The costs of colour: plasticity of melanin pigmentation in an outbreaking polymorphic forest moth J. Ethier 1 *, M. Gasse 1 , K. Lake 2 , B.C. Jones 3 , M.L. Evenden 2 & E. Despland 1 1 Biology Department, Concordia University, 7141 Sherbrooke Street West, Montreal, QC, Canada H4B 1R6, 2 Department of Biological Sciences, University of Alberta, 11455 Saskatchewan Drive, Edmonton, AB, Canada T6G 2R3, and 3 Alberta Environment and Sustainable Resource Development, 8660 Bearspaw Dam Road NW, Calgary, AB, Canada T3L 1S4 Accepted: 10 December 2014 Key words: nutrition, population density, nitrogen, forest tent caterpillar, Malacosoma disstria, polymorphism, melanism, Lepidoptera, Lasiocampidae Abstract Outside the context of industrial melanism, little is known about the physiological and ecological importance of genetic melanic polymorphisms in moths. Melanin pigments are synthesized from amino acid precursors and should therefore be costly to produce in nitrogen-limited insects. A genetic melanic polymorphism is present in adult Malacosoma disstria H€ ubner (Lepidoptera: Lasio- campidae), a widespread forest moth with outbreaking population dynamics. We test the hypotheses that melanin-based colouration is physiologically costly in M. disstria, that expression of melanin- based colouration is a plastic trait which varies with population density and nutrition, and that the genetically based melanic phenotype is disadvantaged under nutritionally poor conditions. Two experiments were used to test these hypotheses. A field study compared pigmentation and pheno- typic frequencies in moths collected from high- and low-density populations. A laboratory experi- ment investigated the effects of larval nitrogen availability on adult pigmentation and phenotypic frequencies. High population density and nitrogen limitation reduced pigmentation and size of all moths, but phenotypic frequencies were not affected in either experiment. The effects of diet on both pigmentation and size were stronger for melanic moths than for typical moths. Our results show that adult melanism in M. disstria is physiologically costly, that colour expression is plastic despite its genetic component, and that the melanic phenotype may be disadvantaged under poor conditions but favoured under good conditions. We suggest that temporal variation in selection and trait plas- ticity help maintain polymorphism stability. Introduction Genetically based melanic polymorphisms are widespread among insects and have been linked to a variety of func- tions, including thermoregulation, sexual selection, mim- icry, aposematism, and immunity (reviewed in True, 2003). Perhaps the best known instance of insect melanic polymorphism is industrial melanism, observed not only in the famous peppered moth, Biston betularia L. (Lepi- doptera: Geometridae), but also in several other moth spe- cies (Kettlewell, 1973; Majerus, 1998; Grant, 1999; Cook et al., 2002; Cook, 2003). However, melanic polymor- phisms also occur in moth populations unaffected by industrial pollution (Owen, 1962; Lorimer, 1979; Sargent, 1985; Cook et al., 2002). Outside the context of industrial melanism, little to nothing is known about the functions of melanin-based colouration, the cost-benefit trade-offs for melanic and non-melanic phenotypes, and the rele- vance of melanic polymorphisms to population dynamics in moths. Stoehr (2006) proposed that melanin production should be more costly in insects than in vertebrate animals. Melanin pigments are nitrogen-rich and must be synthe- sized from amino acid precursors obtained in the diet, and insects have additional requirements for their nitrogenous resources that vertebrates do not have. The insect cuticle is rich in nitrogen, being composed mainly of protein and chitin (Hackman, 1953a,b,c; Wigglesworth, 1957; Ander- sen, 1979). Protein is also a major component of the silk *Correspondence: J. Ethier, Biology Department, Concordia University, 7141 Sherbrooke Street West, Montreal, QC, Canada H4B 1R6. E-mail: jess_ethier@yahoo.ca 242 © 2015 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 154: 242–250, 2015 DOI: 10.1111/eea.12275