The scientists included four populations of German cockroaches, including ones collected in Lexington in 2021 whose ancestors were previously exposed to household insecticides, evidenced by their high levels of resistance to the insecticide used in most household sprays - pyrethroids. They also compared these home-collected populations to a laboratory population, which has been in colony for 80+ years, which means it uniquely has not been exposed to modern insecticides. Four common household spray insecticides were studied under direct spray and contact with dried insecticide residues (30-minute of limited exposure and 24-hour of continuous exposure).
 
Gordon and her colleagues reported that, while direct spraying caused the greatest mortality for most of the cockroach populations tested (as would be expected), even direct spray caused low mortality in the resistant Lexington cockroaches for several of the household spray products.  While effective, it would be nearly impossible to find and directly spray every cockroach in a home.
 
To eliminate an infestation, limited exposure residual efficacy is critical because this is the only route most cockroaches will ever be exposed to insecticides. For Lexington cockroaches who walked on dried insecticide for 30 minutes before being transferred to a clean container, the mortality after a day did not exceed about 20%, regardless of the product. For both the lab and home-collected cockroach populations, survival rates increased when insecticide was applied to porous surfaces, like painted drywall. Even under a best-case scenario for product residues to be effective where the Lexington cockroaches were forced into contact with dried insecticides, the scientists reported a statistical analysis that suggested that at least 8 to 24 hours of continuous exposure was needed to achieve 100% mortality.
 
Based on the results, Gordon and her colleagues concluded that cockroach mortality “substantially decreases for all products when applied as residual contact insecticides,” particularly on porous surfaces. This is a problem because it’s very unlikely people will be able to successfully control cockroaches in their homes using these products alone. As a result, they may resort to using even more insecticide, increasing their risk of pesticide exposure. Exposure to pyrethroid pesticides has been shown to cause skin ir­ritation, nausea, vomiting, dizziness, prenatal and infant neurodevelopmental issues, and hearing loss in minors.
 
As alternatives, the researchers recommend consumer bait products or professional pest control services. The scientists acknowledge that “the high price of professional pest control is just one of many barriers to effective pest control in low-income housing.”

Cockroach photos provided by Dr. Johnalyn Gordon