The monarch butterfly, those iconic orange and black insects seen flying south across Missouri in the fall, are the focus of a lot of effort and money these days. The National Fish and Wildlife Foundation (NFWF) recently provided more than $3 million in grants aimed at increasing and improving monarch habitat in North America. Almost a third of the initial grants have been directed at Missouri, which is smack in the middle of the monarch’s migration flyway.
Why the sudden interest in monarchs? Over the past 20 years, the population of monarch butterflies most Missourians know for their unique, two-way migration, has plunged from more than 1 billion to fewer than 60 million. While monarch abundance can certainly fluctuate markedly between years, this sustained, long-term decline is alarming.
A look at monarch biology sheds light on the survival hardships these unique insects face. The monarchs now flying toward overwintering sites in Mexico are the last of four or five generations that were born in North America since last April.
A good starting point to discuss the monarch life cycle is to begin in the highland forests near Mexico City, where millions of monarchs cluster together on fir trees to survive the winter. As springtime begins, so does the northward migration of these butterflies—the same monarchs that flew up to 3,000 miles from their birthplaces in North America to Mexico the previous fall. Those who survived the winter are faded and tired. What they want most is nectar to restore energy—enough energy to have sex and lay eggs before they die. They have been in sexual diapause (not breeding) since their birth and now they are eager to procreate.
These butterflies are senior citizens by monarch standards—eight or nine months old by the time they produce the first generation in the North American breeding range. The monarchs who breed during the summer will live just three to six weeks.
A female monarch is picky about where she chooses to place the 300 to 500 eggs she can lay over her lifetime. Milkweed plants are the only place a female will lay eggs, and the larvae (caterpillars) that hatch three or four days later will only eat milkweed. The sap from milkweed plants contains toxins called cardiac glycosides, which, when ingested, makes caterpillars unpalatable to most birds.
Lack of available milkweed across the breeding range is one of the key reasons cited for monarch decline. Once abundant, milkweed has become relatively limited due to farming practices, roadside and utility right of way mowing and spraying, and loss of grasslands to development and agriculture. Other contributors to the decline include logging of the monarch’s Mexican habitat and weather.
A newly hatched monarch caterpillar first consumes its own eggshell. Then it becomes a milkweed-munching machine that will increase its weight 2,000-fold as it grows through four molts (skin sheds) over a period of approximately two weeks.
Unfortunately, less than ten percent of monarch larvae will make it to the next life stage—pupation. A caterpillar ready to pupate spins a silky net on a branch, leaf, or almost any place where it feels safe. It attaches itself upside down to the silk button and its body forms a “J” shape. As it wriggles and writhes through one final molt, its skin appears to “unzip” to reveal the light green chrysalis within. Soft at first, the chrysalis soon hardens, protecting the approximately 12-days of dramatic body change that will produce the adult butterfly.
Upon emerging, the job of adult monarchs in summer generations is reproduction. Mating monarchs may stay coupled for hours, occasionally nectaring and even flying while linked. Females lay eggs after their first mating (which can occur at three days after emerging as an adult), and mate multiple times over their short lifespan.
As the growing season progresses, subsequent generations of monarchs tend to move northward following the milkweed. The first cold days of late August in the northern range prompt newly emerged adults of what, by then, is the fourth of fifth generation, to begin preparing for fall migration. It is that generation—the one that trades sex for longevity—that will start the cycle again in the spring.
Lee Phillion is a Missouri Master Naturalist partnering with Missourians for Monarchs.
Special thanks to Journey North for both graphics.