From settled to dispersal: density-dependent behavioral switching in the pea aphid, Acyrthosiphon pisum

Group-living insects like the pea aphid, Acythosiphon pisum, use population density as a key signal to regulate behavior, but the underlying mechanisms of individual responses remain unclear. This study employed a novel dual-color system to precisely track individual aphid behavior across a density gradient. We found that aphid behavioral responses are not linear but are triggered at critical density thresholds, which differ depending on the aphid's initial state (settled or active). As density increased, aphids exhibited significantly greater movement and reduced reproductive output, with a key behavioral shift toward dispersal occurring above 1.25 individuals/cm² (1: 11 group). To investigate the molecular basis for this shift, we analyzed the expression of genes related to three key monoamine neurotransmitters. The results showed a strong positive correlation between population density, locomotor activity, and the expression of the tyramine β-hydroxylase (TBH) gene, which is critical for octopamine synthesis. Conversely, the expression of the serotonin-related gene tryptophan hydroxylase (TPH) decreased, while the dopamine-related gene Tyrosine hydroxylase (TH) showed no clear trend. These findings suggest that aphids utilize a "threshold-triggered" model to rapidly shift from a settled to a dispersal state, a process primarily mediated by the monoamine neurotransmitters system. This provides a new perspective on the self-regulation mechanisms of insect populations.