Wild Ideas: The decline of the monarch

Sidebar: How you can help monarchs

To help monarchs, plant more milkweed, push for monarch conservation and help with monitoring local populations. Citizens are invited to join the Rappahannock butterfly count on July 15. Expertise is not required and basic training in butterfly identification is provided. For more information, contact Don Hearl at 540-825-6660 (540-672-5712 after 5:30) or donh@ess-services.com. 

To learn more about monarchs and other butterflies, check out the following sources:

Historically, millions of monarch butterflies return to their overwintering grounds during Mexico’s Day of the Dead celebration at the end of October. Last fall was different, according to a Nov. 13 article in the New York Times:

“This year, for the first time in memory, the monarch butterflies didn’t come, at least not on the Day of the Dead. They began to straggle in a week later than usual, in record-low numbers. Last year’s low of 60 million now seems great compared with the fewer than three million that have shown up so far this year.”

An iconic butterfly species, the monarch (Danaus plexippus) has been in decline for the past 15 years, according to a study published in Insect Conservation and Diversity (March 2012). Monarchs overwinter in high-elevation fir forests of Mexico and journey up to 4,000 miles north to breed in the summer. During the 2009-10 overwintering season, the study’s authors write, the total area in Mexico occupied by the eastern North American population of overwintering monarch butterflies reached an all-time low and, despite an increase, remained low in 2010-11.

The reasons for the sudden collapse of monarch populations are complex, mirroring the insect’s complex life cycle, but they all the actions of Homo sapiens, starting with habitat loss. The monarch’s habitat has been disappearing through conversion of land to development and crops here in the U.S. and illegal logging in Mexico. Unstable weather due to global warming, with abnormal temperatures or precipitation, is also cited as among the causes of monarch declines.

At the core of the habitat issue here in the U.S. is loss of native plants, specifically milkweed. While adult monarchs will feed on a variety of flower nectars and fruits, their young will only eat milkweed species, in the genus Asclepias. The most common species, prosaically called “Common milkweed” (Asclepias syriaca), is only one among more than 20.

An adult monarch on butterfly weed, in the milkweed genus. While monarch caterpillers will eat only milkweed, the adults feed on nectar and fruit from a variety of plants.Patricia Temples
An adult monarch on butterfly weed, in the milkweed genus. While monarch caterpillers will eat only milkweed, the adults feed on nectar and fruit from a variety of plants.

They all produce toxic chemicals that deter most other animals from eating the milkweed and, in turn, the monarch. According to the same New York Times article, some monarchs can escalate their war on parasites by seeking out more toxic types of milkweed.

Land conversion is not the only threat to the monarch’s habitat. Federal subsidies have incentivized farmers to put every last acre of available land, including part of the U.S. conservation reserve, into corn and other crops used for biofuels. Most of the crops are genetically modified, primarily by Monsanto, to resist the herbicide glyphosate, the prime ingredient in Roundup, produced by the same company. Farmers can then use more lethal doses of glyphosate to destroy every native plant growing on cropland, including milkweed, without destroying the crops.

According to the Insect Conservation and Diversity study, Iowa alone has seen a “drastic reduction” of common milkweed growing in glyphosate-treated fields: a 90 percent loss from 1999-2009, and a 79 percent loss from 2000-09. We’re basically turning our native ecosystems into sterile, toxin-filled monocultures.

There is increasing pushback on the war on milkweed. Last year my conservation email lists lit up with requests from members for sources of the seed so they could plant more milkweed on their land. Conservation organizations have also been pushing to protect public lands where milkweed normally grows. As an Oct. 13 article in the New York Times put it, “After decades of trying to eradicate milkweed, gardeners are being encouraged to plant it in their gardens, and townships and counties are being asked to let it thrive in the roadside ditches.”

Will planting more milkweed here in the U.S. save the monarch? Its fate is tied up with its historic migration route. Not only are the fir forests being decimated by illegal logging, but ironically, people’s love of monarchs have led to increased numbers showing up in the overwintering grounds, disturbing the masses of monarchs that congregate there.

Abnormally high or low temperatures and precipitation caused by global warming have also led to increased monarch mortality, according to the authors of the Insect Conservation and Diversity study, among others. Those monarchs that do manage to find a place to overwinter and survive the weather and the ecotourists are finding their journey north increasingly challenging, as habitat along the way continues to disappear.

What about monarchs in Rappahannock? Data now being collected through the Old Rag Master Naturalists’ butterfly count (as part of the North American Butterfly Association’s annual count) will help chart the butterfly’s progress, but the three years of data collected so far is insufficient to show real trends. What we do know from the count is that alarmingly few monarchs have been recorded in the three years of the count and, unlike many other butterfly species counted, the trend for their numbers is not upward.

Although the number of species recorded in the 2013 count (41) had declined by almost 20 percent from the previous year, the number of individual butterflies counted more than doubled that of 2012 and almost quadrupled that of 2011.

Of the 4,798 individual butterflies counted last year, 2,375 of them (nearly half), were eastern tiger swallowtails. In contrast, only two monarchs — 0.04 percent of the total — were recorded. The previous year, the monarch percentage was up to 0.3 (eight individuals) and in 2011, it was 0.2 (two individuals). (See my column last August for more about last year’s count.)

It’s not surprising that the eastern tiger swallowtails fared better than the monarchs, although the actual numbers of swallowtails are pretty amazing. As across the animal kingdom, it comes down to generalists versus specialists. While the monarch young only feed on milkweed, the eastern tiger swallowtail’s young will dine on the foliage of many trees; if one tree species declines, the swallowtail can just shift to another.

A monarch caterpillar feeding on milkweed.Patricia Temples
A monarch caterpillar feeding on milkweed.

Unfortunately, data from the Rappahannock count is also in line with results from the Fauquier County NABA count, according to Louise Edsall, assistant director and beekeeper at Environmental Studies on the Piedmont, which manages the neighboring count. In an interview last summer following both NABA counts, she said that, as in Rappahannock, fewer than 10 monarchs were counted in Fauquier in both 2012 and 2013.

A participant in Project Monarch, a program of Monarch Watch, Edsall says in 2011 she was raising more than 50 of this species’ from eggs collected on milkweed in the wild: “I tagged over 100 monarchs in 2011. This year [2013] I have yet to find the first egg or caterpillar. So sad.”

She is not alone in being concerned. According to that New York Times article, some experts fear that the butterfly’s spectacular migration “could be near collapse.”

© 2014 Pam Owen

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Pam Owen
About Pam Owen 274 Articles
Writer, editor, photographer, and passionate nature conservationist living in Rappahannock County, in the Blue Ridge Mountains of Virginia. Two favorite quotes: By E.O. Wilson, who coined the term "biodiversity," "Nature holds the key to our aesthetic, intellectual, cognitive and even spiritual satisfaction”; by Douglas Adams, “I love deadlines. I love the whooshing sound they make as they pass by.”