Roach sprays don’t work | Popular Science

In his line of research, Zachary DeVries has seen a lot of German cockroach infestations. Inevitably, when the urban entomologist enters homes that have been infiltrated by the small, oblong pests, he’ll also see something else: Cans or bottles of insecticide spray–the kind you can buy in basically any hardware or grocery store nationwide. “We kept seeing these products around, we kept seeing people use them, and we kept seeing people continuing to deal with these pests despite using these products,” DeVries, an assistant professor at the University of Kentucky, tells Popular Science. 

Maybe the infestations persisted because people weren’t applying to the sprays correctly. Or maybe the products were failing. Over more than a decade of work trying to better understand and eliminate cockroach infestations, Devries found himself asking, ‘are these consumer bug sprays doing anything?’ New research from DeVries and a team of colleagues offers a clear answer to that question, a resounding ‘no’. 

Common consumer insecticide sprays and aerosols are ineffective against German cockroach infestations, according to a study published August 14 in Journal of Economic Entomology. The findings have big implications for the management of one of the most insidious household pests, and for how pesticides are regulated and marketed.

“This is really important work,” says Michael Scharf, a now-retired urban entomologist and insect toxicologist who spent years investigating roach pesticide resistance at the University of Florida and Purdue University. German cockroaches (Blattella germanica) have evolved alongside human settlements for millenia. Though a handful of other roach species can become unwelcome indoor invaders, including the much larger American cockroach, German roaches only live in human structures and are the most prolific and problematic home pest. And they aren’t just unsightly and annoying, they’re also a major public health concern in cities worldwide. Roach saliva, feces, and body parts can trigger allergies and respiratory illness, and the insects are thought to be a significant driver of the disproportionately high asthma rates observed among children living in cities and public housing. 

Bunk buys

Past research has repeatedly demonstrated that the vast majority of free-living German roach populations have evolved resistance to pyrethroids, the class of insecticides most commonly found in consumer pest control products. Yet the new study is notable for actually assessing off-the-shelf sprays and thoroughly evaluating their residual effects under different exposure conditions, says Scharf. “Usually, manufacturers are paying for the research,” he notes–and so very little gets published which portrays specific products in a negative light. This new work is different. It was funded by the federal Department of Housing and Urban Development, and names the particular sprays analyzed, Scharf says–which means the findings can be directly used to advise households suffering roach infestations. 

DeVries and his co-authors tested four consumer products in multiple different application scenarios: Raid Ant & Roach Killer 26; Hot Shot Ant, Roach & Spider Killer; Ortho Home Defense Insect Killer for Indoor & Perimeter; and Spectracide Bug Stop Home Barrier. They found that all of the sprays readily killed susceptible, lab strain German cockroaches that lack the genes for pesticide resistance–but that in field-collected insects taken from three different free-living populations, they were wholly inadequate to manage an infestation. 

Unsurprisingly, when sprayed directly on insects, the products did usually kill their target, says Johnalyn Gordon, lead study author and an entomology researcher at the University of Florida. But, concerningly, some insects survived direct application with certain sprays. And, crucially, all four products demonstrated weak residual effects–meaning roaches exposed to a surface that had been sprayed with the pesticide usually survived. Residual effects, or the ability of a pesticide to kill even when you’re not staring down a bug directly, are key for combatting an infestation, says Gordon.

“It’s all but impossible to directly apply a pesticide to every single cockroach inside a home,” she explains. Most roaches are hiding and inaccessible at any given time. For a product to work, it has to linger and kill bugs that happen to come into contact with it while they’re on the move. Based on Gordon and DeVries’ research, these sprays don’t. 

“If you can find a bug to spray with a product, you can also just hit it with a hammer,” DeVries says. In essence, there’s no more benefit to one of these bug sprays than a well-timed shoe.

The entomologists sprayed each product on three different common household materials: stainless steel, ceramic tile, and painted drywall. Then, they exposed ten roaches to each surface/spray combination for 30 minutes, before moving the roaches to a pesticide-free environment. After 24 hours, they found that less than a quarter of the cockroaches had died across all the test conditions. Painted drywall, the most porous surface, yielded the least deadly outcomes. Yet even on steel and tile, there were some instances where none of the roaches in an experimental scenario died after a 30-minute insecticide exposure. That’s despite marketing materials for all four sprays that claim long-term, residual effects and pest protection lasting between four weeks and 12 months.

The idea with this experiment was to mimic the type of short-term exposure a cockroach might realistically encounter in a home treated with a pesticide spray, Gordon says. However, even this is an ideal exposure scenario that’s likely rare inside a home because, as documented in previous research, German roaches don’t linger on pyrethroid-covered surfaces, notes Gordon. The chemicals irritate the insects, so they avoid them. In most cases, roaches probably don’t spend half an hour at a time crawling through pesticides. 

In additional tests, the researchers set out to determine just how long it would take for the bug sprays to kill 100 percent of the roaches.They found that the insects needed to be in continued contact with a treated surface for between eight and 24 hours before every last one dropped dead. “That’s a long time,” DeVries says, noting that it’s “extremely unlikely” that scenario ever unfolds outside of a lab environment. “All the things we tested, they didn’t work at all. They didn’t control the roaches,” he adds. 

Insecticide downsides

Moreover, applying them inside a home carries potential risks. Compared to other pesticides, pyrethroids are considered relatively safe and non-toxic, when used as directed–they’re not associated with increased cancer risk, for instance. But long-term or repeat exposure has still been linked to some health harms. One study found childhood exposure to pyrethroids may negatively affect cognitive development, another linked pyrethroid levels to hearing loss in teens, another found higher pyrethroid metabolite levels were associated with a slight increased risk of death from all causes and death from heart disease. One case study linked an instance of facial paralysis in an infant with pyrethroid exposure. Incorrect use like overapplication, accidental ingestion, or inhalation carries more immediate risks.

“In our field, we always talk about cost/benefit,” says DeVries. Insecticides are almost always necessary to deal with serious insect infestations, but risks have to be taken into account and balanced with the upsides, he explains. “When you put an insecticide in a home, the risk may be very minor, but there still has to be a benefit outweighing the potential harm. In this case, there is none, there is only cost.” 

Popular Science reached out to the manufacturers of each of the four tested products for comment. ScottsMiracleGro, the parent company of Ortho, and Spectrum Brands, the parent company of Spectracide and Hot Shot, did not respond as of publication time. Raid had not sent a statement at the time of publication. 

Regulation revision

In the U.S., all consumer insecticides are regulated by the Environmental Protection Agency, which publishes guidelines for product testing and labeling. Each one of the studied sprays met EPA requirements in order to be marketed as effective against cockroaches. But the new study highlights that the EPA allows anti-roach insecticides to be green-lit after testing on susceptible, lab-strain cockroaches, which specifically haven’t evolved pyrethroid resistance. 

“It’s a deficiency within testing,” says Gordon. “If resistant populations were included in testing requirements for product registration, that would raise the bar across the board,” she adds. She and DeVries hope their research can help spur a regulatory change, similar to what’s already happened with bed bug treatments, requiring that companies report test results on field-collected insect strains.

Alternate attacks

Repeatedly purchasing ineffective products, combined with the inherent difficulty of combatting a roach infestation in an apartment building can leave a lot of people feeling like cockroaches are an impossible problem to solve. “It can create a situation of hopelessness,” says Scharf. 

But roach infestations aren’t intractable, and there are things that people can do. Taking out trash daily, cleaning up and securely storing pet food, plugging entry points, and fixing water leaks are big initial control steps to minimize resources available to roaches, Scharf notes. And some consumer pesticide products are worth trying. Avoid ‘bug bombs’ or total release foggers, which also don’t work, and carry a sizable risk of pesticide exposure, says DeVries. A better strategy is to buy gels and other bait products that are meant to be ingested by pests. These target the insecticide delivery, minimize the risk of human or pet exposure, and are generally more effective, says DeVries. If one bait starts to fail, he recommends trying another, to keep pesticide resistance at bay. 

German roaches are a uniquely adaptable species. Among other tools in their pesticide-evading evolutionary arsenal, they can evolve detoxifying “Swiss Army” enzymes capable of breaking down multiple types of insecticides, says Scharf. With enough time and exposure, it’s likely that a population of roaches will become effectively immune to any pesticide people throw at it. That’s where the professionals come in. Integrated pest management and carefully rotating through different insecticide treatments are industry-wide practices, which can prevent resistance from becoming established and truly exterminate an infestation, DeVries notes. In most states, landlords and building managers are legally required to ensure a pest-free environment–which often means hiring a professional pest control service. 

We’re locked in a thousands-year-long arms race with German roaches. Fortunately, there are winning strategies out there, they just don’t come in a spray can.