Plant-dwelling insects are in perpetual danger of being accidentally munched on by plant-eating animals. One such insect, the sap-sucking aphid (a common pest in gardens), has an effective escape plan, though: the bugs detect an approaching herbivore's breath and simply drop off the plant before it's eaten.
Researchers at the University of Haifa at Oranim, Israel first noticed this phenomenon when they allowed a goat to feed on aphid-infested alfalfa plants—65 percent of the plant pests simultaneously dropped to the ground just before the vegetation was devoured.
The team suspected that several cues might have motivated the mass dropping, including the sudden shadow cast by the goat, plant-shaking triggered by the munching marauder and/or the herbivore's exhalations. The researchers tested the effects of each cue individually and found that simply casting a shadow on the plants had no effect on the aphids. Vibrations caused by leaf picking caused only one quarter of the insects to flee the plant. By contrast, when the researchers placed a lamb within five centimeters of the foliage (close enough to breathe on it, but not nibble on it), nearly 60 percent of the bugs dropped to the floor, suggesting that breath was the key danger signal.
To better understand the properties of breath that prompt the aphid behavior, the scientists constructed an artificial breath apparatus consisting of a pump that produced a gentle, steady stream of air. When they positioned the instrument within one centimeter of aphids and delivered a two-second stream of room-temperature air, no aphids abandoned the plant. Similarly, adding carbon dioxide or volatile chemicals common in breath had no impact on aphid behavior. It wasn't until the temperature and humidity of the airstream were increased that there were noticeable effects. Altering either parameter alone produced only modest increases in aphid dropping, but the combination of increased warmth (to 35 degrees Celsius) and humidity (at 90-100 percent) caused nearly 40 percent of the aphids to plummet. The results were published online August 9 in Current Biology.
"Tiny insects like aphids are not helpless when facing large animals that rapidly consume the plants they live on," said Moshe Inbar, professor of environmental and evolutionary biology and coauthor of the study, in a prepared statement. "They reliably detect the danger and escape on time."
The researchers noted that human breath also effectively repelled aphids—so how's that for a non-toxic way to rid gardens of this pesky plant pest?
Image of aphids on a soybean plant courtesy of Maine Department of Agriculture, Division of Plant Industry