Eating organic? Roaches disdain key “junk-food” sweetener

About 30 years ago, many cockroach haters began to use baited traps that blended high-fructose corn syrup with insecticide. But within seven or eight years, the traps started losing their clout.

“We first assumed that cockroaches were becoming resistant to insecticide, “says Coby Schal, a professor of entomology at North Carolina State University. “That’s been discovered in more than 1,000 species.”

Upon a closer look, scientists found that the roaches were turning up their noses at glucose, a primary component in the cheap corn syrup that was used to attract roaches to the poisonous bait.

This week, Schal publishes an explanation for the change: a mutation in German cockroaches that alters taste-sensitive nerve cells. The mutation causes these cells to perceive glucose as bitter, not sweet, so the mutated roaches, no dummies, disdain bait containing glucose.

The German cockroach Blattella germanica is small and ubiquitous in buildings. “It does not have a natural world,” Schal says. “You can’t find them outside human structures,” such as barns, restaurants, schools, and especially homes.

Three cheers for glucose!

Normally, animals love glucose, not hate it, Schal says. “It’s no wonder that plants put glucose and fructose [another simple sugar] into nectar, so insects will pollenate them.”

But bitter trumps sweet, because bitter compounds are often toxic. “This makes a lot of sense,” Schal adds. “If you taste something bitter that could kill you, it would make sense to ignore any sweet taste.”

To determine why these roaches shun glucose, Schal, Jules Silverman and Ayako Wada-Katsumata made electrical measurements of sensory hairs near the mouth, which contain neural cells variously attuned to sugar, bitter, water or salt. “We put an electrode over the top of the hair and recorded the neural responses,” says Schal. Because each type of nerve cell gives a typical impulse, it was possible to distinguish the firing of sugar cells.

Among glucose-averse roaches, Schal found, a mutation placed a glucose receptor on the bitter neuron, giving sugar the bitter taste that’s obvious in the movie.

Loathing glucose aids survival, but only when the sugar comes with a dollop of insecticide, Schal says. Otherwise, “this cockroach would be at a huge selective disadvantage, as they would not have access to the calorie-rich diet” that cockroaches crave.

“To the German cockroach,” he says, “that Krispy Kreme donut on the counter is a bonanza, but if it evolves this behavior, it can’t touch that Krispy Kreme.”

Cockroaches: just disgusting, or dangerous too??

Our innate disgust of cockroaches is more than creepy-crawlies paranoia; these arthropods can harm us via allergies and infectious disease.

Disease Transmission

Cockroaches can ferry pathogens from their filthy habitat into your kitchen. They also ingest microbes while grooming, says Schal. “We have shown that the digestive tract of the cockroach contains microbes that represent the environment it lives in.”

Allergies

As a cockroach sheds feces, saliva, eggs and exoskeleton into the environment, those bits can trigger allergic asthma. In the United States:

* 26 percent of the population is allergic to the German cockroach;

* 63 percent of homes contain cockroach allergens; and

* 44 percent of living room floors have cockroach allergens.

Data: Environmental Health Perspectives, 114(4), 522.

Sweet is beautiful — or not…

While mutations usually destroy specific functions, Schal notes, the cockroach mutation actually confers a new ability. “Normally, the gene is non-functional, but in this case the roach hasn’t lost its sweet tooth, it’s gained a bitter tooth for glucose.”

At the same time, the roaches’ sweet receptors become less reactive to glucose. “It’s a two-step whammy,” says Schal. “They now respond to glucose as bitter, and there is a suppression of the response to glucose as sweet.”

This flexibility is another reason to respect roaches, says Schal. “I have been working with them for 30 years, mostly involving the arms race between roaches and us. We lob something at them, and they lob something twice as big back at us.”

The roach’s rapid response to the new baiting strategy is sobering, says Walter Goodman, a professor of entomology at the University of Wisconsin-Madison. “We have not been using baits all that long. The strong selective pressures put on the wild-type roach have led to resistance via the antennal neurons in less than 50 years (possibly 200 generations).”

Although the result may look like learning — after all, the roaches have changed their behavior to survive their new environment, “There is no learning involved,” Goodman says. “The change is in the antenna receptors, not in the brain.”

And these mutations are making some baits obsolete, Goodman adds.

Replacing glucose with another sugar could reinvigorate roach traps, Schal says. “But fructose is very expensive, and maltose is used in beer. Cockroaches love maltose, but we don’t want them competing with beer-makers.”