Social aphids use their antennae to perceive density cue for soldier production

In social insects, local interactions among colony members facilitate information transfer, and allow the whole colony to regulate division of labor and task allocation in an integrated and coordinated manner. In particular, regulation of caste differentiation in response to external cues is important for sustaining social insect colonies. The social aphid Tuberaphis styraci exhibits a caste polyphenism, producing second-instar soldiers and non-soldiers. Previous studies using an artificial diet rearing system identified high aphid density as a crucial cue for soldier production, which acts on embryos in the maternal body and newborn first-instar nymphs to induce soldier differentiation. While direct contact stimuli from live non-soldiers were suggested to mediate the density effect, how the aphids perceive the stimuli has been unknown. Here we investigated how antennal removal of adult females affects the soldier production in T. styraci. Under a high density condition, intact females produced the highest percentage of soldiers, females deprived of both antennae produced the lowest percentage of soldiers, and females deprived of one antenna exhibited an intermediate percentage of soldiers. Scanning electron microscopic observations of the aphids revealed the existence of sensory organs for chemoreception and tactile sensation on the antennae of the mother aphids. These results indicate that T. styraci females use their antennae to perceive soldier-inducing density cue, suggesting that maternal perception of density cue is involved in regulation over caste phenotype of their offspring.