The stench of dying bugs is an effective — and ancient — insect repellent, biologists at McMaster University have found.

David Rollo, a professor of biology at McMaster in Hamilton, found that the corpses of insects and certain other animals emit the same blend of stinky fatty acids, and act as a strong repellent for those same animals.

As an added bonus, while the smell sends the critters running, people can barely detect the scent.

Rollo was studying social behaviour in cockroaches and looking for smells that attract them when he made the discovery. Cockroaches emit pheromone signals when they find a good place to live, to attract other cockroaches.

Rollo was extracting chemicals from the bodies of dead cockroaches to isolate the pheromone involved, when he found a group of fatty acids with the opposite effect.

"It was amazing to find that the cockroaches avoided places treated with these extracts like the plague," said Rollo in a release. "Naturally, we wanted to identify what chemical was making them all go away."

Rollo's team found the specific fatty acids that the dead roaches gave off, and then found the same chemicals in ants and caterpillars.

Furthermore, the same stink signals death in woodlice and pill bugs, which are actually crustaceans, like lobsters and crabs.

This suggests that the smell is an ancient signal, because insects and crustaceans diverged from a common ancestor more than 400 million years ago. The signal probably evolved in the water, since very few crustaceans live on land.

The scientists believe that these signals would warn others that diseased corpses or predators were nearby.

"Recognizing and avoiding the dead could reduce the chances of catching the disease, or allow you to get away with just enough exposure to activate your immunity," said Rollo.

The researchers found that a log treated with the chemicals kept wood beetles away for a month.

The study, published in the journal Evolutionary Biology, suggests that fatty acids could be used for repelling pests from homes or food storage areas, because the effect is so strong and applies to so many different species. The chemicals are also non-toxic and nearly odourless to humans.