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Daily News Blog

02
Jul

Cockroaches Rapidly Develop Resistance to Nearly Every Pesticide, Requiring Alternative Approach

(Beyond Pesticides, July 2, 2019) German cockroaches, the bane of many apartment-dwellers throughout the U.S., can rapidly develop cross-resistance to insecticides they have never been exposed to, according to researchers from Purdue University. “This is a previously unrealized challenge in cockroaches,” said Michael Scharf, PhD, whose findings were published in the journal Scientific Reports. “Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone.” In the face of pesticide resistance, integrated measures that focus structural, mechanical, and cultural pest management practices must become standard practice for this notorious pest.

Dr. Scharf and his colleagues began their study at two separate housing complexes in Indianapolis, IN and Danville, IL. Prior to the study, researchers pre-treated a subset of cockroaches in each building, and selected five insecticides out of 14 commercially available. These insecticides – abamectin, pyriproxyfen, thiamethoxam, lambda-cyhalothrin, and boric acid, were used because cockroaches had already developed significant resistance to others tested, mostly synthetic pyrethroids. Pre-treatment applications of synthetic pyrethroids revealed over 80% of cockroaches surviving.

For the insecticides left with any level of efficacy, researchers established three separate treatment approaches, and stuck with it for six months, with one application each month, to test for additional changes in resistance. In the first, only the insecticide abamectin was applied. In the second, a mixture of insecticides was applied. And for the third, insecticides were rotated each month.

Overall, all the treatment regimens established fared very poorly against German cockroaches. Only the single abamectin treatment showed any promise in one housing complex. At the Indianapolis location, cockroaches were found to be susceptible to the chemical beforehand, and numbers decreased significantly after two applications. However, at the Danville location, roughly 10% were already resistant to abamectin, which permitted their population to flourish after the first application.

Rotating the insecticides used each month resulted in cockroach populations that were relatively stable or increased, while the treatment with a mixture of pesticides resulted in a population explosion. And researchers found that for cockroaches that survived after an application of one chemical didn’t develop resistance merely to that one chemical or even class of chemicals, they also developed resistance to pesticides in other chemical classes. This occurred even if they had never been exposed to these new pesticides in their life.

“We would see resistance increase four- or six-fold in just one generation,” Dr. Scharf said. “We didn’t have a clue that something like that could happen this fast.”

The research results track what farmers are experiencing in agriculture as a result of pressure from agrichemical and insurance companies to spray or grow pesticide-incorporated genetically engineered (PI-GE) crops as a preventative practice. A 2013 study found that “stacking” multiple toxin-producing genes into PI-GE corn crops produced resistance effects in corn earworms similar to what Dr. Scharf and his team discovered. Researchers assumed that earworms resistant to one 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. However, as Dr. Carrière, Yves Carrière, PhD, lead author of the GE study explains, “[O]n the two-toxin plants, the caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain.”

And in crop fields repeatedly doused with herbicides, a 2018 study by scientists at University of Sheffield in the United Kingdom (UK) determined that herbicide use was not only the major, but the singular factor driving weed resistance. Other factors, such as cultural techniques, or herbicide rotations could not ameliorate resistance at all. And like cockroaches, weed resistance to one herbicide was likely to drive increasingly rapid resistance to other, different chemical formulations.

As Dr. Scharf’s work indicates, integrated methods to address pests are critical to an effective treatment plan. And there is no doubt it is important to control cockroaches, as they increase risk of asthma, and can carry a number of human pathogens. But it is simply not enough to spray a pesticide and assume the job is done. In fact, you may be creating an even worse pest problem.

To manage cockroaches, focus in denying them access to the necessities of life –food, water, and shelter. Seal up cracks and crevices that may allow entryway, install doorsweeps to further impede movement, and make sure food and water is never left out, and all surfaces are clean/vacuumed. Throughout the process, monitor populations with traps to gauge areas of activity, and the intensity of the infestation. Once you have done everything you can to deny food, water, and shelter, employ the only insecticide in the study scientists say cockroaches did not develop a resistance to: boric acid. Place boric acid bait gels around areas of activity, but don’t let up on sanitation or structural repairs. For a step-by-step checklist and guide to take care of a German cockroach problem, see Beyond Pesticides ManageSafe entry on this atrocious pest.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Purdue University Press Release, Scientific Reports

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