Determination of larval melanization in the moth, Mamestra brassicae, and the role of melanin in thermoregulation

Abstract

Variability in the colour of lepidopteran larvae has been recorded in a broad range of species, yet little is known of its evolutionary significance, or whether it has a genetic basis. I assess the role of genes and environment in determining the degree of larval melanization in the moth, Mamestra brassicae, and examine functional aspects of larval pigmentation. In particular, whether melanization is of importance in thermoregulation, and whether larvae differing in melanization exhibit concurrent differences in size, rate of development, and fecundity. In the fourth and fifth instars, larval M. brassicae exhibit a continuous range of colour from pale green to black: a classification scheme is described to quantify this variation. Heritability (h2) was measured using regression of brood means against mid-parent values for 36 broods (2339 offspring), and was estimated to be 0.237 0.07 (SD) for fourth instar larvae and 0.421 0.10 (SD) for fifth instar larvae. However, environmental factors mediated development of larval colour: larvae were darker when reared at low temperature (12°C) compared to high (24°C). Direct measurement of larval temperatures using thermocouples inserted into the alimentary canal indicated that dark larvae absorbed more radiant heat, and thus under illumination consistently maintained a higher body temperature than pale larvae. Hence dark larvae are presumed to be at a selective advantage at low ambient temperatures, and increased melanization of larvae reared at low temperatures may be adaptive. I suggest that variation within natural populations may be maintained by fluctuating weather conditions. Dark larvae were found to be smaller, but developed more quickly than light larvae so that weight at pupation, time to pupation, and fecundity (measured by the number of fertile eggs produced) did not differ according to colour. Further studies are suggested to examine the influence of larval colour on thermoregulation, growth rates and predation in the field.