For evolution and maintenance of the social system of insect colonies, investment in the sterile caste should be adequately controlled in response to environmental cues. Recent developments using artificial diet rearing techniques have revealed an underlying mechanism of caste control in a gall–forming aphid, Tuberaphis styraci, which has a soldier caste in the second instar. Statistical analyses of field–collected galls detected a significant positive correlation between aphid density and soldier proportion in the natural colonies of T. styraci. Artificial diet experiments showed that soldiers are produced under crowded conditions. Detailed experiments demonstrated that soldiers are produced in a density–dependent manner rather than in a colony size–dependent manner. From these results, it was concluded that aphid density is the crucial cue that triggers soldier production in T. styraci. This study provides, to our knowledge, the first experimental demonstration of an environmental factor involved in aphid soldier differentiation.