Monarch Danaus plexippus

The Monarch is one of the world’s most charismatic insects. Native to the Americas, since the 19th century it has become increasingly cosmopolitan. The sporadic records in Western Europe were first attributed to vagrants from across the Atlantic; then colonies were confirmed in the Canary Islands and later in Iberia. This expansion has speculatively been linked to global warming, but more certainly is due to the arrival of other colonists: milkweeds, such as the Bristly-fruited Silkweed from South Africa. Milkweeds are the food plants of Monarch larvae and their juices make the Monarch poisonous to predatory birds. The bright orange of the butterfly is really a warning: keep off!

In Portugal, A Rocha observers first spotted the butterfly in October 1998, and since then it has become an increasingly familiar visitor to the Cruzinha gardens. Could they be breeding locally? There is a small stand of milkweed on the local Quinta, and on 11 June last year we spotted a lone adult flitting from plant to plant. Following it through the binoculars, we were able to observe ovipositing and later, sure enough, found four pale eggs, laid singly on the leaves and bristly fruits. The eggs soon hatched into the impressive and brightly striped caterpillars.

It is for the amazing annual migration across North America that these butterflies are best known. They overwinter in Mexico in about 6-12 hectares of Oyamel forests in the Transvolcanic Mountains, where as many as 160 million butterflies can make it hard to see the trees for the butterflies! In the spring these adults set off north, laying eggs on the first available milkweed plants, maybe in Texas. The caterpillars feed and grow and as adults, continue the journey north, laying their eggs. By September, the third and fourth generation adults (which could be anywhere from Maine to Minnesota) start to migrate back down to Mexico. It is possible to lie on your back on the sand on Assateague Island, on the Maryland coast, and count a steady stream of butterflies passing overhead, their navigation system still a mystery to us.

Efforts are underway to protect the overwintering sites from logging, but further threats exist. Milkweed plants are considered to be noxious by many people, and removed. Insecticides may harm the butterfly. Invertebrate predators and parasites on the larvae can also decimate populations. A Rocha USA is monitoring a milkweed patch weekly through the summer for eggs, larvae and adults, within the bounds of the nation’s capital, in conjunction with the National Park Service. This is part of a nationwide citizen science project, the purpose of which is to increase understanding of the population biology of the Monarch and threats to its survival. It also has the effect of drawing attention to the presence of this amazing species in meadows not far from downtown Washington DC.

- Ann Stuart and Will Simonson