How roaches developed disgust at first bite

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Cockroaches that don’t fall for traps’ sweet poisons have evolved taste cells that register sugar as bitter.

In certain groups of the widespread German cockroach (Blattella germanica), nerve cells that normally detect bitter, potentially toxic compounds now also respond to glucose, says entomologist Coby Schal of North Carolina State University in Raleigh. The “bitter” reaction suppresses the “sweet” response from other nerve cells, and the roach stops eating, Schal and his colleagues report in the May 24 Science.

Normally roaches love sugar. But with these populations, a dab of jelly with glucose in it makes them “jump back,” Schal says. “The response is: ‘Yuck! Terrible!’”

This quirk of roach taste explains why glucose-baited poison traps stopped working among certain roaches, Schal says. Such bait traps combining a pesticide with something delicious became popular during the mid-1980s. But in 1993, Jules Silverman, also a coauthor on the new paper, reported roaches avoiding these once-appealing baits.

“This is a fascinating piece of work because it shows how quickly, and how simply, the sense of taste can evolve,” says neurobiologist Richard Benton of the University of Lausanne in Switzerland.

What pest-control manufacturers put in their roach baits now, and whether some still use glucose, isn’t public, Schal says. But humankind’s arms race with cockroaches could have started long ago, “in the caves,” he says. In this back-and-forth struggle, it’s important “to understand what the cockroach is doing from a molecular basis.”

Roaches don’t detect taste with tongues, as people do, but instead use hairlike structures that grow in lots of spots on their bodies. “The cockroach can taste things by stepping into them,” Schal says. Taste nerve cells spangle the roach hairs. Coauthor Ayako Wada-Katsumata presented various flavors and measured the responses from two types of nerve cells, the GRN1 cells, which detect sugars, and the GRN2 cells, which normally warn of bitter compounds such as caffeine.

In roaches that shied away from glucose, the sweet-detecting nerve cells continued to fire when exposed to various sugars. What differed in these roaches were the bitter-detecting GRN2 cells, which responded to glucose as well as to bitter compounds. In these insects, bitter overwhelms the signal from the sweet detector.

Among the scenarios Schal imagines for the origin of GRN2’s glucose aversion are a chance mating with some other as-yet-unidentified roach species that doesn’t eat glucose. Or he wonders if the bait traps triggered the spread of formerly rare genetic variations in roach taste cells left over from before the species moved in with people. Those outdoor ancestors might have evolved a distaste for glucose because so many plants defend themselves with compounds called glucosides, blends of sweetness and something noxious.

An aversion to glucose doesn’t mean distaste for all sugars. The new study found roach enthusiasm for fructose, although Schal has heard that other roaches may be averse to it. What would really surprise him, he says, is a roach distaste for the beery sugar maltose. During roach courtship, a male roach offers maltose as a gift to win female favor. The male fans out his wings so a female can climb onto his back to nibble at maltose he produces. “They use it the way we use chocolates,” he says.

Cockroaches used to love sugar and would mob both jelly and peanut butter. But some roaches have evolved aversions to glucose and avoid jelly.
Courtesy of Ayako Wada-Katsumata