Sugar Is Not Always Sweet To Some German Cockroaches
May 24, 2013

German Cockroaches Sometimes Avoid Bait Because Of Included Sugars

[ Watch the Video: Glucose Aversion in Cockroaches ]

April Flowers for - Your Universe Online

Glucose is a simple form of sugar that is a common ingredient in roach-bait poison. Unfortunately, sugar isn´t always sweet to German cockroaches, especially those that avoid the roach baits.

Entomologists from North Carolina State University show the neural mechanism causing the aversion to glucose in a new study published in the journal Science. Bitter receptors in the roach taste buds are set off by glucose, causing the roaches to avoid foods that bring on this reaction. The aversion has a genetic basis, spreading from parent to offspring. This heritable trait results in increasingly large groups of cockroaches that reject glucose and the baits made from it.

Glucose elicits activity in sugar gustatory receptor neurons in normal German cockroaches. The receptor neurons react when exposed to sugars like glucose and fructose — found in corn syrup, a common roach-bait ingredient. Roaches generally 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. "That causes the glucose-averse roach to close its mouth and run away from glucose in tests."

To test for this aversion, the researchers conducted tests on the roach tongue — the paired mouth appendages called paraglossae. The tests, which showed the unexpected electrophysiological reactions that glucose stimulates both sugar and bitter receptor neurons, confirmed earlier behavioral tests that showed roaches quickly fleeing from glucose when presented with it.

It´s not simply a sugar aversion though, the same roaches were happy to partake of fructose when exposed to it.

When glucose-averse roaches were forced to taste glucose, they refused to ingest the sugar. However, when normal cockroaches were exposed to glucose, they were happy to eat it. To determine this, the researchers combined the glucose with food coloring to make it easier to see if the roaches ingested or rejected the sugar.

Glucose aversion was discovered by Dr. Jules Silverman, the Charles G. Wright Distinguished Professor of Entomology at NC State, who described its inheritance pattern more than 20 years ago. "It is extremely gratifying that we now understand the neural mechanism that underlies this unusual, yet adaptive, behavior," he said.

The glucose-aversion comes at a cost, however. Glucose-averse cockroaches grow more slowly than normal roaches in a laboratory setting where glucose-toxicant mixtures are absent and there are no nutritional stresses. "Now we want to understand how this trait persists in nature, where the food supply is probably limited," Silverman said. "Cockroaches have to adapt to a varied and unreliable food supply, and glucose-aversion places an additional restriction on obtaining adequate nutrition."

The pest-control arms race has mostly been about pests gaining resistance to the insecticides, not the attractant foods included with them. The findings of this study, however, show that the arms race includes behavioral resistance to certain types of food as well.

"Most times, genetic changes, or mutations, cause the loss of function," Schal says. "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."

Ayako Wada-Katsumata is a NC State senior research scholar who performed most of the experiments. She is continuing her research by investigating whether roaches can learn to associate glucose with specific odors and thus use their memory to ignore baits that contain glucose.