Turning in mid-air allows aphids that flee the plant to avoid reaching the risky ground.

Abstract

When forced to drop from the plant, flightless arboreal insects can avoid reaching the risky ground by maneuvering their body through the air. When wingless pea aphids (Acyrthosiphon pisum) are threatened by natural enemies, they often drop off their host plant while assuming a stereotypic posture that rotates them in mid-air, aligning them with their feet pointing downwards. This position may increase their chances of re-clinging onto lower plant parts and avoid facing the dangers on the ground, although its effectiveness in realistic field conditions has not been tested. We performed both laboratory and outdoor experiments, in which we dropped aphids upon host plants to quantify clinging success in plants with different characteristics such as height and leaf size. Live aphids had twofold higher clinging rates than dead ones, indicating that clinging success is indeed affected by the active aerial-righting of dropping aphids. The ability to cling was positively dependent on the plants' foliage cover as viewed in vertical direction from above. Therefore, we released aphids in commercial alfalfa (Medicago sativa) fields with varying plant heights and foliage cover and induced them to drop. Most (up to 75%) of the aphids avoided reaching the ground in taller plants (65 cm), and 17% in shorter plants (21 cm), demonstrating the efficiency of the aphids' response in averting risks: both those of an approaching enemy on the plant and the plethora of new risks on the ground. Evidently, even in complex field environment, the aerial-righting mechanism can substantially reduce the possible risks following escape from a predator.