What’s not to like about monarchs? Their caterpillars are gorgeously striped. Their chrysalises are an otherworldly green bejeweled with glimmering gold. The adults weigh only half as much as a paperclip, yet can migrate thousands of kilometres—and if you’re lucky enough to be in the right place at the right time you can see congregations of thousands. Monarchs can also make blue jays throw up—a delightful tidbit I learned while researching my new book about migration, Is This Panama? (beautifully illustrated by Soyeon Kim!). Sadly, just as the book went to press, bad news arrived: the over-wintering monarch population in Mexico was the lowest ever recorded.
|Roosting monarchs (Agunther)|
Though adult monarchs sip nectar from a variety of flowers, their caterpillars are specialist herbivores, which means they depend on a single type of plant for food. In the monarch’s case, these plants all belong to the milkweed family.
|Between hatching and forming a chrysalis, monarch caterpillars increase 3,000 times in size. These two caterpillars are in their second and third stages of growth, called instars. (Jan Thornhill)|
Milkweeds have evolved elaborate defenses to prevent themselves from being eaten, including uncomfortably hairy leaves, sticky white latex that bleeds from injured tissues, and toxins called cardenolides. Because of the cardenolides, most insects, as well as vertebrates, avoid eating milkweed, but the monarch has evolved to not only tolerate the toxins, it also uses those very poisons to its own advantage by storing them in its own body, which makes even an adult butterfly just as toxic as the plant. The first time a bird, such as a blue jay, eats a monarch, it will vomit a short time later. Though this is of no benefit to the monarch that was eaten, the bird will remember its bad experience and will avoid eating black-and-orange butterflies from then on.
Meanwhile, milkweed plants are continuing to evolve in response to being eaten by caterpillars. Recent genetic research is showing that there is an evolutionary trend in milkweeds towards healing themselves faster than they can be eaten, instead of producing more toxins that might eventually deter caterpillars.
|Monarch caterpillar on milkweed flowers (Jan Thornhill)|
So What Can We Do To Help?
Grow milkweed in your backyard or in your schoolyard:
Grow a “butterfly garden” with plants that provide food for caterpillars and/or nectar for butterflies:
Learn more about monarchs and share what you learn: http://www.monarchwatch.org
Help monarch researchers: http://www.monarchwatch.org/class/studproj/index.htm
Lobby your political representatives to have products made with genetically modified (GM) crops labeled.
*Pleasants, J.M and Oberhauser, K.S.. 2012. Milkweed loss in agricultural fields due to herbicide use: Effect on the Monarch Butterfly population. Insect Conservation and Diversity (March 12, doi: 10.1111/j.1752-4598.2012.00196x)