Researchers have found that insects sense the breath of approaching herbivores and flee plants to survive. A process of elimination isolated the characteristics of their detection system

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.

Temperature and humidity turned out the be the most important factors

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 next question should be whether they vacate completely or come back shortly after.