ORI GIN AL PA PER Soldiers with large weapons in predator-abundant midsummer: phenotypic plasticity in a eusocial aphid

Abstract Eusocial aphids produce sterile individuals (‘‘soldiers’’) that specialize behaviorally and morphologically to protect their colony from predators, while production of soldiers can negatively affect colony growth because of reproductive allocation and opportunity cost. Hence, a cost-saving soldier production strategy is expected to be favored. Here, we hypothesize that, to save the cost, a eusocial aphid Ceratovacuna japonica produces soldiers with smaller weapon in the season when predators are not abundant. The abundance of two specialist lepidopteran predators (i.e., Taraka hamada and Atkinsonia ignipicta) of C. japonica dramatically increased, and aphid colony size sig-nificantly decreased, from July to August. In line with these, the soldiers in August had larger weapons (i.e., frontal horns) than those in June, indicating a correlational increase in weapon size with predation pressure. We predict that a reliable prospective signal indi-cating the coming of midsummer (environmental temperature) induces mother aphids to produce soldiers with larger weapons. Experiments clarified that soldiers produced at 20 C (typical temperature of July to August) had larger weapons than those produced at 15 C (typical temperature of May to July). Such phenotypic plasticity appears to b