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. After years of planting Combat in homes, he noticed that it wasn’t working to lure German cockroaches. 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. A few insects were born with a unique set of genetic mutations that rewired their taste and they lived to tell their descendants. 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. Saccharine death was the end of roach life. They adapted quickly, becoming a nuisance that is still a problem.
Cockroaches’ aversion to sweetness came with costs. Even though meats and nuts are still good, beans and other super-complex starchy foods like beans taste 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. Schal said that he could imagine an infestation at Dunkin’ Donuts. Starved of low-carb options, the mutant roaches might struggle to eat enough. Schal and Wada Katsumata haven’t worked 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 also has a negative impact on the sex life of German cockroaches. 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. Wada Katsumata explained to me that the secretion has a chemical similarity to chocolate and has a comparable appeal. 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 treats are not appealing to sugar-loving 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. If she is interested in trying again, it may take weeks before the female can be ready to get married 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. It was possible that it was at first, because cockroaches had to deal with a tug-of-war between poison-survival, and sexual, according Jessica Ware (entomologist, American Museum of Natural History). The sugar-loving roaches died in their traps while the keto-restricted kin survived without being able to reproduce.
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. The males can conjure up two variations. The males are changing the recipe of their gift so that it contains less maltose and instead uses a complex sugar that is more difficult for roach saliva. Heinen Kay said that they are engaging the female more quickly after she starts to feed, as if preparing for the possibility of her “getting grossed out” and leaving.
All of that adds up, once again, to a losing stance for humans. Our tasty baits seem to be becoming obsolete, and the insects are reproducing perfectly. “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 speed and breadth of German cockroaches is what makes them so special, according to Chow-Yang Lee 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 has been studying these insects for many decades and doesn’t think they are weak. He said that German cockroaches are “probably the most resilient” of their relatives. “They overcome challenges over and over. You cannot help but have a lot of respect.”
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