The Monarch butterfly (1 of 3): Roosting in the “wrong” trees?

The Overwintering Monarch's dependence on introduced Eucalyptus in California

Monarchs on Eucalyptus trees in Sacramento, California. Photo by Hazel Rodriguez/USFWS, Creative Commons Attribution 2.0 Generic license

The massive migrations of Danaus plexippus, the Monarch butterfly, are a well-known and much loved phenomena. Many people have marveled at photos and videos of trees in Mexico and California covered with thousands and thousands of the beautiful insects, their orange and black wings so densely packed they look like the trees’ foliage. Because of plummeting Monarch numbers since 1997, the Union for the Conservation of Nature has designated the Monarch’s migration a “threatened phenomena” that should be protected, and the plight of the butterfly has become, quite deservingly, a cause celebré.

What is less known is that in California, the annual event could be a recent historical development connected to European colonization. At the very least, its scale has been much enlarged by the introduction of Eucalyptus trees, which are the main roosting sites for overwintering Monarchs.

Eucalyptus is a genus from Australia that has become the object of vociferous hatred by anti-“invasive” crusaders—and of ardent defense by others, in part because it is vitally important to Monarchs. The last few years have seen heated battles throughout the Golden State over the fate of Eucalyptus trees in cities, parks and on state land. Many thousands of mature trees have fallen victim to the saws. The debate is not settled, and is fought around values as much as (if not more than) facts.