Partially coupled reaction norms of pupal colour and spot size in a butterfly

Pupal colour plasticity in the butterfly Catopsilia pomona (Lepidoptera: Pieridae)

Pupal colour plasticity (PCP) in many lepidopterans can be adaptive by helping pupae match their background colours. Studies on PCP, most of which have been laboratory-based, have largely used human assessment of colour to categorize pupae as green or brown. This binary categorization limits the understanding of finer pupal colour variations and their function. We conducted a study of PCP in the butterfly Catopsilia pomona by comparing laboratory-reared and wild populations. Considering pupal colour as a continuous variable, we showed that a large proportion of the pupae matched the colours of their substrates, with leaf-borne pupae tending to be greener and off-leaf pupae browner. Pupal colour also responded to the leaf substrate’s finer colour variations, highlighting the importance of treating pupal colour as a continuous variable. Compared to the wild population, the laboratory population had more green pupae on off-leaf substrates. Our study thus illustrates that caution should be used when extrapolating the results from laboratory-based studies to the natural world. In leaf-borne wild pupae, pupation position on the leaf, the thickness of the midrib where pupation occurred and the leaf’s length influenced the pupal colour. Our study underscores the need for further research on PCP as a background-matching strategy in light of predation.