Friday science quickie: a new paper in Current Biology reports an intriguing adaptation: the breath of mammalian herbivores induces aphids to drop off plants, saving them from being eaten along with the leaves.
Three biologists at the University of Haifa in Israel noticed that two species of aphids, the pea aphid (Acyrthosiphon pisum) and Uroleucon sonchi, dropped off their plants when they were about to be ingested by a goat or a lamb. Here’s a photo:
This escape behavior is probably adaptive, because although dropping off the plant risks death by starvation and desiccation, it surely give you more of a chance than being eaten by a herbivore.
The authors tested whether the appearance of a shadow over the plant, or shaking of the plant itself from plucking leaves, could cause this behavior Shaking caused a moderate drop, but nowhere near as big as herbivore breath. Nor did the aphids drop when faced with a natural predator, the ladybug (Coccinella septempunctata). So the authors made an artificial breath machine, which controlled temperature, humidity of the airstream, and presence of chemicals in the airstream:
The result: no chemicals, including carbon dioxide, acetone, etc., increased the aphid drop, nor did they in mixture. Bovine nasal secretions added to the air didn’t do anything, either. Nor did higher airstream temperature or higher humidity at ambient temperature. But the aphid drop really took off when they increased both the temperature and humidity of the airstream. It was the combination of these two factors, then, that the aphids used as a dropping cue.
This fleeing behavior in response to mammal breath is, so far, unique. Other parasites, like ticks and mosquitoes, can detect and home in on exhaled carbon dioxide and body head, but using mammalian (and perhaps bird) cues to flee hasn’t been seen before. Of course, nobody’s really looked for it before (this study resulted from a fortuitous observation), and I’ll bet that there are similar cases in other species.
One question: has this behavior evolved in recent times, when humans introduced foraging cows and goats? Or did aphids have mammalian enemies in more ancient times? The authors don’t discuss this.
Gish, M., A. Dafni, and M. Inbar. 2010. Mammalian herbivore breath alerts aphids to free host plant. Current Biol. 20:R628-R629.