Escaping herbivores by their breath

Moshe Gish, Amots Dafni and Moshe Inbar of the University of Haifa have uncovered a defense mechanism insects use to avoid being eaten by herbivores. Apparently, they can detect animal breath.

You might wonder why insects need to escape from plant-eating mammals. After all, herbivores have no interest in insects, only in leaves. The point is that many tiny insects, such as the 2 to 4 mm long pea aphids used in this study, live relatively sedentary lives on one or a few leaves. When a large animal tries to make a meal of that leaf, it can gobble up the aphids along with the leaf unless they make an escape. It turns out that aphids are remarkably good at knowing when to drop off a leaf just before it disappears down an animal’s gullet.

After determining that it was the animals’ breath that was triggering the aphids to launch themselves off leaves rather than noises, vibrations, shadows or other cues, the researchers set about determining what component of breath was responsible. At this stage of the experimentation, they began to use snorkels to direct their own breaths away from the aphid-infested plants.

At first, the scientists expected to find that carbon dioxide or some other chemical component of the breath was responsible. They found that the answer was much simpler. It was the combination of warmth and humidity in the exhaled breath that caused the aphids to drop from the leaves. The researchers predict that other invertebrates will be found that use the same defense cues.

Image: Pea aphid (Acyrthosiphon pisum)

Whitney Cranshaw, Colorado State University / © Bugwood.org / CC-BY-3.0-US.