A German cockroach races across a kitchen floor. Someone grabs a bottle of essential oil spray, soaks the insect and watches it die. The product worked, but a new University of Kentucky study suggests the larger infestation probably did not notice.
German cockroaches - the type normally associated with indoor infestations - live in the narrow spaces people cannot easily reach. They slip behind refrigerators, enter walls and wait in cabinets until the room goes dark. Killing the insect in plain sight may feel like progress, but according to one Martin-Gatton College of Agriculture, Food, and Environment (CAFE) researcher, it does little to reach the population hiding nearby.
"If I could find every cockroach and they were just all sitting in a line, I could just use a fly swatter," said Zachary DeVries, Ph.D., UK associate professor of entomology and Bill Gatton Foundation Endowed Professor. "But that's the problem. We don't see all the cockroaches. You see one, and there are 20 more somewhere else."
Researchers at Martin-Gatton CAFE tested four commercial essential oil-based products sold for cockroach control in a Scientific Reports study. They used a laboratory population that is sensitive to insecticides and a home-collected population that is highly resistant to insecticides.
When researchers sprayed the products directly on the cockroaches, more than 96% died within seven days and nearly every test reached 100% mortality.
"They died immediately," said lead author Daniela Jackson, a 2025 UK graduate in Agricultural and Medical Biotechnology, now with the University of Pittsburgh Medical Center. "If the study had stopped there, the products would have looked highly effective."
But cockroach control depends on what happens later, after the sprays dry and hidden insects cross the treated areas. The researchers applied three of the products to ceramic tile and pine wood, then allowed the residues to age for 24 hours, one week or one month.
Nearly every residual treatment killed fewer than half of the cockroaches, and most killed fewer than 10%. The best result reached 78%, well below the 90% performance level described in the EPA federal guidance for pesticide effectiveness claims.
"Those numbers may still give the products too much credit," DeVries said. "Cockroaches were forced to remain on the treated surfaces for four hours, far longer than they would likely stay in one place inside a home. In fact, video tracking experiments revealed that fresh residues caused cockroaches to avoid treated tiles, although most of that repellent effect disappeared within 24 hours, coinciding with decreases in efficacy at later aging trials."
Essential oils evaporate quickly, which helps explain the divide between the direct-spray and residual results along with the decline in repellency after only 24 hours of aging. The spray can kill an insect it covers, but little may remain after the surface dries. For a pest that moves mainly at night and spends much of its life hidden. Jackson said it is a major problem.
"Many essential oil-based insecticides qualify as minimum-risk pesticides and are exempt from the U.S. Environmental Protection Agency's standard registration process," Jackson said. "That lower barrier can encourage new products and competition, but effectiveness claims may receive less scrutiny. A container can promise to kill cockroaches even when users must find and soak each insect."
That gap can fall hardest on people living in low-income housing, where German cockroaches are common and professional pest control may be difficult to afford.
"Families can spend limited money on a product that leaves them exposed to cockroach allergens linked to asthma and other health concerns. It's not reasonable to expect residents to track down every roach and spray it to meet what the label says," DeVries said.
For a German cockroach infestation, both Devries and Jackson recommend insecticidal baits rather than essential oil or conventional sprays. Cockroaches eat the bait and ingest the insecticide, a method that has remained the standard approach for several decades.
To learn more about the Department of Entomology, visit entomology.mgcafe.uky.edu.
Research reported in this publication was supported by the U.S. Department of Housing and Urban Development as part of an award KYHHU0090-24 totaling $999,979 with 100% funded by HUD.
Research reported in this publication was supported by the National Institute of Environmental Health Sciences of the National Institutes of Health under Award Number R25ES027684. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch Project under award number 7010213. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the Department of Agriculture.
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