Over the past month I have seen a number of monarch butterflies (Danaus plexippus) both around my house and up at Harrison Hills Park. The fields at Harrison Hills are loaded with butterfly weed and milkweed and also a great diversity of flowering plants (nectar sources not only for monarchs but for many important insects and birds! Check out Deborah’s website about the wildflowers of Harrison Hills Park ( https://sites.psu.edu/harrisonhills/ )). For the past eight years I have also nurtured an expanding patch of milkweed on the east side of my house in the hope that monarchs will find it and lay their eggs on it so that their caterpillars will then use it a food source. I have even “seeded” the plants with monarch caterpillars hatched and raised by Doc and Linda Mueller, but have not had successful chrysalis formations or butterfly emergences. I have seen numerous tussock moth caterpillars on my milkweed (they really chew up the leaves!) but no monarch caterpillars except those few we have introduced.
Monarch Watch (an organization that monitors the yearly surge and retreat of monarchs across North America) reports that the unusual March and April 2018 weather patterns may have had a surprisingly positive impact on the migrating monarchs. While most of the rest of the country (especially the East) was unseasonably cold this March and April, Texas was, as has been the ongoing trend over the past 20 years, unseasonably warm (in fact March 2018 in Texas was 5.3 degrees F warmer than average!). The surrounding cold weather caused the migrating monarchs from Mexico to stay in Texas for longer than usual, and fortunately, they responded with a population boom possibly assisted by the warm temperatures! Although limited by the available milkweed, the Texas monarchs multiplied and then, as the weather in April began to moderate, surged out across the eastern and midwestern United States.
Monarch migration maps at JourneyNorth.org show a steadily expanding line of migration of the butterflies: In March they were in the southern-most parts of the country, between Texas and Florida, by April they had traveled north to a line roughly between Oklahoma and South Carolina, and by May they had moved up past a line between Kansas and Maryland. By early June they had reached a line that stretched from Iowa to Ohio, and by late June had finally gotten to Pennsylvania, New York and New England.
The monarchs were reproducing all along the way (so milkweed was critical at each location!). At each stop each mated female laid three to four hundred eggs on the milkweed (spreading her eggs out over a large number of plants), and then the adult monarchs died. The eggs hatched in three to five days depending on the temperature, and the emerging larvae (the “caterpillars”) fed first on their egg capsules and then began to eat the milkweed leaves. They molted five times during their larval life stage and increased their body mass more than two thousand times. The caterpillars take between 9 and 14 days to go through their five instar growth phases.
The monarch 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 in their body tissues the milkweed’s cardenolides (a cardiac glycoside that can cause the heart of a vertebrate to stop its contractions!). These cardenolides make the larvae (and, eventually, the adults) poisonous to most vertebrates.
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.
So at each location, the caterpillars are seen a week or so behind the initial appearance of the arriving adults, and the adults from these caterpillars who then push on northward emerge from their chrysalises about a month later. By late July, migrating adults and caterpillars in many stages of coming and going are all over Western Pennsylvania. It takes anywhere from three to six mating/development cycles for the monarchs to reach the northern edges of their North American range!
Pennsylvania is just one of stops on this seasonal northward surge and eventual southward retreat 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 developing milkweed in New York and New England. I remember in the early 1980’s seeing clouds of monarchs along the New York State Thruway 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 in late summer, though, will turn around and begin the long journey back south. These late summer/early fall born monarchs are part of the overwintering cohort that tries to find their way to the coniferous forests in the mountains of the Mexican states of Michoacán and Mexico. These overwintering monarchs live 8 or 9 months (compared to 2 to 5 week life span of the “summer” monarchs) and will be the individuals that push back north into Texas next February and March where they will mate and lay eggs thus starting the migration cycle all over again!
One of the most accurate ways to access the monarch butterfly population is to count them when they are in their overwintering forests or to measure the area of the forests that have monarchs. In the winter of 1996-1997 18.19 hectares of forest were covered with overwintering monarchs. Last winter, though, there were only 2.48 hectares of forest with monarchs! Monarchs are reaching dangerously low population densities!
Some great local news! Deborah spotted several large (possibly 5th instar) monarch caterpillars on milkweed and multiflora rose plants at Harrison Hills Park! Also, Jane Glenn has reported monarch eggs on her milkweed in New Kensington, and Doc and Linda Mueller report that they have “wild” monarch caterpillars on their milkweed with abundant eggs and that they just received this year’s shipment of mail-order monarch caterpillars (orders were delayed because of unexpectedly large demands for monarch caterpillars!).
I hope that our final migration and overwintering count improves over last year! So many people around the country are trying to pitch in to save the monarch! Plant some milkweed! Plant native plants (for nectar fueling sites!)! Use fewer pesticides! All of Nature will benefit from these steps, and we will help bring the monarchs back!