Do insecticide sprays really work against cockroaches? Research from the US shows that even against non-resistant strains, their performance is poor.
German cockroaches are a major pest in affordable housing, particularly in low socioeconomic areas of the US. Pest control services are unaffordable for many people, which leaves them with the option of using DIY pyrethroid spray products available from hardware stores. But do these consumer sprays actually work? Latest research from the US confirms that they certainly do not.
In a paper published in the Journal of Economic Entomology in August 2024, researchers from the University of Kentucky and Auburn University tested the effectiveness of liquid and aerosol pyrethroid insecticide sprays against cockroaches collected from real-world infestations. The labels indicated that these products had been designed to be sprayed on surfaces where cockroaches were likely to appear, with cockroaches picking up a lethal dose as they moved across the surface.
The research team tested the residual performance of the products by exposing cockroaches to a variety of common household surfaces that had been sprayed, each for a total of 30 minutes. After this time less than 20% of the cockroaches were killed. When the cockroaches were confined to the sprayed surfaces, most products took between 8 and 24 hours to kill the cockroaches. With some products, the cockroaches weren’t killed until after five days’ exposure to the treated surfaces.
The researchers noted that the type of surface also played a role in the effectiveness of the products, with drywall (plasterboard) showing the worst results. A population of non-resistant cockroaches were minimally affected by the residual sprays on the drywall, indicating that the porous nature of the surface is likely to have contributed to the poor performance when compared with results on ceramic tile and stainless steel. Given that consumer spray products are very likely to be applied to skirting boards and painted drywall, this is a particularly important observation.
As pest managers know, cockroaches are unlikely to rest for extended periods on surfaces treated with insecticides. So even the best-case efficacy scenario where cockroaches remain on a treated surface for an extended period is unlikely to occur. The products only seemed to show some level of effectiveness when they were sprayed directly onto the cockroaches themselves. However, due to the severity of cockroach infestations experienced in many urban residential areas, this is not an option that is likely to deliver any relief.
The underlying problem is simple: cockroaches have developed resistance to pyrethroids, and it is highly unlikely that these consumer products will have been tested on pyrethroid-resistant strains. The US Environmental Protection Agency (and indeed most regulatory authorities) do not require insecticide products to be tested on cockroaches recently collected from the field or with demonstrated insecticide resistance. In fact, there is little independent testing of these kinds of consumer products at all.
Combined, these factors paint a dim picture for cockroach control via insecticide sprays. “Based on our results, pyrethroid-based DIY products are likely to fail against German cockroaches inside the home, especially if they are being used as residual products,” wrote Johnalyn Gordon, lead author on the study and a postdoctoral associate at the University of Florida.
In Australia, efficacy tests are required for the registration of consumer products, but there is no requirement for them to have been tested on pyrethroid-resistant cockroaches.
The researchers note that despite challenges, cockroach control in these challenging situations remains an achievable goal. More promising options for cockroach control include gel or liquid baits, which attract cockroaches to a food source containing a slow-acting insecticide. This approach has been proven to be highly successful in the work of Prof. Dini Miller, professor at Virginia Tech, whose 2019 research investigated the use of gel baits in low-income housing projects in the US. However, this is probably not a DIY option as it requires a certain level of knowledge, so success in German cockroach control will remain the domain of professional pest managers for the foreseeable future.
In conclusion, the researchers noted that “Without residual efficacy, as shown in the consumer aerosol and spray products tested, we expect these products to add little to no value to cockroach control.”
Further reading: Johnalyn M Gordon, et al. Common consumer residual insecticides lack efficacy against insecticide-susceptible and resistant populations of the German cockroach (Blattodea: Ectobiidae), Journal of Economic Entomology, 2024;, toae158, https://doi.org/10.1093/jee/toae158
