In a finding that could revolutionize pest control, researchers have discovered the identity of the “perfume” produced by female cockroaches when they are feeling amorous. When the scientists set out traps wafting synthetic versions of the compound, male cockroaches came scurrying within seconds.
The study, which marks the end of a decades-long search for the pheromone, appears in Friday's issue of the journal Science, published by AAAS, the nonprofit science society.
There hasn’t been a particularly effective way to attract the tenacious pests until now, so pesticide is currently the antiroach weapon of choice, according to author Coby Schal of North Carolina State University.
By themselves, cockroach traps probably won’t eradicate a whole cockroach population, but they should help with detecting and monitoring the insects, especially in places where even a single bug is too many, such as schools, operating rooms and food processing centers.
Schal also proposed that adding the pheromone to bait laced with insecticide might help reduce cockroach populations via the “domino effect.” Cockroaches have about two or three days after eating the poisoned food before they die. In the meantime they could pass along the insecticide via their feces, which baby cockroaches eat.
The Blatella blues
Cockroaches like to muck around in human and animal waste as well as food, so they’re carriers of all kinds of germs. The cockroach’s own proteins are also associated with asthma and allergies in humans, particularly among inner-city children and the elderly.
The German cockroach, Blatella germanica, is the most common and often the most difficult to control.
“This cockroach gives all the others a bad name,” said coauthor Wendell Roelofs of Cornell University.
As many other insects do, the female cockroach emits a sex pheromone into the air when she is ready to mate. Often a single mating provides the female with enough sperm that she can store it and produce batches of eggs for the rest of her life without mating again.
Once she becomes sexually mature, the female typically stands on a high surface during the nighttime, raises her wings and emits the pheromone. Males will approach moments later.
Researchers have had a fair amount of success isolating pheromones from other cockroach species.
Roelofs remembered a poolside meeting at a Jamaican hotel 20 years ago, when Schal presented a piece of filter paper soaked with pheromones from the American cockroach, a large sewer-dweller. Cockroaches emerged from a nearby sewer “by the thousands.”
But the German cockroach has been confounding researchers for 30 years, according to Roelofs.
For decades, nobody knew where its pheromone-producing gland was. Then, in 1993, Schal’s team identified it in the German cockroach’s last abdominal segment, called the “pygidium.” Only a few cells in each cockroach produce the pheromone in minute quantities.
But, the substance secreted by the gland was too fragile to survive the standard heating process that researchers used to purify it from a mixture of components.
This method, gas chromotography, is essentially the same one that high-school students learn in chemistry class. The basic idea is that if you heat a mixture, different components will evaporate at different temperature. If you can detect each component separately as it evaporates, then you can study each one separately.
A midnight run
To begin the study, Schal’s students dissected thousands of cockroach abdomens in his lab in North Carolina and sent the tiny and precious samples up to Roelofs’ lab in New York. Meanwhile, co-author Satoshi Nojima worked for two years in the New York lab to develop a low-temperature gas chromotography technique for purifying the pheromone from the samples.
Nervous about wasting even a smidgen of the valuable material, Nojima practiced his technique with other substances until the very last minute. Two weeks before he left for Japan to start a new job, he still was working 22 hours a day and sleeping in the lab in order to perfect his approach.
The night before he left the country, he finally purified the pheromone from the real sample and then drove the 5 micrograms (millionths of a gram) of material across the state, where the compound’s chemical identity would be determined.
“He left without knowing whether it was going to work,” Roelofs said.
The technique was a success, and the compound turned out to be a member of the quinone family, which includes compounds used for immune defense and communication in other insects. The researchers named the compound “blatellaquinone.”
Heeding the call
The chemistry experiments gave the researchers the information they needed to produce a synthetic compound that seemed to be a close match to the natural cockroach pheromone. But was it exactly the same?
Lab experiments showed that male cockroaches in a Y-shaped tube would consistently walk down the arm of the tube that contained the compound.
To see if the compound worked in a real-life setting, Schal’s team took pheromone-laced Mason jars — coated inside with petroleum jelly to prevent trapped cockroaches from climbing out — and placed them around a cockroach-infested pig farm. Adult males, but not adult females or immature “nymphs,” flocked to the jars.
Beyond bug bomb
Most people who don’t see any cockroaches in their homes would probably like to leave it at that and not think too much about spraying or setting out roach bait. But in other settings, it’s essential to detect even the smallest sign of the pests.
“The current tools we have for monitoring are basically sticky traps that aren’t very efficient. We suggest that this pheromone can be used to detect cockroaches in hospitals, nursing homes and public schools, where you’d first want to determine whether there is an infestation before applying insecticide,” Schal said.
Pheromone-based traps should also be useful in situations where cockroaches are always present to some degree, such as zoos, botanical parks and pet shops. In these cases, groundskeepers are likely to spray only when the infestation reaches a certain level, which they could determine by monitoring the traps.
Vile though it seems to us, the cockroach lifestyle has allowed these organisms to persist through millennia of natural selection. It seems fitting, therefore, that the best tool for combating the pest is not a new invention by humans but something that comes from the cockroach itself.