By Catherine Clabby

Evidence is abundant that cockroach infestations inside homes, most commonly urban housing, increases some children’s risk of allergic reactions and even asthma attacks.

But confusion reigns over how to effectively evict the creepy pests, says North Carolina State University urban entomologist Coby Schal.

shows a balding man at a microscope
Coby Schal in his laboratory at North Carolina State University. Copyrighted photo by Mark Hall, courtesy North Carolina State University

Experiments have convinced Schal that some widely-marketed insecticide products, including so-called bug bombs, simply don’t work. On top of being poor performers, the products also can leave behind chemical insecticide residue inside homes that may be harmful to kids.

Treatments more in step with cockroach biology and behavior are effective and should be deployed to shield children from preventable illness, Schal told members of the North Carolina Healthy Homes Outreach Task Force meeting in Chapel Hill this week.

“If you have allergies to pollen, you are told to stay indoors. But if your home has cockroaches, you can’t leave,” the scientist told the group.

Healthier homes

Schal this week spoke to people well aware that asthma has become more common in recent decades among children in the United States, particularly among kids living in urban areas. Exposure to cockroach feces, body parts and saliva are partly to blame because compounds in them produce allergic reactions in some kids.

five species of common roaches
The five species of common household roaches: A, the German roach, or Croton bug, Blattella germanica (length 9/16 inch). B, the American cockroach, Periplaneta americana (length 1 3/8 inches). C, the Australian cockroach, Periplaneta australasiae (length 1 1/4 inches). D, the wingless female of the Oriental roach, Blatta orientalis (length 1 1/8 inches). E, the winged male of the Oriental roach (length 1 inch). Image credit: Robert Snodgrass’ Insects, their ways and means of living, 1930,  courtesy Wikimedia Creative Commons.

One task force member recounted that he had visited a home earlier this week to investigate likely sources of elevated lead detected in the blood of a child living there. A cockroach infestation was so bad that when he opened a kitchen cupboard, insects fell on him.

On the other hand, research is growing that reducing the numbers of German and American cockroaches in infested homes helps. One recently published study that Schal participated in showed that just one treatment with pesticide-laced baits in infested homes decreased reports of wheezing, chest tightness and coughing among kids with asthma, along with fewer doctor and emergency room visits.

Families looking to fight cockroaches at home frequently reach for do-it-yourself bug bombs, also called total release foggers (TRFs), Schal said. The products are not very expensive and are easily found on store shelves. The fact they often carry names linked to military or combat themes may bolster their image.

“People think they can’t possibly be bad because they sound so strong,” Schal said, even though use of foggers was linked to illness, most of low severity, but also one infant death reported in a 2008 Morbidity and Mortality Weekly report.

“The companies do know that a lot of these products don’t work, but they make so much money,” Schal said. “The marketing side says: ‘Keep it on market, it’s great.’ The innovating side says, ‘Keep it on until we come up with something better.’”


In a not-yet published study members of his laboratory conducted in five Raleigh apartment buildings, the foggers did not reduce cockroach populations. They did, however, spread chemical residue within those dwellings.

Applying the gel-like bait cut cockroach numbers by close to 96 percent and reduced measurements of residents’ allergens by about 96 percent after six months. And it did not spread chemical residue.

“In the case of TRFs, with roach populations not changing, it was a waste of time and effort to measure allergens, said Schal.”We know from multiple studies that allergens only decline when cockroaches do.”

He credits the difference in performance to poor and good applications of scientific insights.

Foggers expel sprays that are mixtures of insecticide, frequently the chemical pyrethroid. Resistance to pyrethroid is widespread among German cockroaches that frequently invade homes in the U.S., Schal said.

chart compares the different forms of roach prevention
Comparison of different forms of roach control. Courtesy: Coby Schal, NCSU

Those chemicals, once airborne, tend to settle on horizontal surfaces in homes such as floors, while cockroaches prefer to scamper along vertical planes, such as walls and cabinets. Nothing draws the cockroaches to touch the chemicals, and when they do encounter the chemicals, only the exteriors of their bodies touch poison.

“In other words, you treat the environment and hope cockroaches will walk over it,” the professor said.

Instead, with a tool resembling a caulking gun, people can apply gel-like bait mixtures into the places where research has shown cockroaches gravitate to. Bait can be tucked into dark cracks and crevices cockroaches like, especially near food and water. Bait mixtures can stick to vertical surfaces and don’t contain pyrethroids. If appetizing to the bugs, ingredients within the bait actually draw bugs to the poison.

“They ingest it,” Schal said. “That makes the bioavailability so much greater.”

Complex organisms

Schal stressed to people meeting at the University of North Carolina Institute for the Environment that not all cockroaches are harmful. Some actually benefit the state’s ecosystems.

One wood roach with the scientific name Parcoblatta lata is a favored meal of the endangered red-cockaded woodpecker, for one. Schal was among the research team that produced a synthetic version of a sex pheromone that attracts males of that variety, creating a means to get more bird food near where the woodpeckers dwell.

But Schal is also aware that roaches may pose health risks beyond the walls of our homes. His study of cockroach infestations in North Carolina hog farms found that when the insects are able to ingest antibiotic-resistant pathogens when dining on hog manure, they later excrete them. That raises the possibility that roaches could eventually carry unwelcome microbes to places people could get exposed, a topic Schal is trying to study further.

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And Schal does not assume for a second that what works today to fight the bugs will work tomorrow. Cockroaches are adaptable.

His research, for example, has found an explanation for the fact that a growing number of cockroaches are avoiding sugar, which is often an ingredient in cockroach baits. Natural selection appears to be favoring insects with a genetic mutation that produces an aversion to glucose in their tastebuds. The numbers of such insects will likely expand within populations exposed to sugar-containing baits.

Such insights are exactly what environmental health workers need: practical and affordable ways to reduce exposures to cockroaches, said Neasha B. Graves, Community Outreach and Education Manager with the UNC institute. And they will pass the information on.

They can provide information to families they serve on removing asthma triggers,” Graves said.

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Catherine Clabby (senior environmental reporter) is a writer and editor. A former senior editor at American Scientist magazine, Clabby won multiple awards reporting on science, medicine and higher education...