Environmental factors influencing pupal colour determination in Lepidoptera. II. Experiments with Pieris rapae, Pieris napi and Pieris brassicae

Abstract The control of the environmentally determined polymorphism for pupal coloration has been investigated in Pieris rapae, Pieris napi and Pieris brassicae. Light stimuli are the most important factors involved in the determination of pupal colour. In particular, yellow and blue spectral wavelengths determine the production of green and brown pupae, respectively, and it is argued that this correlation results from yellow light being closely associated with green foliage and blue light with sites away from green vegetation. Other, non-optical factors, such as relative humidity, may also influence pupal colour determination, but are of less importance than light stimuli. Light as a controlling factor in pupal coloration is considered relevant because the larvae of these species select pupation sites during daylight, when light cues would be of greatest use in distinguishing between green and brown environments. Diapausing pupae of P. rapae have a greater tendency to be brown than green, but the reverse is true in P. brassicae. It is uncertain whether this correlation between pupal colour and diapause is a consequence of behavioural or of physiological differences between larvae destined for pupal diapause and those that develop directly. In P. rapae and P. napi, green pupal pigmentation requires the secretion of a hormonal factor from a centre in the larval head. In the absence of this hormone, brown pupal cuticle is produced.