Signs of Summer 11: Monarchs and Milkweed

Female Monarch (photo by K.D.Harrelson (Wikimedia Commons))

Over the past few years I have written three or four essays about monarch butterflies and emphasized their dependence upon milkweed plants. The various species of milkweed that grow across North America are the only plants on which the monarch can lay its eggs. The chemicals (the “cardenolides”) that the monarch caterpillar accumulates from feeding on the milkweed make it and also the adult butterfly it will turn into poisonous to, and thus protected from, most potential vertebrate predators. Monarch populations have declined greatly over the past decade. Human impacts on old field and edge ecosystems where milkweed can flourish have greatly reduced the distribution of this important plant and are at least one of the factors that is causing the observed population declines of this beautiful butterfly.

Just outside my back room where my writing desk is located there is a stand of fifteen very hearty milkweed plants. They started flowering in early July and were covered with honey bees, bumble bees, tiger swallowtail butterflies, and fritillary butterflies for several weeks. Through most of July, though, I did not see any monarchs, and the milkweed leaves showed no evidence of any caterpillar predation.

Doc Mueller is a colleague at Penn State New Kensington. Last year Doc and his wife Linda raised some painted lady butterflies from caterpillars, and this year they have decided to raise monarchs. Last fall I gave Doc some milkweed seed pods which he stored out in his garage all winter to cold stratify. He planted the milkweed seeds in flower pots this spring and they germinated beautifully (something I have never accomplished. My thumb is no shade of green at all!). He put the potted milkweed shoots in the entryway to his house and then ordered some monarch caterpillars from an on-line biological supply company. The caterpillars arrived and ravenously took to the milkweed shredding leaf after leaf turning them into caterpillar biomass and a prodigious pile of black fecal pellets.

Photo by D. Sillman

Doc and Linda quickly recognized that they had more caterpillars than their milkweed would support, so they gave me eight caterpillars to put out on my plants. I was concerned about matching up these tiny caterpillars (which were probably in the third or maybe fourth stages of the five instar series they would have to develop through) to the great, thick leaves of my milkweed plants, but as you can see from the picture to the left where there is a will (or a hunger) there is a way! The leaves on my plants slowly took on the characteristic ragged edges and gouged out tips and sides that are hallmarks of monarch inhabited milkweed.

I don’t know how many of my eight caterpillars will make it to adulthood. In the wild some 90% of the caterpillars succumb to disease, invertebrate predators, parasites, or parasitoids. The dense populations of birds in my yard are also a worry: as far as I know none of them have every experienced eating monarch caterpillars, and they may just try one or two before they realized their toxicity! I don’t have nearly enough caterpillars, though, to teach all of these birds a culinary lesson!

I looked over my milkweed plants carefully this morning. There was extensive leaf damage from feeding caterpillars but no visible chrysalises. A number of the lower leaves, though, are curled up into tight clumps that would be just the place to make a very protected cocoon! I did see my first monarch of the summer fluttering around the side of the house. I can’t say for sure that it is one of “my” monarchs, but why not?

I heard from Doc this past weekend: four of his caterpillars made chrysalises and that three of them have already emerged as butterflies! He moved the milkweed pots outside so that the butterflies could stretch their wings and fly away. Although it is hard to watch them leave after so many weeks of careful watching, it was time to let them choose to go north or south.

Photo by D. Sillman

Pennsylvania is just one of stops on the seasonal North American northward surge and southward retreat of monarchs. Some of the adult monarchs that hatch here will continue on north to lay more eggs on the later growing milkweed in New York and New England. Other monarchs that mature here will turn around and begin the long journey back south. They will lay their eggs on some late season, southern milkweed and then that next batch of adults (or maybe the next cohort that comes after that) will head to the coniferous forests in the mountains of the Mexican states of Michoacán and Mexico where they will overwinter. Maria Franco, one of our Penn State colleagues, grew up in a town close to these butterfly forests. She remembers as a child seeing clouds of monarchs flying past. At the time, though, she was not aware of their communal overwintering site up in the nearby mountains. One of these Januaries Deborah and I are going with Maria and her husband Javier (also a Penn State colleague) to see these great forests covered with millions and millions of monarchs!

I would encourage everyone to either plant some milkweed somewhere on your property or at least tolerate the milkweed that is growing there voluntarily. The loss of the seasonally growing strata of milkweed across North America is cutting great holes in the support system for the monarchs and could lead to their extinction. Barbara Kingsolver wrote about the plight of the monarchs and their fractured migratory pathways in her recent novel “Flight Behavior.” I recommend it to all of you as not only an excellent read but also a very solid piece of speculative science.

Happy summer, everyone!