As the bed bug (Cimex lectularius) has re-emerged as a common household pest in recent years, boric acid has been a common tool used by pest management professionals and homeowners alike to try to control it. But, a new study in the Journal of Economic Entomology shows that the route of contact makes a difference: Simple external exposure to boric acid (such as in a dust application) is minimally effective against bed bugs, but bed bugs that ingest boric acid die swiftly. (Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org)

By Andrew Porterfield

After years of apparent dormancy, the bed bug (Cimex lectularius), has made a comeback in Europe and the United States. While the insects do not transmit disease, they are a nuisance, infesting bedding and other furniture and leaving clear symptoms of biting. As scientists try to determine a reason for their resurgence, pest management professionals and members of the public have searched through a number of eradication methods, including chemical pesticides and other treatments, with varying (and often negligible) success.

Andrew Porterfield

One common treatment has been to use boric acid on bed bugs, as a dust, spray, bait, paste, gel, or liquid. Dust may be the most common boric acid treatment, but it’s efficacy is not known. Some pest control advisors and agricultural extension agencies have even recommended not using boric acid at all. To help determine the efficacy of boric acid on C. lectularius, a team from North Carolina State University led by entomology professor Coby Schal, Ph.D., examined two ways to deliver boric acid: contact with dust or by ingestion. They also compared results to boric acid treatments on common German cockroaches (Blatella germanica), which boric acid can kill by both routes.

In a study published in September in the Journal of Economic Entomology, Schal and his team report that boric acid can kill a significant number of bed bugs—but only if the chemical is ingested. Bed bugs that eat boric acid at concentrations greater than 2 percent die quickly, and concentrations as low as 0.5 percent caused 100 percent die-offs, although at a slower pace. However, external contact with high concentrations of boric acid dust did not have a significant effect on bed bugs. By comparison, boric acid effectively killed concentrations of German cockroaches, regardless of the route of contact.

The researchers conducted four experiments on the bed bugs and roaches.

• The first experiment involved feeding boric acid to 10 adult male bed bugs, at concentrations between zero and 5 percent. The bed bugs were fed for seven days.
• The second experiment involved exposing bed bugs to boric acid dust. Here, 118 starved and 120 fully fed bugs were placed in dishes containing boric acid dust, and mortality was measured for 14 days. In the second experiment, the researchers also looked at whether boric acid particle size had any effect on bed bug mortality, homogenizing some boric acid powders.
• The third experiment involved exposing German cockroaches to boric acid dust. One group of roaches had glued mouthparts (to prevent ingestion), while the second group was free to feed.
• The fourth experiment involved injecting boric acid directly into the bed bug’s body (specifically, the hemocoel) to determine toxicity of the chemical to bed bugs.

Bed bugs had no aversion to feeding on boric acid at concentrations up to two percent, though the number of fully engorged bugs dropped to 80 percent at 5 percent concentrations.

Simple exposure only to the dust showed a 33 percent mortality rate among bed bugs after 14 days of observation. Increasing concentrations tenfold only increased unfed male mortality by 10 percent, and there was no difference between fed and unfed bugs. Nor did reducing the size of boric acid particles have any effect on bed bug mortality. In fact, more than 85 percent of fed bugs survived external boric acid exposure.

Injecting bed bugs did cause high mortality rates (up to 95 percent at higher concentrations), indicating a toxicity of boric acid to bed bugs that the insect’s cuticle is effective at staving off.

Ingestion, by comparison, was easily induced in bed bugs and highly effective at killing the bugs. A single blood meal with 0.5 percent boric acid killed 80 percent of all bugs in seven days. Concentrations above 1 percent killed all bed bugs within four days.

By also testing B. germanica, which can be killed by boric acid ingestion or exposure, the researchers found some clues as to how boric acid may work on insects. “It appears that this otherwise insecticide-susceptible strain of bed bugs possesses mechanisms that prevent boric acid from compromising or penetrating the cuticular barrier,” the researchers write.

Exactly how boric acid does its job remains a mystery, which presents challenges in designing baits to attract and ultimately kill bed bugs. But boric acid is a preferable chemical candidate for use in bed bug baits, since it is soluble and stable in water, is not as toxic to mammals as other chemicals, is relatively low cost, and doesn’t appear to trigger resistance by insects.

As for the use of boric acid in dust applications, though, the study shows such efforts are likely to be minimally effective against bed bugs. The researchers note that boric acid dust is widely available to consumers and pest management professionals and is effective against other urban pests, hence its common use. But, for managing bed bugs, methods to induce ingestion of boric acid will need to be devised.

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.