Density-dependent induction and suppression of soldier differentiation in an aphid social system.

We investigated the mechanism underlying the control of soldier production in colonies of a social aphid, Tuberaphis styraci, which has a sterile soldier caste in the second instar. High aphid density was shown to induce soldier production in T. styraci. Analysis of natural colonies revealed that the soldier proportion tended to increase with aphid density but reached a plateau. Artificial diet experiments identified a similar plateau of soldier proportion under high-density conditions. In order to gain insights into the controlling mechanism of soldier production, the effect of soldiers on reproducing adult aphids was examined using the artificial diet system. It was experimentally demonstrated that soldier production was suppressed by coexisting soldiers, whereas coexisting non-soldiers facilitated soldier production. These results suggested that caste ratio in the colony of T. styraci is controlled by positive and negative feedbacks consisting of density-dependent induction and suppression of soldier differentiation.