Monarch butterflies, once strictly limited to the New World, are now found in Europe too. Source: Kenneth Dwain Harrelson, Wikimedia Commons
The famous migratory butterfly, the monarch (Danaus plexippus), is native to North and Central America and northern South America. (Elsewhere in South America, it is replaced by the southern monarch [Danaus erippus], a similar but essentially non-migratory species.) But, while the adult butterfly can feed on the nectar of a wide range of flowers, the larva of this species is extremely limited in its food sources. It can consume only milkweed leaves (Asclepias spp.), a genus originally limited to the New World, as well as those of its next-of-kin, the false cotton bush (Gomphocarpus spp.), originally from South Africa. However, the monarch never managed to find the way to South Africa. So, for tens of thousands of years, the monarch was strictly limited to the New World where milkweeds abound.
Expansionist Ideas
The narrow leaf false cotton bush (Gomphocarpus fruticosus) is one of the food plants that have allowed the monarch to settle in new territories. Source: Xemenendura, Wikimedia Commons
That’s no longer the case! Humans have planted various species of false cotton bush and milkweed as ornamental plants, and a few have become naturalized well outside of their natural range. Tropical milkweed (Asclepias curassavica) and narrow leaf false cotton bush (Gomphocarpus fruticosus), for example, are well established in most subtropical regions around the world and that’s been the case since at least the beginning of the 19th century.
Monarch butterfly distribution today . Source: monarchlab.org
The northern monarch (D. plexippus) has taken advantage of the widened distribution of its host plants to expand its range. It first established a foothold in the Hawaiian Islands in the 1840s and from there spread throughout the islands of the South Pacific in the 1850s and 1860s. It first appeared in Australia and New Zealand in the 1870s and is now firmly entrenched in both countries, even fairly common in some areas.
So much for the Pacific. It somehow crossed the Atlantic as well, first showing up in the Canary Islands, off the coast of Africa, in the 1880s, then spreading to other islands in Macaronesia (Madeira, the Azores, etc.).
By Wind, Wing or Sail?
No one knows for certain how this dispersion took place. Did the butterfly travel as a stowaway on boats from one continent to another? Did it fly on its own? (Since adults can travel 1,400 miles [2,200 km] in the wild, that’s far from impossible). Or were they carried across oceans by the wind? We may never know.
Now Continental Europe
Monarch butterfly and caterpillar on tropical milkweed (Asclepias curassavica). Source: www.jungledragon.com
Monarch butterflies have been showing up sporadically in Europe for hundreds of years, but at first, showed no inclination of trying to settle down permanently. Then in 1980, a population of monarchs was found naturalized in southern Spain (Malaga) and it has since spread along the west coast of Portugal and the southern coast of Spain, two regions where tropical milkweed (Asclepias curassavica) and narrow-leaf false cotton bush (Gomphocarpus fruticosus) are well established. It is also seen in summer and early fall in France and the British Isles. In fact, one female laid eggs on a milkweed in Kew Gardens in London, but the gardeners there brought them indoors to hatch, so the next generation never had a chance to start a local generation.
Butterfly weed (A. tuberosa) is a hardy species that is grown as an ornamental in Europe, but probably not extensively enough to support a local population of monarch butterflies. Source: Aaron Carlson, Wikimedia Commons
Indeed, the monarch butterfly may never be able to fully settle in temperate Europe due to the lack of cold hardy milkweeds there. Some hardy species are now being grown as perennials, but only in gardens. Butterfly weed (A. tuberosa) and swamp milkweed (A. incarnata), for example, are known to European gardeners, but have not naturalized, which limits the possibilities for breeding monarch to settle there permanently.
Still, the many European readers of this blog can monitor the presence of this insect in their gardens in summer and early fall. The monarch is easy to recognize not only because of its flamboyant orange color with black veins, but also because it is the largest butterfly in Europe.
Whether the monarch only maintains a tenuous foothold in Europe or fully establishes itself remains to be seen. And whether its presence on European soil is a good thing or not is also questionable. Only time will tell!
Fortunately, numbers picked up considerably in 2014-2015 and jumped massively this winter (2015-2016), covering 4.01 hectares for an estimated 140 million monarchs. While this is still 30% below the long-term average of 6 hectares, it is still a massive improvement and corresponds to about the average number this century.
Last summer (2015), the weather couldn’t have been better. Where milkweeds still do grow, they were well-watered and healthy all across the monarch’s route north, just what they need to reproduce abundantly. Plus the news that monarchs need milkweeds as a food source is becoming common knowledge, thanks to programs like the 
