A strain of cockroaches in Europe has evolved to outsmart the sugar traps used to eradicate them.

American scientists found that the mutant cockroaches had a "reorganised" sense of taste, making them perceive the glucose used to coat poisoned bait not as sweet but rather as bitter.

A North Carolina State University team tested the theory by giving cockroaches a choice of jam or peanut butter.

They then analysed the insects' taste receptors, similar to our taste buds.

Researchers from the same team first noticed 20 years ago that some pest controllers were failing to eradicate cockroaches from properties, because the insects were simply refusing to eat the bait.

Dr Coby Schal explained in the journal Science that this new study had revealed the "neural mechanism" behind this refusal.

Jam v peanut butter
In the first part of the experiment, the researchers offered the hungry cockroaches a choice of two foods - peanut butter or glucose-rich jam [known as jelly is the US].

"The jelly contains lots of glucose and the peanut butter has a much smaller amount," explained Dr Schal.

"You can see the mutant cockroaches taste the jelly and jump back - they're repulsed and they swarm over the peanut butter."

In the second part of the experiment, the team was able to find out exactly why the cockroaches were so repulsed.

The scientists immobilised the cockroaches and used tiny electrodes to record the activity of taste receptors - cells that respond to flavour that are "housed" in microscopic hairs on the insects' mouthparts

"The cells that normally respond to bitter compounds were responding to glucose in these [mutant] cockroaches," said Dr Schal.

"So they're perceiving glucose to be a bitter compound.

"The sweet-responding cell does also fire, but the bitter compound actually inhibits it - so the end result is that bitterness overrides sweetness."

Highly magnified footage of these experiments clearly shows a glucose-averse cockroach reacting to a dose of the sugar.

"It behaves like a baby that rejects spinach," explained Dr Schal.

"It shakes its head and refuses to imbibe that liquid, at the end, you can see the [glucose] on the side of the head of the cockroach that has refused it."

Dr Elli Leadbeater from the Institute of Zoology in London said the work was exciting.

"Usually, when natural selection changes taste abilities, it simply makes animals more or less sensitive to certain taste types.

"For example, bees that specialise on collecting nectar are less sensitive to sugar than other bees, which means that they only collect concentrated nectar. Evolution has made sugar taste less sweet to them, but they still like it.

"In the cockroach case, sugar actually tastes bitter - an effective way for natural selection to quickly produce cockroaches that won't accept the sugar baits that hide poison."

Dr Schal said this was another chapter in the evolutionary arms race between humans and cockroaches.

"We keep throwing insecticides at them and they keep evolving mechanisms to avoid them," he said.

"I have always had incredible respect for cockroaches," he added. "They depend on us, but they also take advantage of us."

George Beccaloni
Curator of cockroaches, Natural History Museum
The process of natural selection would strongly favour any chance genetic change that caused a cockroach to avoid the bait and therefore death. Since individuals with the trait would have a greater chance of surviving and reproducing, their descendants with the trait would in time replace those that lacked the trait in the cockroach population.

This is the same process that has led to the evolution of antibiotic resistance in disease-causing bacteria, and warfarin resistance in rats.

The discovery of natural selection was one of the greatest scientific breakthroughs of all time and this year sees worldwide celebrations commemorating the 100th anniversary of the death of Alfred Russel Wallace, the naturalist who co-discovered natural selection with Charles Darwin in 1858.