(Beyond Pesticides, March 22, 2022) German cockroaches collected from U.S. residential homes have evolved resistance mechanisms so strong that many can consume ten times the pesticide required to kill a laboratory-susceptible strain and still not die. These are the findings of recent research published in the Journal of Economic Entomology, which focused on determining cockroach resistance levels to commonly used gel bait insecticides in infested Southern California homes. The findings underscore the importance of an integrated approach to cockroach management that recognizes and responds to pest ecology, rather than search for an ever-elusive silver bullet.
Researchers collected five different strains of German cockroaches from various locations around Southern California, two from public housing and three from apartment dwellers. All sites had long-standing cockroach infestations, with varying treatment histories that generally included significant use of common gel bait insecticides. Tests were conducted on male cockroaches as they are gregarious foragers and thus more susceptible to baited food; it was indicated that if an insecticide cannot kill a male, it is highly unlikely to kill a juvenile or female roach. A separate group of cockroaches reared in the laboratory and never exposed to insecticides was used as a baseline for comparison.
This never-exposed laboratory strain of roaches were then exposed to varying levels of commonly used bait insecticides, including fipronil, clothianidin, indoxacarb, abamectin, hydramethylnon, and deltamethrin. Researchers determined lethal doses (LD) that killed 50% of the laboratory strain, as well as the dose that killed 95%.
Scientists then exposed the residential strains to commercial products containing the insecticides listed above. Mortality was recorded 14 days after exposure. Responses varied significantly between different residential strains, and while all baits completed killed off the laboratory strain, no pesticide was able to achieve 100% knockdown across the board.
Referred to as â€śdiagnostic doses,â€ť 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 that dose. While a mere 0-3% of fipronil, 13-27% of clothianidin, and 13-63% of indoxacarb exposed roaches died at the 3 x LD95 rate. Only abamectin and hydramethylnon recorded high mortality rates from this exposure. Scientists then took it a step further and exposed the cockroaches to ten times the LD95. At this rate, upwards of 80% of deltamethrin-exposed roaches still lived, while with fipronil that rate killed off 20-70%. The clothianidin and indoxacarb exposed roaches exhibited a significant negative correlation between survival time after exposure to 10 x LD95 and mortality, while with those exposed to fipronil and hydramethylnon the correlation was insignificant. Scientists say this indicates that resistance is more physiological for the former products, while the insignificant correlation may indicate the development of cockroach aversion to the latter two baits.
Only abamectin exhibits a knockdown that would suggest a level of effectiveness in a cockroach infestation. However, researchers add caution to that finding by referencing a 2019 study that found rapid increases in abamectin resistance in field settings. In that study, roughly 10% of cockroaches in a certain site were resistant to abamectin. But after an application, the 10% that did not die were able to rapidly repopulate. These scenarios drive home the flaws in a product-centric approach to cockroach management.
In order to be successful, bait insecticides must consistently achieve knockdown rates near 100%. But as the present study shows, even doses ten times higher than what should successfully kill a cockroach can leave a breeding population to repopulate. Â
In the 2019 study, researchers tested one active ingredient that was not tested in the present study: boric acid. No evidence was found that cockroaches have developed widespread resistance to boric acid, likely to due its mode of action.
In its powder form, boric acid can be placed along cracks and crevices that cockroaches walk on. It can dry out and desiccate insects, but is generally most effective once consumed, as it acts as an acute stomach poison. The product is found in some commercial pest products, formulated with a food attractant. The powdered form, however, can be more effective when used in proper context. Cockroaches are social animals that regularly groom themselves and each other. Leaving a thin line of boric acid for cockroaches to crawl over will get the boric acid on their feet, which they will subsequently groom off. Cockroaches groom by running their legs and antennae through their mouths, resulting in ingestion of the boric acid stuck to their feet. Young cockroaches feed off the waste material of older cockroaches, providing an add-on route of exposure to the original boric acid meal, and cockroaches generally eat other dead cockroaches, providing yet another route once the target cockroach is dead, making it an effective source-sink.
But even a product as effective as boric acid is unlikely to eliminate an infestation unless other approaches are also integrated. 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 is never left out, and all surfaces are regularly clean/vacuumed. Cracks, crevices, and other entryways into oneâ€™s home or apartment should be completely sealed; consider products like doorsweeps and fine-meshed screens to further impede movement. 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, boric acid gels and dusts can be applied to manage the remaining infestation.
Think about this impact of these actions from the cockroachâ€™s perspective. By monitoring with traps youâ€™ve identified problem areas and the major sites of infestation in your home. By sealing up entryways, youâ€™ve cut off the infestation from reinforcements. By impeding movement youâ€™ve slowed down the ability of the remaining cockroaches to find new mates. By applying thin dusts of boric acid near where youâ€™ve located the infestation, every movement is potentially deadly. By denying access to food and water, youâ€™ve created a situation where the only food available will be boric acid bait poisons. Such as approach requires a bit more forethought, 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 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: Journal of Economic Entomology