One More Reason to Hate Cockroaches
The insects’ sweet tooth should have made them easy to kill. But they outsmarted us with warp-speed evolution.
In the centuries-long war between humans and cockroaches, the most bitter blow was dealt roughly 40 years ago. Tired of chasing after the pests with noxious sprays and bombs, researchers started to infuse their poisons with delicious flavors that could compel roaches to approach of their own accord, and then feast upon their own demise. The secret was sugar: Cockroaches, like us, simply couldn’t resist their sweet tooth.
The advent of these baits “revolutionized pest control,” says Coby Schal, an entomologist at North Carolina State University. Manufacturers were sure that they had, after centuries of strife, gained a decisive upper hand. And victory was sweet.
But not even a decade passed before the battlefield shifted once again. In the late 1980s, the manufacturers of Combat, a popular roach bait, received a perplexed call from a pest-control operator in Florida. He’d been planting Combat all over homes for years, but suddenly, it was failing to seduce German cockroaches to their deaths. One of the company’s researchers, Jules Silverman, plucked several roaches from a Gainesville apartment—and was flabbergasted to find that the insects were no longer tempted by Combat’s corn syrup and instead scuttled away in disgust.
Silverman had stumbled upon an evolutionary accident. Lured irresistibly to sugar-laced poisons for years, most of the roaches in the apartment had died. But a few insects, born with an unusual set of genetic changes that rewired their sense of taste, were no longer attracted to the baits—and, unlike their sugar-addicted kin, lived long enough to pass their mutations on to their offspring. Populations of bait-snubbing cockroaches have since been discovered in other parts of the world, even as far away as Russia, each of them apparently evolving their aversion independently, Schal told me. Faced with saccharine death, roaches adapted at warp speed, turning a liability on its head—yet another reason why they remain some of our most persistent pests.
Cockroaches’ aversion to sweetness came with costs. Meats, nuts, and super-complex starchy foods, such as beans, still taste mostly fine. But anything that contains a pure infusion of the simple sugar glucose, or that rapidly breaks down into it, registers to the mutant cockroaches as horrifically bitter, says Ayako Wada-Katsumata, an entomologist at North Carolina State University. That’s likely a problem for bugs in the sugar-addicted Western world, Schal said, because they “eat whatever we eat”—candy, pastries, and packaged snacks galore. “Imagine an infestation in a Dunkin’ Donuts,” Schal told me. Starved of low-carb options, the mutant roaches might struggle to eat enough. Wada-Katsumata and Schal haven’t done much work in doughnut shops. But their experiments in the lab do show that when baits are scarce and sugary foods flow free, the mutant cockroaches get rapidly outcompeted by their glucose-loving cousins.
The Atkins-esque diet has taken a toll on German cockroaches’ sex lives too. Prior to Sugargate, the insects had a standard courtship protocol: Males extruded a fatty, sugary “nuptial gift” from a gland on their back to tempt prospective mates into a tryst. Chemically speaking, the secretion is “similar to chocolate,” with comparable allure, Wada-Katsumata told me. If tasty enough, the nuptial gift could coax lady roaches into sitting down for an extended snack—five, six, seven seconds, perhaps longer—enough time for her suitor to initiate an hour-plus-long mating embrace, at the end of which he would deliver a package of sperm.
But the male’s precoital treat holds no appeal for sugar-loathing females. It’s chock-full of maltose, a type of sugar the female’s saliva rapidly converts to glucose. “So at first she is interested,” Wada-Katsumata told me. Within a couple of seconds, though, the taste turns foul—prompting her to skedaddle, her eggs still unfertilized. Weeks may pass before the female is ready to couple anew, if she ever becomes interested in trying again. “She learns that the courtship process is not good because of the bitter taste,” Wada-Katsumata said.
This sounds, in theory, like “it should have been great for humans,” says Justa Heinen-Kay, an evolutionary biologist at the University of Minnesota. And perhaps at first it was, as cockroaches were forced to navigate a “tug-of-war” between poison-survival and sex, says Jessica Ware, an entomologist at the American Museum of Natural History. Sugar-loving roaches were doomed to die in traps, while their keto-dieting kin perished without spawning another generation of pests.
And yet, of course: Faced with this conundrum, cockroaches have tinkered with an evolutionary work-around. By studying lab-reared populations of German cockroaches, Wada-Katsumata and Schal have found that sugar-averse females seem to be producing saliva that’s less effective at converting maltose to glucose, making the taste of nuptial gifts less noxious—while still helping them steer clear of glucose-rich baits. Males have conjured up at least two adaptations to match. They’re tweaking the composition of their gift to contain less maltose in favor of a more complex sugar that’s tougher for roach saliva to break down. And they seem to be engaging the female faster after she begins to feed—as if steeling themselves against the possibility that “she’ll get grossed out and leave,” Heinen-Kay told me.
All of that adds up, once again, to a losing stance for humans. Many of our tasty baits are turning obsolete—and the insects appear to be reproducing just fine. “It reminds us how quickly pests can adapt,” Ware told me, especially under immense pressure from us. This certainly isn’t the first time that our meddling has caused other animals to evolve rapidly over centuries or even just decades: Stripped of tree cover amid rampant deforestation, some of New Zealand’s stone flies jettisoned their ability to fly; under pressure from ivory poachers, elephants in Mozambique have begun to birth tuskless calves.
But the feats of German cockroaches are particularly notable for their speed and breadth, says Chow-Yang Lee, an urban entomologist at UC Riverside. And although other animals may eventually collide with the limits of their adaptive flexibility, cockroaches—already infamous for their ubiquity and near indestructibility—seem to be just warming up. That’s probably part of the reason these roaches are everywhere: on every continent save for Antarctica, ubiquitously plaguing us in and around our homes.
Trap manufacturers haven’t yet given up on concocting baits to accommodate the insects’ new dietary quirks—fattier, saltier, or more savory ones may be available soon. But it may only be a matter of time before the roaches find the loopholes in those new lures too. Lee, who’s been studying the insects for decades, doesn’t dare underestimate their pluck. Among their kin, German cockroaches “are probably the most resilient of all,” he told me. “They overcome challenges over and over. You cannot help but have a lot of respect.”