Cockroaches Avoid Sugar Treats to Outsmart Baited Traps: Insect Evolution

Cockroaches are harmless, but they are pests. They're almost impossible to kill with their hard exoskeleton and fast movements. Now, it turns out that they're even more difficult to squash. A strain of cockroaches in Europe has now evolved to outsmart the sugar traps that are used to eradicate them.

For normal German cockroaches, sugar is a tempting treat. Glucose elicits activity in sugar gustatory receptor neurons, which react when exposed to sugars like those found in corn syrup, which is a common roach-bait ingredient. Yet for some reason, many cockroaches seemed to be ignoring this bait. In order to find out why, researchers conducted tests on the roach tongue.

A cockroach tongue isn't like our own tongue. It's composed of paired mouth appendages called paraglossae. After examining the roach tongue, researchers found that there were unexpected electrophysiological reactions. More specifically, glucose stimulated both the sugar and bitter receptor neurons--this means that the bait tasted bitter to the roaches. Yet that didn't mean fructose was off the menu. The roaches that disliked glucose happily ate sweet fructose in its stead.

"We don't know if glucose actually tastes bitter to glucose-averse roaches, but we do know that glucose triggers the bitter receptor neurons that would be triggered by caffeine or other bitter compounds," said Colby Schal, one of the researchers, in a news release. "That causes the glucose-averse roach to close its mouth and run away from glucose in tests."

Yet there is a cost to this glucose aversion. In the absence of glucose-toxicant mixtures, glucose-averse cockroaches grow more slowly than normal roaches in laboratory settings. That means under normal settings in the wild, it's likely that these roaches would be smaller still.

Currently the study points to the fact that roaches are rapidly evolving to outsmart traps that are created by pest-control companies. The fact that these insects have specially adapted to avoid glucose, though, is extremely interesting--and shows how dynamic these creatures truly are.

"Most times, genetic changes, or mutations, cause the loss of function," said Schal. "In this case, the mutation resulted in the gain of a new function-triggering bitter receptors when glucose is introduced. This gives the cockroach a new behavior which is incredibly adaptive. These roaches just got ahead of us in the arms race."

The findings are published in the journal Science.