Cockroaches are becoming even harder to kill than currently assumed. New research published Wednesday has found that store-bought roach sprays are practically useless at controlling infestations of German cockroaches in the wild, thanks largely to widespread resistance to the most common pesticides in use today.
There are 4,500 known species of cockroaches, though only around 30 that actually live near people, and even fewer that are persistent pests. These pest roaches aren’t just gross food-stealing nuisances, though—they can also cause asthma or spread disease. The most common roach that people will be plagued by, and one of the few that actually need to live inside our homes, is the German cockroach (Blattella germanica). Roaches have a sometimes inflated reputation for being unstoppable pests (they’re probably not the most likely bug to survive a nuclear apocalypse). But they are genuinely hardy survivors, and they’ve only gotten better at evading our weapons.
Studies have found that pest roaches today are widely resistant to pyrethroids, for instance, the most common class of pesticides used in consumer products. But there’s been surprisingly little research directly looking at how these products fare against real-world roach populations. In 2019, University of Kentucky scientist Zachary DeVries (previously profiled by Gizmodo) and his team published a study that found total release foggers—also known as bug bombs—were complete trash at treating German cockroach infestations. DeVries and his colleagues have continued to study the topic and in their latest research, led by Johnalyn Gordon, the scientists put four common store-bought brands of pyrethroid-based roach sprays to the test: two aerosol sprays (Raid and Hot Shot), and two liquid sprays (Ortho and Spectracide).
Roach Resistance
The scientists tested the sprays on four different groups of German roaches: a lineage of roaches isolated over 70 years ago, before they would have been exposed to these pesticides, and three populations collected from recent infestations in North Carolina and Kentucky. They exposed the roaches to the products under three conditions: directly spraying them, spraying surfaces commonly seen in households (such as tile or drywall), letting the surface dry out, then keeping the roaches in place for 30 minutes, and lastly keeping them in place on the sprayed surfaces for as long it would take for the roaches to all die. The latter two conditions were intended to mimic the sort of residual exposure that sprays leave behind.
As expected, the frozen-in-time roaches easily succumbed to the spray products under almost every scenario. Continuous exposure to sprayed surfaces also proved eventually fatal for all of the roach populations, though it would take between 8 to 24 hours for them to die completely. But the real world roaches weren’t quite so easy to take down with only limited exposure.
“So what we found was that there was some reduced efficacy with directly spraying the pyrethroid-resistant populations that we tested. We also saw significantly decreased efficacy with limited exposure to those product residues on both porous and nonporous common indoor surfaces with our field-collected populations,” Gordon told Gizmodo. All in all, these roaches only had an average 20 percent or less mortality rate across all product and population combinations under limited exposure. The findings were published Wednesday in the Journal of Economic Entomology.
While directly spraying roaches in your eyeline might kill them most of the time, you won’t be able to tag the countless more hiding in the nooks and crannies of your home. The team’s earlier work has also shown that German roaches aren’t likely to stick around for hours on a sprayed surface, waiting to die. So the most common application of these products is also the one most likely to fail. And these sprays aren’t risk-free either, since the chemicals inside them can potentially sicken people and pets.
Given these and other findings, the researchers say that people struggling with infestations should avoid spray products in lieu of better options. Professional exterminators that use a variety of control tools, an approach known as integrated pest management, are particularly effective, if often too expensive for many people to afford. But even store-brand roach gels and baits can effectively rein in a roach problem, especially if people rotate the products they’re using over time. These products, unlike sprays, can reach the hidden hordes of roaches eager for a meal and are safer as well.
“Everyone deserves access to effective cockroach control and to live in a cockroach-free home, and products available and marketed for cockroach control should be able to control cockroaches,” the researchers wrote in their paper.
Gordon is now a postdoctoral associate at the University of Florida, where she will continue studying roaches and their close termite relatives. She, DeVries, and others are also next planning to study the real-world effectiveness of consumer roach baits and to follow up on some of their results here.
