Extended lifespan and overlapping of generations in a gall-forming social aphid, Quadrartus yoshinomiyai

The evolutionary relationship between sociality and extended lifespan has been studied in many taxa. We investigated the seasonal population dynamics and individual lifespan in a gall-forming social aphid, Quadrartus yoshinomiyai, whose wingless adults defend the colony after ceasing reproduction. The galls of this species are completely closed for over a year, which facilitates monitoring aphid mortality rates in natural galls. Gall foundresses, which were born before winter and formed galls in April, were alive until December, indicating that they can survive for a year. The second-generation wingless adults, born in May or June of the first year, were alive in mature galls collected in March or April of the second year. Morphometric analysis revealed an overlap of three generations in a mature gall; the appendages of the second-generation wingless adults were smaller than those of the third-generation wingless adults. Our results suggest that the extended lifespan, favored in a completely closed gall where extrinsic mortality is very low, promotes the overlap of generations and post-reproductive colony defense by the wingless adults. An extended post-reproductive lifespan might also be favored if the cost of death by the potentially rapid spread of infectious diseases in the completely closed space exceeds the cost of living without reproduction.