Colony size affects the induction of sterile soldier production in the eusocial aphid Ceratovacuna japonica (Hemiptera: Aphididae)

Eusocial aphids produce sterile defensive individuals (soldiers) for colony defense. Because soldier production leads to a trade-off between colony growth and colony defense, to realize stable colony growth, the aphids should produce soldiers in response to a changing predation risk. However, the factors affecting soldier production have been revealed in only three species of eusocial aphids, which altogether comprise 40 species. In this study, we investigated the induction of soldier production in the eusocial aphid Ceratovacuna japonica (Hemiptera: Aphididae) by observing colony growth and the reproductive schedule of C. japonica on its secondary host plant Sasa senanensis (Poales: Poaceae) in the laboratory. We hypothesized that soldier production depended on the number of aphid individuals on the host plant. Our results showed that individuals of C. japonica that had just founded their colony on a leaf of the host plant produced only normal individuals, thereby maximizing the population growth rate. Then, after the number of aphid individuals has increased on the host plant, the adults started to produce soldiers for colony defense. Further, newborn aphid individuals that were moved from a leaf having a colony with soldier production to a new leaf without a colony foundation did not produce soldiers after they grew up. This result suggests that C. japonica can flexibly produce soldiers in response to a change in the number of aphid individuals on the host plant.