Altruistic housekeeping in a social aphid

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Abstract
Aoki (1977) discovered that some aphids have a sterile soldier caste, which functions altruistically to protect the colony from predators. For the first time we show experimentally that the soldier caste of an aphid, the gall-forming Pemphigus spyrothecae, fulfils a second, non-defensive, altruistic role: the soldiers actively clean their gall. This `housekeeping' behaviour, in which defecated honeydew, shed skins and dead aphids are removed from the gall, is costly to the soldiers and essential for gall survival. The colony's waste is actively manipulated to the entrance of the gall by the soldiers and pushed out. Different techniques of manipulation are used for objects of different type and size. This species, and perhaps many aphid species, have a complex repertoire of altruistic behaviours involving housekeeping and defence, which suggests a social system approaching that of the major groups of eusocial insects.