, lead author of the study. “We tested the underlying assumptions of the models in lab experiments with a major pest of corn and cotton.”
One critical assumption of the pyramid strategy is that the crops provide redundant killing, Dr. Carrière explains. “Redundant killing can be achieved by plants producing two toxins that act in different ways to kill the same pest,” he says, “so, if an individual pest has resistance to one toxin, the other toxin will kill it.”
In the real world, things are a bit more complicated, Dr. Carrière’s team documented. “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,” Dr. Carrière says. For their experiments, the group collected cotton bollworm ””also known as corn earworm or Helicoverpa zea”” a species of moth that is a major agricultural pest, and selected it for resistance against one of the Bt toxins, Cry1Ac. In 2008, researchers found the first evidence of resistance in the cotton bollworm only 7 years after the GE crop was first introduced.
Researchers assumed that caterpillars resistant to the first Bt toxin would survive on one-toxin plants, but die when consuming two-toxin plants because they had not yet developed resistance to the new formulation. As Dr. Carrière explains, “[O]n the two-toxin plants, the caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain.”
These findings show that the crucial assumption of redundant killing does not apply in this case and may also explain the reports indicating some field populations of cotton bollworm rapidly evolved resistance to both toxins.
Moreover, the team’s analysis of published data from eight species of pests reveals that some degree of cross-resistance between Cry1 and Cry2 toxins occurred in 19 of 21 experiments. Contradicting the concept of redundant killing, cross-resistance means that selection with one toxin increases resistance to the other toxin.
According to the study’s authors, even low levels of cross-resistance can reduce redundant killing and undermine the pyramid strategy. Dr. Carrière explained that this is especially problematic with cotton bollworm and some other pests that are not highly susceptible to Bt toxins to begin with.
The team found violations of other assumptions required for optimal success of the pyramid strategy. In particular, inheritance of resistance to plants producing only Bt toxin Cry1Ac was dominant, which is expected to reduce the ability of refuges to delay resistance.
Refuges consist of standard plants that do not make Bt toxins and thus allow survival of susceptible pests. Under ideal conditions, inheritance of resistance is not dominant and the susceptible pests emerging from refuges greatly outnumber the resistant pests. If so, the matings between two resistant pests needed to produce resistant offspring are unlikely. But if inheritance of resistance is dominant, as seen with cotton bollworm, matings between a resistant moth and a susceptible moth can produce resistant offspring, which hastens resistance.
Bruce Tabashnik, PhD, coauthor of the study explains that optimistic assumptions by the U.S. Environmental Protection Agency have led to greatly reduced requirements for planting refuges to slow evolution of pest resistance to two-toxin Bt crops. As Dr. Carrière explains, “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.”
Pest resistance is an inherent part of pesticide use. Farmers do not have to remain stuck on a pesticide treadmill that demands ever greater amounts of synthetic inputs and rewards chemical suppliers at the expense of farm profitability and the environment. A better option is to adopt organic agricultural practices, an ecologically-based management system that prioritizes cultural, biological, and mechanical production and natural inputs. By strengthening on-farm resources, such as soil fertility, pasture and biodiversity, organic farmers can minimize and even avoid the production challenges that chemical inputs such as synthetic pesticides, fertilizers and antibiotics are marketed as solving.