Human efforts to kill cockroaches with a sugary poison had an unexpected consequence: It shortened the bugs’ sex lives.
But now, some roaches appear to have tweaked the recipe for the sweet substance that males use to woo females — allowing the bugs to thrive and multiply once more.
This discovery, which scientists describe in the journal Proceedings of the Royal Society BOffers a window into a surprising adaptation from humanity’s most formidable foes.
“Cockroaches are more than just pests,” says Jessica Ware, an entomologist at the American Museum of Natural History who was not involved in the study. “It’s a beautiful evolutionary ecology example.”
Blatella germanica, the German cockroach, has evolved to live only in human environments. This is our top indoor pest, a species that manages to adapt to our efforts to eradicate them.
Coby SchalAn entomologist at North Carolina State University, is very familiar with these insects — and with the traps of roach romance.
In his lab, he collects a female German cockroach in a dish. A few minutes later, he added another roach.
“There we go, I just introduced a man,” she says. Each insect is the size of a lima bean.
A pheromone repels such female roaches like an intoxicating perfume, attracting males. It seems to be working here “Look, he’s starting to follow the woman,” Shall observes.
Once the two make contact he grows his wings. This exposes a gland on his back, from which he secretes a “nuptial gift” – a sweet chemical slurry that the females eat. But to make it up to the lap, she has to mount the male.
“It puts the woman in a perfect position,” Schall explains. “When she’s feeding, the male has this telescoping penis. He extends that penis to the end of the female and inserts it.”
The penis has a hook, which the male uses to lock onto the female’s genitals for 90 minutes – the time it takes him to produce a sperm package, which he transfers to the female.
So when it comes to making baby roaches, having a tasty wedding gift is key That’s why these gifts are often full of glucose, a simple sugar that is a basic source of energy for many living things.
The female cockroach receives the wedding gift
Credit: Ayako Wada-Katsumata
And it is this love of glucose that pest controllers began to exploit in the mid-1980s. They made bait with an insecticide that was loaded with glucose or other sugars that quickly turn into glucose. The result was immediate: lots of dead German cockroaches.
But then, a few years later, a pest-control agency noticed something unsettling. “The cockroach population in Florida apartments was out of control,” says Schall.
It appears that some roaches have developed glucose aversion. They do not like glucose or sweets. It was bittersweet. “And so,” says Shall, “they refused to take the bait.”
“In the last 10 years, the use of bait has increased dramatically,” he says. “And so, I suspect that the population of glucose-averse cockroaches has increased dramatically.”
This change gave the German cockroaches a big leg up in their arms race against us. But it also created a problem for them: any glucose in a man’s wedding gift is now abhorrent to these glucose-averse women.
“Now this female, she immediately stops feeding and takes the male down,” Schaal says. “She simply walks away. So this poor male has lost the opportunity to mate with the female. Suddenly, this adaptive trade-off goes awry in the context of sexual interaction.”
Wife rejects cockroach wedding gift
Credit: Ayako Wada-Katsumata
But if there’s one thing you can trust, it’s that roaches breed more roaches. So when faced with an existential crisis like the inability to seduce a partner, they always seem to find a solution. In this case, their solution consisted of two genetic changes.
“First,” says Schall, “the man has changed the chemistry of the wedding gift he gives the woman.”
That is, he tinkered with the recipe, reducing the amount of glucose and another simple sugar. And he increased the amount of a sugar called maltotriose, which women like and which doesn’t easily break down into glucose. This means the gift stays sweet.
The second change involves how long it takes for the male to lock onto the female’s genitals. Usually, this takes three to four seconds. But the men were able to shave more than a second “before the woman sensed the presence of glucose in her mouth,” Schaal said.
“Overall, the male solution is to buy more time but speed up his mating effort,” he says.
consciousAn entomologist who was not involved in the research praised the new work.
“It shows how this widespread behavior (of wedding gifts), which has probably evolved over millions of years, has been dramatically changed by humans in a short period of time,” she says.
So perhaps now is a good time to reformulate the bait, perhaps by adding some type of fat, says Schaal. However, even if we succeed in knocking out the roaches once more, it probably won’t be long before their numbers begin to climb again.