Brush with death: Assessing the impact of predators and alert cues on the strength of anti-predator behavior in pea aphids
The pea aphid (Acrythosiphon pisum, ‘aphid’) is a phloem-feeding pest of pea and alfalfa plants. Aphids respond to predators with anti-predator behaviors that include kicking, walking away, and/or dropping from plants. Cues that induce these behaviors include the size and movement of predators, especially lady beetles, and the alarm pheromone (E)-β-farnesene (EBF). The multi-colored Asian lady beetle (Harmonia axyridis) is a voracious predator of aphids and other soft-bodied insects. To determine whether EBF exposure and predator presence have a synergistic effect on pea aphid dropping behavior, small populations of A. pisum on fava beans (Vicia faba) were exposed to “predators” in the presence (+) or absence (-) of crushed aphid cue. Forty-eight two-week old V. faba plants with 20 pea aphids of various nymphal stages were placed in six treatment groups. Aphids in Group 1 and 2 were disturbed for 1 minute by live H. axyridis tethered via the elytra to a small-point paintbrush (+/-) cue. Lady beetle legs and mouthparts were active. Groups 3 and 4 were disturbed by dead H. axyridis tethered to a paintbrush (+/-) cue, and Groups 5 and 6 were disturbed by the paintbrush only (+/-) cue.
There was a highly significant effect of treatment and cue on the percentage of aphids dropping from the plant. Group 5 (paintbrush plus cue) was significantly more effective in eliciting aphid dropping than both Group 1 (live lady beetle plus cue) and Group 3 (dead lady beetle plus cue). Group 4 (dead lady beetle minus cue) and Group 6 (paintbrush minus cue) caused the least aphid dropping; surprisingly, there was not a significant difference between them and Group 2 (live lady beetle minus cue). While we used a paintbrush to scare all groups and attempted to produce a standardized level of disturbance, the tethered lady beetles were more cumbersome to manipulate than the paintbrush. This may have contributed to the paintbrush being “scarier” than a live lady beetle predator. In addition, the volatiles in the glue used to tether the lady beetles, although dry, may have interfered with aphid response to the cue. These caveats notwithstanding, our results suggest that when EBF is present, visual cues have a minimal impact on aphid dropping.