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.

Pupal colour–pupation substrate correlation in butterflies

“…Correlation between substrate choice and density, larval developmental time, pupal weight, and sex are indicated by GLM analyses (see Table S2). species is (Mayekar & Kodandaramaiah, 2017, 2022. Furthermore, in experiments where the species was forced to pupate in plastic containers, no brown pupae were formed (Mayekar & Kodandaramaiah, 2022).…”

“…Our previous experimental work on M. mineus involving systematic manipulations has shown that pupal colour in this species is plastic (Mayekar & Kodandaramaiah, 2017, 2021, 2022). Furthermore, in experiments where the species was forced to pupate in plastic containers, no brown pupae were formed (Mayekar & Kodandaramaiah, 2022). Thus, it is highly unlikely that genetic polymorphism explains the pupal colour variation in M. mineus .…”

“…Brown pupae are exclusively formed on off‐leaf substrates, whereas green pupae are found on both leaf and off‐leaf substrates. The proportion of brown pupae is greater at high larval density, which may be related to larvae being forced to pupate away from leaves under crowded conditions (Mayekar & Kodandaramaiah, 2022). Larvae forming brown pupae develop faster than larvae forming green pupae, and this may be because larvae that mature first experience higher effective density on leaves (Mayekar & Kodandaramaiah, 2022).…”

Effects of temperature on monarch caterpillar pigment variation in nature

“…Because of the clear link between colouration and fitness, several of the biotic and abiotic factors that mediate intraspecific variation in colour pattern have been characterised. In lepidopterans, where the bulk of this research has been conducted (Davis, 2014), rearing density altered pupae colour in the butterfly Mycalesis mineus (Mayekar & Kodandaramaiah, 2021), but did not affect wing pigmentation in the wood tiger moth Parasemia plantaginis (Nokelainen et al, 2013) or in the monarch butterfly Danaus plexippus (Atterholt & Solensky, 2010).…”

“…Because of the clear link between colouration and fitness, several of the biotic and abiotic factors that mediate intraspecific variation in colour pattern have been characterised. In lepidopterans, where the bulk of this research has been conducted (Davis, 2014), rearing density altered pupae colour in the butterfly Mycalesis mineus (Mayekar & Kodandaramaiah, 2021), but did not affect wing pigmentation in the wood tiger moth Parasemia plantaginis (Nokelainen et al, 2013) or in the monarch butterfly Danaus plexippus (Atterholt & Solensky, 2010). With respect to abiotic factors, among other examples, geometrid moths were darker at higher latitudes (Heidrich et al, 2018), Himalayan butterfly Pieris canidia was darker at higher elevations (Gautam & Kunte, 2020) and drier habitats produced darker larvae of the moth Chiasmia clathrata (Välimäki et al, 2015).…”