The development of 4th stadium Psacothea hilaris larvae under various feeding and starving regimens was investigated to determine whether there is a threshold weight for metamorphosis. In larvae fed ad libitum, 56% of individuals spent a mean of 13 d in the 4th stadium and 18 d in the 5th stadium before pupation, whereas the rest remained in the 4th stadium for 24 d and pupated. When starved upon ecdysis to the 4th stadium, no larvae molted to the 5th stadium and most eventually died without pupation. In contrast, when larvae were fed for 1 d and then starved, 47% prematurely metamorphosed into small pupae after a mean of 18 d in the 4th stadium. Pupation success from the 4th stadium increased as the feeding period was extended, reaching 97% in the larvae that had fed for four days. All these pupae gave rise to small but morphologically normal adults. The relationships between the weight changes of experimental larvae during starvation and their pupation success suggested the threshold weight for metamorphosis to be 180 mg. The adaptive significance of premature pupation in starved P. hilaris is discussed as a life history strategy under unpredictable food conditions for larval growth.
A temporally structured model that enables simulation of the development of the west Japan type yellow-spotted longicorn beetle, Psacothea hilaris (Pascoe), at different locations was developed. Life history parameter values incorporated into the model were estimated by laboratory rearing experiments. To validate the present model, the development of eggs laid monthly from June 1 through November 1 was simulated under dynamic temperature and photoperiod conditions at Ayabe City. The individuals laid on June 1 did not enter diapause but emerged in early August of the same year. On the other hand, about 2/3 of the individuals laid on July 1, and all those laid on August 1 and September 1 entered diapause (or quiescence), and started to emerge in late May of the following year. Individuals laid on October 1 and November 1 overwintered as young larvae (1st-3rd stadia) and eggs, respectively, and the majority of these emerged in late July-early August. Interestingly, the remaining individuals entered diapause in the 2nd year and emerged in June of the 3rd year. Analyses of these simulation results suggested that concentrated emergence of P. hilaris can occur twice in one year (in late May-early June and in late July-early August) at Ayabe, and this is fairly concordant with known adult prevalence at this location considering the long lifespan of adults. It was also suggested that although P. hilaris at Ayabe has basically a univoltine life cycle with a facultative diapause, varying proportions of the population appear to have a bivoltine life cycle or a semivoltine life cycle depending on the meteorological conditions of that year. The life cycle of P. hilaris is suggested to be very flexible and adaptive to environmental fluctuations.
Abstract. The interactive effects of temperature (20 °C or 25 °C) and photoperiod (LD 12 : 12 h or LD 15 : 9 h) on diapause induction and termination are investigated in the west‐Japan type yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe) (Coleoptera: Cerambycidae). Larval diapause of P. hilaris is induced under three diapause‐inducing conditions (20 °C–SD, 20 °C–LD and 25 °C–SD), and the diapause larvae are transferred to one of four conditions (20 °C–SD, 20 °C–LD, 25 °C–SD or 25 °C–LD) for observation of pupation, which indicates termination of diapause. The intensity of diapause induced under the three conditions increases in the order 20 °C–SD < 25 °C–SD < 20 °C–LD, when assessed by the time course of pupation after the transfer. On the other hand, the effectiveness of the temperature–photoperiod combinations to terminate diapause is in the order 25 °C–SD (ineffective) < < 20 °C–LD < 25 °C–LD < 20 °C–SD. Among the temperatures (5, 10, 15 and 20 °C) examined, 15 °C is the most effective in terminating diapause under the short day; diapause in most larvae appears to have been completed in 15 days.
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