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Cockroaches outsmart sugary traps

The head of a male German cockroach, showing the four major external chemosensory paired appendages. The antennae extend upward (black compound eyes behind them), the longer maxillary palps and shorter labial palps extend downward, and the paired paraglossae are on either side of the mouth. Each appendage contains many sensory hairs, some of which function in gustation.
[Image courtesy of Ayako Wada-Katsumata and Andrew Ernst]

A new study in the May 24 issue of Science reveals how cockroaches outsmart the sugary traps designed to catch and kill them.

The rapid evolution of this clever feeding behaviorówhereby cockroaches walk right by insecticide-filled traps, even though they are coated in delicious sugaródemonstrates the ability of the cockroach sensory system to change in response to its environment. This is important for survival.

Scientists understand that sensory systems help insects navigate their environment. But how insect sensory systems adapt to rapid environmental changesólike the presence of poison in a once-safe foodóremains unknown.

Ayako Wada-Katsumata at North Carolina State University and colleagues were interested in understanding this process in the German cockroach, which people have been tempting with sugar-coated traps since the mid-1980s. It only took the German cockroach a few years to develop a dislike for these traps, however. Such savvy insects are called glucose-averse (GA) cockroaches.

Like other insects, cockroaches use tiny hair-like sensors on their mouths to "taste" food. These sensors are full of gustatory receptor neurons, or GRNs, some of which are turned on in the presence of sugar. This signals cockroaches to feed. Other GRNS are turned on when food is bitter. This signals cockroaches to stop feeding altogether.

Wada-Katsumata and colleagues used special tests of neuron activation to study the responses of gustatory receptor neurons in control and GA German cockroaches. They exposed both groups of cockroaches to six different "tastes."

Control cockroaches behaved as expected; their sugary GRN was stimulated by sugary tastes and their bitter GRN was stimulated by bitter ones.

In GA cockroaches, however, the researchers observed something unusual: Exposure to sugar actually repressed the sugary neuron's response. Furthermore, it stimulated the bitter neuron, telling the cockroach to stop eating, which it did.

This suggests that sugar is processed as a deterrent, or a bad thing, in GA German cockroaches, aiding these insects in avoiding manmade traps.