Quantitative short-day photoperiodic response in larval development and its adaptive significance in an adult-overwintering cerambycid beetle, Phytoecia rufiventris

The chrysanthemum longicorn beetle, Phytoecia rufiventris, overwinters in the adult stage and reproduces in spring. Larvae of this beetle develop during summer inside a host stem or root. In the present study, photoperiodic control of larval development and its adaptive significance were examined in this beetle using an artificial diet. Larvae showed a short-day photoperiodic response at 25 °C with a critical day length of around 14 h; larvae reared under short-day conditions pupated, whereas those reared under long-day conditions entered summer diapause with some supernumerary molts and did not pupate. A similar response was found at 30 °C, but with a shorter critical day length. Below the critical day length, a shorter day length corresponded to a shorter larval period. Larvae transferred from long-day conditions to various photoperiods showed a similar quantitative response. Field rearing of larvae starting at various times of year showed that pupation occurs within a relatively short period in early autumn. Field rearing of pupae and adults at various times indicated that only pupation in early autumn results in a high survival rate until winter. Earlier or later pupation led to a low survival rate due to death before overwintering in the adult and pupal stages, respectively. Thus, in P. rufiventris, timing of pupation regulated by the quantitative short-day photoperiodic response is vital for survival. Relatively lower developmental threshold in the pupal stage supports this hypothesis.