Roaches evolving to avoid sugary baits

Cockroach loses its sweet tooth! A strain of cockroaches have outsmarted human efforts to kill them by evolving taste buds that make sweet insecticide baits taste bitter, scientists have found.
North Carolina State University entomologists discovered the neural mechanism behind roaches' aversion to glucose, the simple sugar that is a popular ingredient in roach-bait poison.
In a study on German cockroaches, glucose set off bitter receptors in roach taste buds, causing roaches 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," said 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," Schal said.

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 behavioural tests that showed roaches quickly fleeing from glucose when presented with it.
Glucose-averse roaches that were forced to taste glucose refused to ingest the sugar, akin to a child who spits out her bitter-tasting food. Normal cockroaches, meanwhile, were happy to eat glucose.
Researchers learned this by combining the glucose with food colouring and watching it get ingested or rejected by the normal or glucose-averse roaches, respectively.

Study co-author Dr Jules Silverman, the Charles G Wright Distinguished Professor of Entomology at NC State, discovered glucose aversion and 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, behaviour," he said.
There is a cost, however, to cockroaches with glucose aversion. In the absence of glucose-toxicant mixtures, glucose-averse cockroaches grow more slowly than normal roaches in laboratory settings where there are no nutritional stresses.
The study was published in the journal Science.