Effect of density on pupal colour plasticity in the butterfly Mycalesis mineus (Lepidoptera, Nymphalidae)

Abstract

We tested the effect of larval density on pupal colour plasticity in the butterfly Mycalesis mineus. Pupal colour of this species is known to be a plastic phenotype, being either brown or green. Colour is correlated with pupation substrate: pupae on leaves are almost exclusively green, whereas off-leaf substrates (stem, soil, etc.) include both green and brown pupae. We hypothesised that brown pupae are more frequent at high larval densities and tested this hypothesis using two experiments. In the first experiment, we reared larvae at different densities in plastic containers, which provided a restricted choice of pupation substrates. There was no influence of density on pupal colour. In the second experiment, we simulated more natural conditions, wherein larvae were reared in nylon mesh sleeves and had a greater choice of pupation substrates. Here, there was a strong correlation between larval density and the proportion of brown pupae. We discuss why results from the two experiments may have differed and what can be concluded about factors influencing pupal colour. We also discuss the importance of choice of experimental protocols, especially in relation to how realism trades off with practicality and control.