Monarch butterflies (Danaus plexippus) are best known from their migratory North American range, where they feed on approximately 30 species of milkweed (Asclepias). Less well-known is that monarchs have become established and form non-migratory populations in locations around the world, including most Pacific Islands, the Caribbean, Central and South America, and the Iberian Peninsula. In most of these non-migratory populations, monarchs have become narrow host plant specialists: they typically have access to only a single milkweed species, which in many cases is an evolutionarily novel host (e.g. Gomphocarpus spp.). We used a greenhouse rearing experiment to determine if monarch populations across the globe show patterns of local adaptation to their milkweed host plants. Monarchs from six populations (eastern and western North America, Hawaii, Guam, Australia, Puerto Rico) were reared on six milkweed species (Asclepias syriaca, A. incarnata, A. speciosa, A. fascicularis, A. curassavica, Gomphocarpus physocarpus) in a fully factorial design. We recorded larval growth rate, survival, time to eclosion and pupation, eclosion mass, adult wingspan, and levels of cardenolide sequestration.
Results/Conclusions
We found evidence for local adaptation in larval growth and survival metrics, though the magnitude of this local adaptation effect was small compared to inherent differences between host plants and monarch populations. Interestingly, the ancestral North American populations outperformed all other populations regardless of host plant, and larval performance was lowest when derived non-migratory populations were reared on their ancestral North American hosts. Our results are consistent with recent studies suggesting that neutral processes – relaxed selection and genetic drift – may play an important role in driving patterns of host plant specialization.