Bad Breath Prompts Insects to Keel Over : Discovery News

Plant-dwelling insects have a unique defense system that zeroes in on smelly animal breath.

Some insects drop off of plants when they detect warm, humid mammal breath.

The insects' reaction prevents them from being eaten by animals that feast on their plant homes.

The findings could lead to the development of new, non-toxic insect repellents.

People may feign falling over backwards after smelling bad breath, but new research shows some insects actually do keel over when they detect warm, humid mammal breath.

It turns out the dramatic instinct, which causes insects living on plants to fall to the ground, is a life-saving strategy that often prevents the bugs from being eaten by potent breath-releasing mammals as they feast on the plant.

The study, published in the latest issue of Current Biology, is the first to prove that any kind of defensive behavior exists against incidental predation by herbivores.

"Tiny insects like aphids are not helpless when facing large animals that rapidly consume the plants they live on," said lead author Moshe Inbar of the University of Haifa. "They reliably detect the danger and escape on time."

For the first part of the study, Inbar and his team allowed a goat to feed on potted alfalfa plants infested with aphids. The researchers were amazed to see that 65 percent of the aphids dropped to the ground just seconds before they would have been eaten along with the plant.

"As soon as we started to work on this problem, we suspected that the aphids responded to our own breath," said Inbar, who added that he and his colleagues later wore snorkels to prevent their own breath from affecting the experiment results.

The mass dropping of bugs might have been caused by other factors, however, such as shaking of the plant by the goat while it ate or rubbed against the alfalfa, and shadows produced by the goat's presence. So the researchers invented a special leaf-picking device that shook the plant, without, of course, emitting breath. The aphids didn't keel over.

Shadows also had no effect on the aphids' dropping behavior. Even shadows created by ladybugs, an enemy of aphids, didn't inspire that kind of synchronous response.

The scientists next allowed a restrained lamb to approach aphid infested broad bean seedlings. The breath of the lamb did the trick: aphids dropped off the seedlings left and right.

"It was now obvious that herbivore breath is the key player in conveying to the aphids the message of imminent obliteration," according to the researchers.

To further investigate what qualities in mammal breath cause this bug reaction, the team constructed an artificial breath apparatus that they tested out on the aphids. Carbon dioxide and isolated volatile organic compounds in the produced breath led to no bug response.

When the airstream was warm and humid, however, aphid drop off rates shot up to 87 percent, suggesting that the aphid's sensory system on ambient humidity might be key in their breath detection abilities.

"We predict that this sort of escape behavior in response to mammalian breath may be found among other invertebrates that live on plants and face the same threat," the researchers concluded.

Other insects pay attention to our breath too.

Scientists at the University of California, Riverside, for example, recently did extensive work on how mosquitoes use carbon dioxide, emitted in human breath, to find blood meal targets.

Project leader Anandasankar Ray, an assistant professor in the university's Department of Entomology, and his team hope to foil the mosquitoes by developing repellents that he said will "block mosquitoes' ability to detect carbon dioxide in our breath, thereby dramatically reducing mosquito-human contact."

The aphid work may also pave the way for the development of non-toxic insect repellents.