10/16/2026
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.
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