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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

A Good Year for Monarchs? Monarch butterfly, Danaus Plexippus


Monarchs and sunflowers -  pretty spectacular!


Each summer about this time we devote an episode to catching up with the wondrous monarch butterfly. As seasons go, this one at my home in Columbia, MD seems to be one of the best in recent years for these long-distance travelers. My first monarch sighting took place in early July and since then almost on a daily basis I have seen several gliding about the landscape, sipping nectar from blossoms, and searching for just the right milkweed leaf on which to deposit eggs. Presently, my small patch of butterfly milkweeds is being devoured by one large, one mid-sized, and one tiny monarch caterpillar, abetted by several milkweed leaf beetles like ones we met in a previous episode. Last week during a brief moment of sunshine, I watched a female deposit an egg on the underside of a leaf, ergo the caterpillar show isn’t over yet. As most of you know, during spring and summer these peripatetic beauties depart their winter refuge in Mexico and continue their journey north for months, reaching milkweed patches in the northern US and southern Canada. As summer wains, days grow shorter, temperatures cool, and these vagabonds begin the epic journey thousands of miles south to their wintering sites in the mountains of Mexico, truly remarkable.

After imbibing nectar, the monarch finds just the right leaf on which to place an egg. Under the watchful gaze of a milkweed leaf beetle, a tiny monarch caterpillar consumes a leaf but soon it will grow into a behemoth capable of consuming milkweed seedpods. Within a dazzling chrysalis, a caterpillar becomes a butterfly ready to feed, fly, and spawn the next generation.

Are monarchs really enjoying a better year in 2018, or is this simply a figment of my “glass half full” mentality? Let’s catch up a bit with some facts regarding the status of monarchs in 2018. We start with how they fared last winter in the mountains of Mexico. According to Monarch Watch, 9 monarch colonies were spotted in the Monarch Butterfly Biosphere Preserve in Mexico during the winter of 2017–2018. The area occupied by these beauties was 2.48 hectares, roughly a 15% decline from the hectares occupied by monarchs during the winter of 2016-2017 and only a fraction of the all–time high of more than 18 hectares recorded in the winter of 1996–1997, the year that Los del Rio topped the charts with Macarena. Despite a decline in monarchs overwintering in Mexico, Professor Chip Taylor of Monarch Watch suggested that due to a combination of spring climatic events, monarchs returning to the US may have been bottled up in Texas where warm temperatures elevated larval survival and development. This resulted in a bumper crop of monarchs migrating north a bit later in the season. Here in the mid-Atlantic generous rainfall in June, July, and August has produced lush stands of milkweeds along highways, field edges, and at monarch waystations. Let’s hope benign weather favors milkweeds and monarchs the remainder of this year and boosts populations of migrating monarchs throughout eastern North America.

Dramatic decline in overwintering populations of monarchs in Mexico portends a gloomy fate for Eastern Monarchs unless this trend can be reversed. Image credit: Center for Biological Diversity

Monarch guru Lincoln Brower believes several factors conspire to reduce populations of monarchs. Illegal logging of trees in the mountains of Mexico has reduced the critical overwintering habitat for monarchs. Without this refuge the monarchs cannot survive winter. Scientists suggest that more severe storms associated with climate change may also threaten monarchs. In the winter of 2015–2016, according to one report, a late winter storm froze or killed more than 7% of overwintering monarchs. This translated into several million fewer monarchs making their way north for the annual migration. Here in the US, urban sprawl and the use of herbicides in agricultural production greatly reduce populations of milkweed plants that are vital for the survival of monarch caterpillars. There is also concern that planting the exotic tropical milkweed Asclepias curassavica in southern states may interrupt the primal migration of monarchs and cause them to take up residence in the south, eschewing their age-old migration northward.

In the waning weeks of summer, go to the meadow and enjoy these beauties, and next spring plan to include milkweed in your perennial gardens. Be sure to consult a reference to learn what milkweeds work well in your geographic region. Here in Maryland, species including common milkweed, Asclepias syriaca, swamp milkweed, Asclepias incarnate, and butterfly weed, Asclepias tuberosa, are good choices. We all can play a role in conserving these remarkable vagabonds.


The excellent references, “Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?” by Lincoln Brower and colleagues, and “Foraging Dynamics of Bird Predators on Overwintering Monarch Butterflies in Mexico” by Lincoln Brower, were consulted for this story. Information used in the episode also came from the Monarch Watch website and several of the other websites listed below. To learn more about monarchs, their migrations and perils, and how to conserve them, please visit the following websites: