Cockroaches Evolve Yet Another Way to Outsmart Humans

German cockroaches (image), which are found throughout many human settlements, have apparently evolved an aversion to glucose in order to avoid roach poisons that often contain this ingredient. In a study published in the May 24, 2013 issue of Science, North Carolina State (NC State) University entomologists describe the neural mechanism behind the aversion to glucose, the simple sugar that is a popular ingredient in roach-bait poisons. Glucose now sets off bitter receptors in cockroach taste buds, causing cockroaches to avoid foods that bring on this taste-bud reaction. This aversion has a genetic basis and it eventually spreads to offspring, resulting in increasingly large groups of cockroaches that reject glucose and any baits made with it. In normal German cockroaches, glucose elicits activity in sugar gustatory receptor neurons, which react when exposed to sugars like glucose and fructose – components of corn syrup, a common roach-bait ingredient. Generally, roaches have a sweet tooth for these sugars. “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,” says Dr. Coby Schal, the Blanton J. Whitmire Distinguished Professor of Entomology at NC State and the corresponding author of the paper. “That causes the glucose-averse roach to close its mouth and run away from glucose in tests.” In the study, the researchers conducted tests on the roach tongue, the paired mouth appendages called paraglossae. The tests showed the unexpected electrophysiological reactions that glucose stimulates both sugar and bitter receptor neurons, confirming behavioral tests that showed roaches quickly fleeing from glucose when presented with it. But it’s not just a sugar aversion.
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