Wily Cockroaches Find Another Survival Trick: Laying Off the Sweets

Everyone knows that cockroaches are the ultimate survivors, with enough evolutionary tricks up their carapaces to have thrived for 350 million years and to have completely adapted to the human species.

 But the nature of the adaptation that researchers in North Carolina described on Thursday in the journal Science is impressive even for such an ancient, ineradicable lineage, experts say. Some populations of cockroaches evolved a simple, highly effective defense against sweet-tasting poison baits: They switched their internal chemistry around so that glucose, a form of sugar that is a sweet come-hither to countless forms of life, tastes bitter.

The way the roach’s senses changed, experts say, is an elegant example of quick evolutionary change in behavior, and offers the multibillion-dollar pest control industry valuable insights into enemy secrets, perhaps even revealing some clues for the fight against malaria-carrying mosquitoes, which are far more dangerous to human health than roaches.

“This is a fantastic discovery,” said Walter S. Leal, the head of the entomology department in the College of Agricultural and Environmental Sciences at the University of California, Davis. (Dr. Leal was not part of the research.)

“Sometimes,” he said, “the science is beautiful but you don’t know whether there is going to be an application five years from now, 10 years from now or 100 years.” But in this case, he said, the impact is both fundamental and practical.

Ayako Wada-Katsumata, Jules Silverman and Coby Schal, all at North Carolina State University, who wrote the report in Science, set out to explain a well-known phenomenon: Some populations of German cockroaches (the ones that apartment dwellers see scurrying around in the kitchen at night) avoid poison bait that is laced with glucose, which is supposed to attract them.

This behavior, discovered by Dr. Silverman, “first appeared in the early ’90s,” said Jim Fredericks, chief entomologist at the National Pest Management Association, shortly after exterminators — who now prefer to be called pest management professionals — started using poison baits instead of spraying as the main method of battling roaches. To get around the problem, the industry developed new baits, but the change in roach behavior was a puzzle.

Grzegorz Buczkowski, an entomologist at Purdue University who was not involved in the research, said the industry was always developing new poisons, because roaches and other pests become resistant to their effects, just as bacteria become resistant to antibiotics.

“We lose baits all the time,” he said.

But in this case, the problem was not a poison that had become ineffective. The cockroaches just seemed to avoid any bait that had glucose.

Dr. Silverman showed that this behavior was inherited, not something an individual roach learned during its brief life. And a few years ago the North Carolina researchers decided to investigate what caused the change.

Instead of taste buds, roaches have taste hairs on many parts of their bodies. The three North Carolina researchers concentrated on those around the mouth area and on two types of nerve cells that sense tastes and respond by firing electrical signals to the brain. One responds only to sugars and other sweet substances; the other responds only to bitter substances. Whenever a molecule of something sweet attaches to a sweet detector, it fires electrical impulses and the roach brain senses sweetness, which makes it want to eat whatever it is tasting. Whenever a molecule of something bitter attaches to the bitter detector, that cell fires and the brain senses bitterness, which makes the roach want to avoid that substance.

But somehow the roaches had changed so that the glucose made the bitter detector fire.

“Basically,” said Dr. Buczkowski, “when cockroaches taste glucose, they’re repelled by it because it tastes bitter to them.”

Dr. Schal said the next step was to figure out the details of the genetic mutation that had occurred. Perhaps a mutation changed the molecules that detect bitter substances so that they would be sensitive to glucose, too. Or a different sort of mutation could have caused the dedicated bitter neurons to have lots of standard glucose detectors, which did not exist on those neurons before — a shift that also would have made the insects register sweet glucose as bitter.

The research may be relevant far beyond roach control, perhaps helping to explain the behavior of mosquitoes that spread malaria, Dr. Schal said.

“The mosquito changed its behavior,” he said, “and no longer rests on walls that are treated with insecticide. Instead it tends to rest on the ceiling, or it tends to rest on the outside walls that are not treated with insecticide.

“We still don’t understand the cellular, the neural mechanism responsible for this change in behavior of the mosquito,” he said, so the approach that yielded results with the cockroach could offer useful insights.