I saw my first monarch butterfly (Danaus plexippus) three weeks ago. I was driving along on one of my favorite back roads between my house and campus, and I spotted it fluttering out over a diverse old field that was full of butterfly weed and milkweed. By early August I usually see a few monarchs around our house, too, but this year Deborah and didn’t see our first “local” monarch until almost the last day of the month! Monarch Watch (an organization that monitors the yearly surge and retreat of monarchs across North America) reports very low numbers of monarchs in the northern edges of their range this year. They blame storms that killed large numbers of the butterflies even before they could start out from Mexico on their northward trek and also the hot, dry summer weather. They also worry that temperature and day length cues that start the monarchs out on their migration in the early spring may no longer be precisely timed to the developmental cycles of the milkweed and the nectar producing plants growing along the migration route. The monarchs arrive in a habitat where there is no nectar for the adults and where the milkweed is not ready for the monarch eggs and larvae. The elegant efficiency of the monarch migration and its timing with their sustaining plants is beginning to crumble.
Pennsylvania is just one of stops on the seasonal North American northward migration of monarchs. Some of the adult monarchs that hatch here in mid-summer might, in a typical year, continue on north to lay more eggs on the later growing milkweed in New York and New England. I remember in the early 1980’s seeing clouds of monarchs along the New York Interstate just west of Syracuse! These butterflies may have had a part of their life cycle tied to the fields of Western Pennsylvania.
Other monarchs that mature here especially in late summer, though, will turn around and begin the long journey back south. They will lay their eggs on some late season, southern milkweed and then the next batch of adults (or maybe the 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.
Two years ago the census of the monarchs in these overwintering forest was alarming low! There were only 35 million individual monarchs in a shrunken forest range of just 3 acres. Twenty years ago the overwintering forest covered 45 acres with an estimated population of over a billion individuals! There is good news and bad news for this past year. The monarch population has recovered from its dangerously low numbers (this past winter there were 140 million individuals spreading over 10 acres of forest), but the population has still not returned to safe, sustaining levels (in 2002, for example, a single storm killed 500 million monarchs! The population must be large and robust enough to withstand that kind of natural stress!).
At each milkweed stop the monarchs carry out the same basic cycle: Each female after mating lays three to four hundred eggs on the milkweed (spreading her eggs out over a large number of plants). Then the adult monarchs die. The eggs hatch in three to five days depending on the temperature, and the emerging larvae (the “caterpillars”) feed first on the egg capsule and then begin to eat the milkweed leaves. They molt five times during their larval life stage and increase their body mass more than two thousand times.
The eggs and the larvae are under intense predation and parasite pressures. More than ninety percent of the eggs and caterpillars will fail to survive. Eggs are eaten by ants, earwigs and snails, and larvae are eaten by beetles and other insects (like paper wasps) or killed by parasitoid wasps, bacteria, or fungi. Since the larvae feed exclusively on milkweed leaves they accumulate the milkweed’s cardenolides (a cardiac glycoside that can cause the heart of a vertebrate to stop its contractions!) in their body tissues. These cardenolides make the larvae (and, eventually, the adults) poisonous to most vertebrates. Relatively few monarch caterpillars or adult butterflies, then, will be consumed by vertebrate predators. Research from scientists at Cornell University (published this past winter in the Proceedings of the Royal Society B (November 2015)) showed that monarchs have evolved enzymes that are themselves unaffected by the cardenolide toxins and are thus able to freely accumulate the milkweed toxins in their bodies.
The end stage caterpillar then forms a chrysalis within which the tissues and organs of the larvae dissolve and are reformed into the structures of the butterfly. This metamorphosis takes between nine and fifteen days. The adult butterfly then emerges, mates, and continues on its migration and cycle.
Doc and Linda Mueller have once again shared some of their carefully hatched and tended monarch caterpillars with Deborah and I. On the Sunday before classes started, we put nine of the caterpillars out on my dense cluster of milkweed plants on the west side of my house. The milkweed had had very little evidence of insect activity (most leaves were intact, etc.) before our adding the caterpillars, but even just two days after caterpillar introduction the leaves were showing high levels of shredding and skeletonizing! After five days I spotted one of the caterpillars now at least four times larger, working his way across a thick stem to get to some untouched leaves. My hope is that we get some chrysalises and some adults this year!
The Center for Biological Diversity has requested that the U. S. Fish and Wildlife Service classify the monarch as a “threatened” species. This would allow a better funded effort to be organized to try to re-establish the vital milkweed and nectar plant “stops” along the monarch’s traditional migration routes. It would also allow better coordination with Mexican authorities to try to control the illegal logging in the critical forests of Michoacán and Mexico. It could also help to fund research into the documentation of the ongoing effects of climate change on this once abundant but now increasingly rare butterfly species.