(Beyond Pesticides, June 1, 2022) A new evolutionary strategy spreading among the German cockroaches is making them more difficult to kill than ever before. In a recent publication in Nature Communications Biology, scientists determined that cockroaches are developing an aversion to sugar baits containing glucose, with impacts that are changing their behavior and altering their mating rituals. “We are constantly in an evolutionary battle with the German cockroaches,” said study co-author Coby Schal, Ph.D., of North Carolina State University. “Evolution can be sped up tremendously in the urban, human environment because the selection force imposed on insects, especially inside homes, is so intense.”
Issues with the German cockroaches
At issue with the German cockroaches is a trade-off between natural and sexual selection. Natural selection or, in this case, human-induced natural selection, has led cockroach females to become averse to baits containing glucose sugars. While many are now familiar with the fact that the vast majority of the German cockroaches are resistant to nearly every synthetic pesticide, with some resistant to upwards of 10x the label application rate, less reported is the pests’ growing resistance to sugar-laced baits. Sugar-containing baits have been employed for decades and, over time, the German cockroaches that are able to survive in locations where sugar baits were employed developed a distaste for the otherwise attractive solution.
As the present study shows, the implications of this development are affecting a critical life stage of the German cockroach: mating. Roach copulation is initiated by males approaching a female and exposing a specialized gland on its abdominal segment, which subsequently excretes a sugary concoction that is intended to attract the female. Females feeding on the gland are thus at the right position for males to mate. Copulation is generally more successful when females feed for longer periods of time; a successful courtship can last up to 90 minutes.
It follows that bait aversion has come to an evolutionary reckoning with cockroach mating rituals. For female cockroaches, something tastes a bit off. “We’re seeing glucose-averse female the German cockroaches turning down this nuptial gift – and the chance to mate – and wanted to understand more about the mechanism behind it,” said Ayako Wada-Katsumata, Ph.D., study coauthor and research scholar at NC State. Male excretions contain a range of sugars and amino acids that females rapidly break down into glucose with their saliva. But with natural selection dictating a need to avoid these sugars, female cockroaches are averse to glucose taste bitterness when feeding on the solution, resulting in a short and failed courtship.
Difference between male and female cockroaches
According to the study, glucose-averse females mated at a significantly lower rate than wild-type male cockroaches (lab-reared cockroaches without sugar aversion). There was no significant difference seen when glucose-averse males mated with glucose-averse females. However, even within glucose-averse pairs, their courtship was much shorter than wild-type mating events. Looking closer at the nuptial secretions, scientists found that by adding fructose to the secretion of wild-type males, they could increase their mating success with glucose-averse females.
To manage the German cockroaches, focus on denying them access to the necessities of life –food, water, and shelter. Seal up cracks and crevices that may allow entryway, install door sweeps to further impede movement, and make sure food and water is never left out, and all surfaces are clean/vacuumed. Throughout the process, monitor populations with traps to gauge areas of activity, and the intensity of the infestation. Once you have done everything you can to deny food, water, and shelter, employ the only insecticide that cockroaches have not developed resistance to boric acid.