(Beyond pesticides, March 22, 2022) German cockroaches collected from US residences have developed resistance mechanisms so powerful that many can consume ten times the pesticide needed to kill a susceptible strain in the lab and not die. These are the results of recent research published in the Journal of Economic Entomology, which focused on determining cockroach resistance levels to gel insecticides commonly used in infested homes in Southern California. The findings underscore the importance of an integrated approach to cockroach management that recognizes and responds to pest ecology, rather than searching for an ever-elusive silver bullet.
The researchers collected five different strains of German cockroaches from various locations in Southern California, two from public housing and three from apartments. All sites had long-standing cockroach infestations, with varying treatment histories that generally included heavy use of common gel insecticides. Tests were conducted on male cockroaches as they are gregarious foragers and therefore more sensitive to baited food; it has been stated that if an insecticide cannot kill a male, it is very unlikely to kill a juvenile or female cockroach. A separate group of lab-reared cockroaches never exposed to insecticides was used as a basis for comparison.
This never-exposed strain of laboratory cockroaches was then exposed to varying levels of commonly used insecticide-baits, including fipronil, clothianidin, indoxacarb, abamectin, hydramethylnon, and deltamethrin. The researchers determined the lethal doses (LDs) that killed 50% of the lab strain, as well as the dose that killed 95%.
The scientists then exposed the residential strains to commercial products containing the insecticides listed above. Mortality was recorded 14 days after exposure. Responses varied widely between different residential strains, and although all baits completely killed the lab strain, no pesticide was able to achieve 100% knockdown at all levels.
Called “diagnostic doses,” each cockroach strain was then directly treated with three times the lethal dose that killed 95% (LD95) of the laboratory strain. With the synthetic pyrethroid deltamethrin, no cockroaches died at this dose. While only 0-3% of fipronil, 13-27% of clothianidin, and 13-63% of cockroaches exposed to indoxacarb died at the rate of 3 x LD95. Only abamectin and hydramethylnon recorded high mortality rates following this exposure. The scientists then went a step further and exposed the cockroaches to ten times the LD95. At this rate, more than 80% of cockroaches exposed to deltamethrin were still alive, while with fipronil this rate killed 20-70%. Cockroaches exposed to clothianidin and indoxacarb had a significant negative correlation between survival time after exposure to 10 x LD95 and mortality, while in those exposed to fipronil and hydramethylnon the correlation was insignificant. The scientists say this indicates that the resistance is more physiological for the former products, while the insignificant correlation may indicate the cockroaches developing an aversion to the latter two baits.
Only abamectin exhibits a reversal that would suggest a level of efficacy in a cockroach infestation. However, the researchers add a caveat to this finding by referring to a 2019 study that found a rapid increase in abamectin resistance in the field. In this study, about 10% of cockroaches at a certain site were resistant to abamectin. But after one application, the 10% that didn’t die were able to repopulate quickly. These scenarios highlight the flaws of a product-centric approach to cockroach management.
To be successful, insecticidal baits must consistently achieve near 100% knockdown rates. But as this study shows, even doses ten times higher than what should be successful in killing a cockroach can leave a breeding population repopulating. A
In the 2019 study, the researchers tested an active ingredient that was not tested in the current study: boric acid. No evidence was found that cockroaches developed generalized resistance to boric acid, possibly due to its mode of action.
In its powdered form, boric acid can be placed along the cracks and crevices that cockroaches walk on. It can dry out and desiccate insects, but is generally more effective when consumed, as it acts as an acute stomach poison. The product is found in some commercial pest control products formulated with a food attractant. The powdered form, however, may be more effective when used in an appropriate setting. Cockroaches are social animals that regularly groom themselves and each other. Leaving a thin line of boric acid that the roaches can crawl over will get the boric acid on their feet, which they will then clean off. Cockroaches groom themselves by running their legs and antennae through their mouths, which results in ingesting the boric acid stuck to their feet. Younger cockroaches feed on the waste products of older cockroaches, providing an additional route of exposure to the original boric acid meal, and cockroaches typically eat other dead cockroaches, providing yet another route once the cockroach target is dead, making it an effective -sink source.
But even a product as effective as boric acid is unlikely to eliminate an infestation unless other approaches are also incorporated. An approach that responds to pest ecology recognizes that pests, like all life, need food, water and shelter to survive. Make sure food and water are never forgotten and all surfaces are regularly cleaned/vacuumed. Cracks, crevices and other entrances to the house or apartment must be completely sealed; consider products like door sweeps and fine-mesh screens to further impede movement. Throughout the process, monitor populations with traps to assess areas of activity and intensity of infestation. Once you have done all you can to refuse food, water, and shelter, boric acid gels and dusts can be applied to manage the remaining infestation.
Think about this impact of these actions from the perspective of the cockroach. By monitoring with traps, you have identified problem areas and major infestation sites in your home. By sealing the entrances, you have cut off the reinforcement infestation. By interfering with movement, you’ve slowed down the remaining cockroaches’ ability to find new mates. By applying fine dusts of boric acid near where you have located the infestation, every move is potentially deadly. By denying access to food and water, you have created a situation where the only food available will be boric acid bait poisons. Such an approach requires a little more foresight, but is significantly more effective than one that focuses solely on chemical use while ignoring pest ecology.
For a step-by-step checklist and guide to taking care of a German cockroach problem, see the Beyond Pesticides ManageSafe entry on this atrocious pest.
All unattributed positions and opinions in this article are those of Beyond Pesticides.
Source: Journal of Economic Entomology