Boric Acid Kills Mattress Bugs, However Solely When They Eat It

With the bed bug (Cimex lectularius) becoming a common household pest again in recent years, boric acid has been a widely used tool pest control professionals and homeowners use to try to control it. However, a new study in the Journal of Economic Entomology shows that the route of contact makes a difference: simple external exposure to boric acid (e.g. when applying dust) is minimally effective against bed bugs, but bed bugs that ingest boric acid die quickly . (Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org)

By Andrew Porterfield

After years of apparent calm, 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, infecting bedding and other furniture, and leaving clear symptoms of biting. As scientists attempt to pinpoint a cause for their resurgence, pest control professionals and members of the public have sifted through a range of eradication methods, including chemical pesticides and other treatments, with varying (and often negligible) success.

Andrew Porterfield

A 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 its effectiveness is unknown. Some pest control advisors and agricultural counseling agencies have even recommended not using boric acid at all. To determine the effectiveness of boric acid in C. lectularius, a team at North Carolina State University, led by professor of entomology Coby Schal, Ph.D., investigated two ways boric acid could be released: contact with dust or ingestion. They also compared the results with boric acid treatments on common German cockroaches (Blatella germanica), which boric acid can kill both ways.

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 in concentrations greater than 2 percent die quickly, and concentrations as low as 0.5 percent caused 100 percent death, albeit more slowly. However, external contact with high concentrations of boric acid dust had no significant effect on bed bugs. In comparison, boric acid was effective in killing concentrations of German cockroaches regardless of the route of contact.

The researchers conducted four experiments with bed bugs and cockroaches.

• The first experiment involved feeding 10 adult bed bugs with boric acid at concentrations between 0 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 beetles 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 an impact on bed bug mortality and homogenized some boric acid powders.
• The third experiment involved exposure of German cockroaches to boric acid dust. One group of cockroaches had mouthpieces taped (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 the chemical’s toxicity to bed bugs.

Bed bugs had no aversion to feeding boric acid in concentrations up to two percent, although the number of crammed bugs dropped to 80 percent at concentrations of 5 percent.

Simple exposure to dust only showed a bed bug death rate of 33 percent after 14 days of observation. A ten-fold increase in concentrations only increased mortality in unfeed males by 10 percent, and there was no difference between fed and unfeed beetles. Reducing the size of boric acid particles also had no effect on bed bug mortality. In fact, more than 85 percent of the fed beetles survived external boric acid exposure.

Injecting bed bugs caused high mortality rates (up to 95 percent at higher concentrations), suggesting that boric acid is toxic to bed bugs, which is effective at repelling the insect’s cuticles.

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

By testing B. germanica, which can be killed by ingestion or exposure to boric acid, the researchers found some evidence of how boric acid may affect insects. “It appears that this bed bug strain, which is otherwise susceptible to insecticides, has mechanisms that prevent boric acid from affecting or penetrating the skin barrier,” the researchers write.

How exactly boric acid does its job remains a mystery that poses a challenge in developing baits that attract and ultimately kill bed bugs. However, boric acid is a preferred chemical candidate for use in bed bug bait because it is soluble and stable in water, is not as toxic to mammals as other chemicals, is relatively inexpensive, and does not cause insect resistance.

As for the use of boric acid in dust applications, the study shows that such bed bug efforts are likely to be minimally effective. The researchers find that boric acid dust is widely available to consumers and pest control professionals and is effective against other urban pests, which is why it is widely used. However, for the treatment of bed bugs, methods must be developed to induce the uptake of boric acid.

Andrew Porterfield is an author, editor, and communications consultant for academic institutions, corporations, and nonprofits in the life sciences. He frequently writes on agricultural issues for the Genetic Literacy Project. He lives in Camarillo, California. Follow him on Twitter @AMPorterfield or visit his Facebook page.

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