Seasonal environmental fluctuations provide formidable challenges for living organisms, especially small ectotherms such as butterflies. A common strategy to cope with harsh environments is to enter diapause, but some species avoid the unsuitable conditions by migrating. Despite a growing understanding of migration in the life cycles of some butterfly species, it is unknown how individuals register and store environmental cues to determine whether and where to migrate. Here, we explored how competition and host plant availability during larval development affect patterns of DNA methylation in the migratory painted lady (Vanessa cardui) butterfly. We identify a set of potentially functional methylome shifts associated with differences in the environment, indicating that DNA methylation is involved in the response to different conditions during larval development. By analysing the transcriptome for the same samples used for methylation profiling, we also uncovered a non-linear relationship between gene body methylation and gene expression. Our results provide a starting point for understanding the interplay between DNA methylation and gene expression in butterflies in general and how differences in environmental conditions during development can trigger unique epigenetic marks that might be important for behavioural decisions in the adult stage.

Organisms are forced to adapt to seasonal variation in both abiotic and biotic factors. Butterflies and moths (lepidopterans) have adopted strategies to cope with unfavourable conditions by either entering a state of dormancy and metabolic inactivity (diapause) or by migrating to regions with more favourable conditions, or a combination thereof (Urquhart, 1976;Williams, 1930). While the mechanistic underpinnings of diapause in lepidopterans have been investigated in some detail (e.g.,