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.
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.