IMAGE The head of a German cockroach with hairy appendages capable of sampling tastes from potential food, like this dyed drop of water. [Courtesy of Ayako Wada-Katsumata and Andrew Ernst]
Cockroaches like sweets but the sugar-coated traps designed to lure and kill them haven’t been as effective as they once were, and a study in the 24 May Science explains why.
Insects navigate their environment—including determining whether food in their path is safe or harmful—using sensory systems. How these systems adapt to rapid environmental changes, like the presence of a poison in a food that was once safe, remains unknown.
Now, researchers studying a species of cockroach clued in to avoid sugary traps have determined the mechanism for this change. Like other insects, cockroaches use tiny hair-like sensors on their mouths to “taste” food. These sensors house gustatory receptor neurons, or GRNs, some of which are activated in the presence of sugar, and others that are activated when food is bitter; activation of sugary GRNs causes cockroaches to feed while activation of bitter GRNs stops feeding altogether.
Ayako Wada-Katsumata and colleagues at North Carolina State University focused their work on the German cockroach; starting in the mid-1980s, it was rigorously controlled by baits that incorporate a stimulant (glucose) and a deterrent (insecticide), and just a few years later it evolved a behavioral change: glucose aversion. Glucose-averse cockroaches steer clear of manmade traps despite the tempting sugary coating.
The researchers used electrophysiological tests to evaluate the responses of gustatory receptor neurons in normal and glucose-averse German cockroaches exposed to different “tastes.” In the glucose-averse cockroaches, the researchers observed something unusual: Exposure to sugar actually suppressed the sugar GRN response, and it stimulated the bitter GRN, preventing the cockroach from feeding.
This suggests that glucose is processed as a deterrent in glucose-averse German cockroaches, aiding these insects in avoiding manmade traps. The rapid evolution of the glucose-averse feeding behavior in response to environmental changes demonstrates the plasticity of the cockroach sensory system.
“Most times, genetic changes, or mutations, cause the loss of function,” said corresponding author Coby Schal, the Blanton J. Whitmire Distinguished Professor of Entomology at North Carolina State, in a press release from that university. “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.”
and jelly with and without glucose. [Video courtesy of Ayako Wada-Katsumata]
Links
Read the abstract, “Changes in Taste Neurons Support the Emergence of an Adaptive Behavior in Cockroaches,” by Ayako Wada-Katsumata and colleagues.
Listen to related Science podcast interview with co-author Coby Schal.