Aphids use unreliable cues to optimize their response to a reliable cue and avoid being incidentally eaten

Obtaining more information about the environment usually helps animals reach optimal decisions. Such additional cues may reduce uncertainty, yet if they might be interpreted in several contrasting ways, the animal needs to discern their exact source precisely to avoid the costs of choosing wrongly. Plant-dwelling insects, wishing to avoid being ingested along with their host plant by a grazing mammalian herbivore, have been shown to use the heat and humidity of mammalian breath as a reliable indicator of mammalian presence. Many insects, e.g. aphids (Hemiptera: Aphididae), then drop from the plant despite possible dangers on the ground. Other cues, such as plant vibration or visual movement detection, do not cause a dropping response on their on, as they are not as reliable: they might also be caused by environmental conditions unfavorable for a falling insect.

We tested whether aphids can differentiate between the two possible sources of the unreliable cues and modulate their response to the reliable cue accordingly. When the two cues were temporally synchronous, the dropping response increased, while asynchronous cues decreased the dropping response and induced aphids to assume a body posture lowering their chance of being dislodged by wind. Cue duration also allowed aphids to distinguish between a mammalian herbivore and environmental wind. Higher cue intensities increased the amplitude of the aphids' response, but did not change the overall pattern. Aphids thus use different parameters of unreliable cues to verify their origin and respond optimally.