From weeds to insect pests to disease, resistance to efficient, economical crop protection products is threatening their efficacy and raising costs for crop producers. So why do regulations or the chemical industry always seem to have a delayed reaction to resistance? A big part of the problem involves dueling definitions, suggests Tim Dennehy, manager, Global Insect Resistance Management, Bayer Seeds.
“Academics have leaned toward defining resistance as identifying 10% of the population in the lab being resistant to the product, regardless of what is seen in the field,” he says. “The industry has consistently wanted to say, ‘We can’t use the ‘resistance’ word unless there is complete failure in the field and usually repeated field failure.’“
Dennehy, a former Cornell University and University of Arizona researcher, says the latter is ludicrous, because a chemical company can’t claim that it is taking care of growers’ needs as promised if their product failure is hurting profits. At the same time, he points out that resistance found in the lab is no guarantee that it is in the field.
“The problem is that different people use the same word differently,” says Bruce Tabashnik, Entomology Department head, University of Arizona. “Resistance could indicate a product is failing in the field and not providing value to the grower, or it could indicate a minor change in a portion of the pest population with no immediate yield effect, but warning of a worse problem on the horizon.”
Tabashnik, along with fellow University of Arizona entomologist Yves Carriere and Michigan State entomologists Mark Whalon, David Mota-Sanchez and Robert Hollingworth, have tackled the problem. Their paper, “Defining Terms for Proactive Management of Resistance to Bt Crops and Pesticides” attempts to take the discussion of resistance from disagreement to dialogue. The goal is a more rapid and effective response to potential resistance problems in the field.
Western corn rootworm resistance to the Cry3Bb1 trait is a textbook example of the problem. Identified in the lab in 2008, possible product failure began showing up in the field in 2009, yet early seed industry response often cited agronomic mismanagement instead of possible resistance to the Bt rootworm trait. By 2011, resistance had been successfully documented, and by 2013, resistance had expanded to several traits.
Tabashnik suggests that this delay in recognizing and responding to potential resistance may have doomed single-trait effectiveness and threatens the long-term efficacy of pyramided traits. “When industry and researchers use different terms, it can impede direct action,” he says. “When a public sector scientist says, ‘we’ve found resistance here and need to take action,’ but industry says that’s not resistance, it leaves the EPA in a quandary.”
In their paper, Tabashnik and his coauthors suggest definitions for 50 key resistance terms, beginning with resistance defined as a genetically based decrease in susceptibility to a pesticide. More importantly, they further define resistance into three categories: field-evolved, laboratory-selected and practical resistance.
Field-evolved resistance defines the population as exposed to the pesticide in the field. Laboratory-selected resistance is recognized as being confined to the lab and does not suggest reduced control of pests in the field. Practical resistance, of greatest importance to growers, reflects a reduction in pesticide efficacy as measured by the percentage reduction in pest damage due to the pesticide.
“What is great about this paper is that the authors have taken us to a new level of clarity,” Dennehy says.
“With these definitions, the public sector researcher can say, ‘we have resistance, but it has not yet reached practical resistance,’” Tabashnik says. “We would like to take proactive steps to prevent that.”
The EPA, which requested the definitions, is responsible for requiring proactive steps. Co-author Whalon serves as the liaison between the Entomological Society of America and the EPA and works closely with the industry on resistance concerns.
“Having a common language so public sector researchers, industry and the regulator can agree what resistance means is essential,” Whalon says.
He thinks these definitions may also make it easier for chemical companies to communicate with growers about resistance concerns. He notes that these companies, which are naturally protective of their products, can react with avoidance or denial when public-sector researchers or others point out negative features such as possible resistance.
“Growers listen to industry people, who often have good information but are often reluctant to talk about resistance relative to their own product,” Whalon says. “They want to frame the discussion in a way to maintain business while still fighting the problem. There can also be a breakdown between the dealer and the grower. If we are using a common language, communication can be much more straightforward.”
Dennehy stresses that the customer needs to be the focus. “That requires clarity, honesty, courage and a lot of candor,” he says. “Growers need to understand the discussion and weigh in. If resistance impacts grower profitability, we have failed as an industry. These terms allow us to talk in a manner consistent with the reality of how serious resistance problems actually are.”