A new study digs into the pros and cons of two different methods for testing potential bed bug repellents: a harborage test and a barrier test. Repellency results can differ when bed bugs (Cimex lectularius) are searching for shelter versus when they are searching for a bloodmeal. (Photo by Gary Alpert, Harvard University, Bugwood.org)
By Andrew Porterfield
Bed bugs (Cimex lectularius) have returned as a common pest, thanks to global travel and resistance to insecticides. Repellents like DEET or icaridin can fend off the insects for several hours, and still other natural repellents (like cinnamon oil or margosa extracts) are used against other arthropods and could be used against bed bugs.
Andrew Porterfield
But testing for such repellents’ effectiveness has not been evaluated very thoroughly. In fact, in Europe, no agreed-upon testing methods are available for evaluating the ability of repellents to halt bed bug infestations, and no officially authorized bed bug repellents exist in the European Union.
Moreover, researchers from the German Environment Agency found that testing methods may vary depending on whether the bed bugs are foraging for food or looking for shelter. Anne Krüger, Ph.D., Erik Schmolz, Ph.D., and Arlette Vander Pan, Ph.D., entomologists at the agency, found that two test systems (one for harborage and the other a barrier test) showed significant variations in repellent effectiveness. Their work was published this month in the Journal of Economic Entomology.
Left to right: Anne Krüger, Ph.D., Erik Schmolz, Ph.D., and Arlette Vander Pan, Ph.D.
In their study, the researchers looked at how test systems evaluated five types of repellents—cinnamon oil, DEET, icaridin, margosa extract, and permethrin. The first test system was based on bedbug harborage, looking at shelter-seeking behavior with untreated and repellent-treated harborages. In this test, two shelters made of filter papers lying on top of each other were placed in a crystallizing dish. The bedbugs were released in the middle of the dish and allowed to choose a harborage.
The second system was a barrier test, in which bed bugs were attracted by carbon dioxide and heat (simulating a possible blood meal) to walk across filter papers treated with the repellents. In this test, bed bugs were released in a clear acrylic cylinder out of a kitchen towel paper bag, acting as a harborage. The bugs could stay there or crawl to a metal container emitting heat and carbon dioxide. On the way, they had to cross filter paper treated with a repellent.
There were significant differences between harborage and barrier tests. Repellency overall was much lower in the barrier tests than the harborage tests, with the sole exception of DEET, which repelled 97 percent of bed bugs 24 hours after application. For the harborage tests, DEET and icaridin showed high repellency, and even cinnamon oil showed at least 99 percent repellency of bed bugs 24 hours after application.
A new study digs into the pros and cons of two different methods for testing potential bed bug repellents: a harborage test and a barrier test. Shown here is the harborage test setup, in which bed bugs were placed in a dish with two harborages, each consisting of a pair of filter papers (b and c), one treated with repellent and the other untreated. (Image originally published in Krüger et al 2021, Journal of Economic Entomology)
A new study digs into the pros and cons of two different methods for testing potential bed bug repellents: a harborage test and a barrier test. Shown here is the barrier test setup, which lures bed bugs across filter paper (f) containing repellent, using heat and carbon dioxide as a lure to simulate a host. (Image originally published in Krüger et al 2021, Journal of Economic Entomology)
The high repellency was cinnamon oil was a surprise, the researchers say. “We had tested cinnamon oil previously in experiments with different ant species, and it had remarkably high repellency levels. This was the reason for using cinnamon oil in the bed bug repellent experiments.”
Another surprise arose from the barrier test, “where significantly more females crossed the repellent than male bed bugs,” the researchers say. “Female bed bugs are supposed to be responsible for bed bug dispersal because they tend to hide in new harborages and leave shelters more often for host and harborage seeking. The higher tolerance against repellents shows that efficacy testing should be done with both sexes.”
Both testing systems were effective at evaluating repellents and underscore the need for accepted testing methods that could also determine repellent action against certain bed bug behaviors (like seeking shelter or food). “People tend to improvise with DIY methods to protect themselves against bed bugs,” say the researchers. “The authorization of repellent products (required in the European Market) can help to sort out inefficient or unsafe products. Recognized test methods give manufacturers safety that their tests will be taken into account in the authorization process. Such test methods are not only beneficial for authorities and manufacturers but also for consumers who can then rely on the efficaciousness of products they purchase and use.”
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.
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