Human attempts to kill cockroaches with sugary poison have had an unintended consequence: It has cramped 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 be fruitful and multiply once more.
The discovery, which scientists described in the journal Proceedings of the Royal Society B, offers a window into a surprising adaptation from one of humanity's most intractable foes.
"Cockroaches are more than just pests," says Jessica Ware, an entomologist at the American Museum of Natural History who wasn't involved in the research. "This is a beautiful evolutionary ecology example."
Blattella germanica, the German cockroach, has evolved to live only in human environments. It's our top indoor pest, a species that keeps managing to adapt to our attempts to eradicate them.
Coby Schal, an entomologist at North Carolina State University, is very familiar with these insects — and with the trappings of roach romance.
In his lab, he deposits a female German cockroach in a dish. After a few minutes, he adds another roach.
"There we go, I just introduced a male," he says. Each insect is the size of a lima bean.
A pheromone wafts off female roaches like this one like an intoxicating perfume, which lures males in. It seems to be working here. "See, he's starting to follow the female," Schal observes.
Once the two make contact, he raises his wings. This exposes a gland on his back, from which he secretes a "nuptial gift" — a sweet chemical slurry that the female consumes. But to lap it up, she needs to mount the male.
"That places the female in a perfect position," Schal explains. "While she feeds, the male has this telescoping penis. He extends that penis to the end of the female and inserts it."
The penis has a hook on it, which the male uses to lock onto the female's genitalia for 90 minutes — the length of time it takes him to create a sperm package, which he then transfers to the female.
So when it comes to making baby roaches, having a tasty nuptial gift is key. That's why these gifts are often full of glucose, a simple sugar that's a basic source of energy for many living things.
Female cockroach accepts nuptial gift
Credit: Ayako Wada-Katsumata
And it's this love for glucose that pest controllers started exploiting in the mid-1980s. They made baits laced with an insecticide that were brimming with glucose or other sugars that quickly break down into glucose. The results were immediate: lots of dead German cockroaches.
But then, a few years later, a pest-control company noticed something unsettling. "A population of cockroaches in a Florida apartment just could not be controlled," says Schal.
It turns out that some of the roaches had evolved a glucose aversion. Glucose no longer tasted sweet to them. It was bitter. "And therefore," Schal says, "they refused to eat the bait."
"In the last 10 years, there's been a dramatic increase in the use of baits," he says. "And therefore, I suspect that populations of glucose-averse cockroaches have increased dramatically."
This alteration gave German cockroaches a major leg up in their arms race against us. But it also created a problem for them: Any glucose in the male's nuptial gift is now repulsive to these glucose-averse females.
"Now this female, she immediately interrupts feeding and dismounts the male," says Schal. "She simply walks away. So this poor male has just lost his opportunity to copulate with the female. Suddenly, this adaptive trade becomes maladaptive in a context of sexual interaction."
Female cockroach rejects nuptial gift
Credit: Ayako Wada-Katsumata
But if there's one thing you can count on, it's that roaches beget more roaches. So when facing an existential crisis like the inability to woo a partner, they always seem to find a workaround. In this case, their solution consists of two genetic changes.
"The first one," says Schal, "is that the male changed the chemistry of the nuptial gift that he offers the female."
That is, he's tinkered with the recipe, reducing the amount of glucose and another simple sugar. And he's elevated the amount of a sugar called maltotriose, which the females love and which doesn't break down into glucose as readily. This means the gift stays sweet.
The second change involves the amount of time it takes for the male to lock onto the female's genitalia. Usually, it takes three to four seconds. But the males have managed to shave off more than a second "before the female senses the presence of glucose in her mouth," says Schal.
"Overall, the male solution is to buy more time but speed up his copulatory attempt," he says.
Ware, the entomologist who wasn't involved in the research, applauds the new work.
"It shows how this elaborate behavior [of this nuptial gift], which evolved presumably over hundreds of millions of years, has in just a short period of time been altered dramatically by humans," she says.
So perhaps now is a good time to reformulate the bait, maybe by adding some kind of fat, says Schal. However, even if we succeed in tricking the roaches once more, it probably won't be long before their numbers begin to crawl upwards again.