Bad breath fells aphids

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.

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Gish, M., A. Dafni, and M. Inbar. 2010.  Mammalian herbivore breath alerts aphids to free host plant.  Current Biol. 20:R628-R629.

13 thoughts on “Bad breath fells aphids

  1. I wonder what other insects might do this if any? The pea aphid has a world wide distribution it seems. Was it always thus or did it spread advantageously with humans & the spread of agriculture, being better able to survive than other aphids perhaps? I suppose this could potentially mean using blasts of breath-like gas mixtures to control them as pests… I will try breathing on every aphid I now meet!

    1. Oh, I don;t know–aphids are basically protein-concentrators.

      Whatever their metabolic physiology was, herbivorous dinosaurs, parareptiles, therapsids, etc. would have exhaled water-saturated air at increased temperatures, so this may predate ungulates by a long shot.

      By the way, I bet it has to be mouth-exhaled breath, because on the way out the nose a lot of heat and water is reclaimed.

      1. Heh, actually I thought of the protein angle. But this is a biology site, so I try to be nice to the specialists by keeping down the speculation in that area.

        Since you got me started: presumably an herbivore would not be much dependent on the aphid availability over the season.

        Mouth breath, huh? That would come when they reach for feed, so a good signal.

      2. Also, that is basically the outdoor picnic comfort around here. “Hey, it’s nutritious!”

        When you are in the mountains you mostly get mosquitoes though, so it’s more like recycling your own proteins.

  2. Someone should place aphid infested plants in a non-air-conditioned gym and test the effect.

  3. Assuming they did the experiments on native aphids/plants in Israel, there would have been a very rich assemblage of ungulate herbivores in the Levant until just a few thousand years ago –and pretty much an overabundance of livestock since then, to give an uninterrupted ecological lineage of heavy breathers.

    For more info, check out this one: “Distribution and Extinction of Ungulates during the Holocene of the Southern Levant” — http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005316

  4. But do the herbivores regulate their exhalation dependent on how much sauce they want with their veggies?

  5. I smell a money-making opportunity. Market what is basically a glorified humidifier as a preventative/control measure for house plants.

    Call it Magic Goat Breath.

  6. I am surprised that this article says that the feeling of mammalian breath seems to be unique to aphids. I remember an episode of the Crocodile Hunter in which a millipede allowed Steve Irwin to touch it, but as soon as he breathed on it, it retracted and curled up. I’ve since noticed the same behavior in ants (they reverse direction) and the spiders that hang over my doors in Madison, WI. When I want the spiders to retract and clear the doorway so that I can get through, I’d just huff on them gently. They then quickly scurry up the the corners of the doorway.

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