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Here in the northeast we saw a lot of monarch butterflies, both adults and caterpillars, in the summer and fall of 2019, more than have been seen for many years.
That produced surprise and enthusiasm among observers, and the many sightings raised hopes that the 20-year decline in monarch numbers had slowed or even reversed.
But World Wildlife Fund-Mexico recently released the official count for this winter, and it showed a significant decline, not an increase; less than half as many butterflies were seen in Mexico this winter compared to the previous winter. What happened? Why didn’t the numerous monarchs we saw throughout our region increase the abundance of the overall population?
First of all, the thrilling abundance of monarchs in 2019 was unusual. Monarchs that survive the winter in Mexico and migrate northward lay their eggs in Texas and other southern states, and the next generation develops from these eggs and continues the migration northward. Additional generations develop during the summer in the northern part of the range, expanding the size of the population. It just happened that at each stage in 2019 weather conditions were ideal for monarch growth and reproduction; excellent weather in 2019 – a rare occurrence – allowed for an unusual buildup of the number of adults.
We know that their numbers surged in 2019, but what happened to all those monarchs when they left the northeast? Though abundant in our region, monarchs were not equally numerous everywhere else. The regional differences are important because, based on recovery rates in Mexico of tagged individuals, monarchs from the northeast reach the Mexican overwintering grounds at a much lower rate than do monarchs from the Midwest. In short, our northeastern monarchs contribute less to the surviving and overwintering North American population than do monarchs from the Midwest. And in the Midwest, reproduction of monarchs has been harmed by loss of habitat, especially the decline in milkweed abundance because of industrialized agriculture and pesticide usage. Habitat in the northeast is less affected by these factors.
Climate change plays a significant role, too. Last fall’s warmth delayed the southward migration of monarchs, and that delay decreased their survivorship through the long journey. Importantly, drought along the migratory route in Texas and northern Mexico eliminated many nectar sources, right where monarchs must feed heavily to build up their energy (fat) supplies for the coming winter.
Sadly, the long-term decline in monarch abundance continues, despite an occasional good year such as we saw in 2019. Their overall numbers are determined by availability and quality of habitat (with milkweeds), freedom from pesticides, and weather conditions (influenced by climate change). Year 2019 was a fluke for the northeast, but it serves as a reminder that long term trends are better indicators of their status than single good or bad years and that critical determinants of monarch abundance are available habitat and weather throughout the butterfly’s full annual cycle.
Ernest 
Williams is an ecologist who taught at Hamilton College for 34 years. He has a summer camp on Twitchell Lake. He also provided the photos for this post.
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Depressing news, concisely and clearly presented.
Thank you Mr. Williams for a very interesting article , love seeing the Monarch Butterly and I am trying to do my part my sitting aside a portion of my garden for them.
Ernest, what a surprise, a fellow Twitchell Lake friend with a great piece published in the Almanack. Congrats, very interesting and informative, taking me back to your guided talks and walks along the Twitchell Inn Road to show us the life cycle of the Monarch with the milkweed growing there. I got some pictures of Monarchs in the process of coming out of the cocoon and ready to fly south to Mexico, I think you said, or Texas. That still amazes me that this fragile creature can do that! I put that in a category I call a “wonder of nature.” We have a ton of milkweed in our condo frontage here in Grafton, MA, and it keeps spreading. I will mention to you that a Lapidologist- is that the word for butterfly scientist?- by the name of William W. Hill spent several years at Number Four out toward Lowville, past Stillwater, studying ADK butterflies, and had his work published in Verplanck Colvin’s 7th Report on the Adirondack Survey (1880). Hill’s work was done from 1875 to 1878- have you seen that, an addendum to Colvin’s survey info. You can Google it and it was published as a separate work, now a free download. Also, Ed Pitts, Stillwater historian, has an article coming out in the May-June Adirondack Life on the butterfly work of Hill. Anyway, great to read your piece, hope you are staying safe, and look forward to seeing you at Twitchell this summer!
Noel
Noel Sherry
31 Church Street, Grafton, MA 01519 (residence rest of the year)
9 Twitchell Lake, Big Moose, NY 13331 (summer)
Thanks, Noel; I’ll look forward to seeing you in healthier, less constrained times. I’ll also look up what Hill reported. Quickly, butterflies and moths – scaly-winged insects – are called lepidoptera, named from the Greek for scale (lepido-) and wing (pter-), so those who study them are known as lepidopterists. And butterflies emerge from an uncovered pupa generally known as a chrysalis; a cocoon is the silk and leaf covering that most moths produce around their pupas.
We look forward to seeing how abundant monarchs will be this summer in the Adks, as well as to getting back to more normal – but safe and healthy – lives for all of us.
The GMO crop farmlands of the upper midwest actually had a strong migration https://youtu.be/bNDHIUmEFqg. But the western Great Plains didn’t and that shortfall was the reason numbers were lower in Mexico. Drought in Texas was not severe and nectar sources remained abundant.
Hi Ernest!
The Northeast must have produced more monarchs in the past when the landscape was more agricultural before WWII. The same reasons why monarchs are declining seem to be shared by other species needing young forest & grassland habitat. Dozens of neotropical migratory birds that need to feed their young caterpillars, bobwhite quail, meadowlark, bobolink, ruffed grouse, New England Cottontail and so on.
The region needs more early successional habitat. The closed-canopy forest, and now subsequent replacement with unsuitable sprawl, looks to be a major cause of the core of monarch breeding to have shifted to the Corn Belt.
Marcus,
I agree about the need for early successional habitat for many species of concern; the decline of grassland birds is well known. You also raise a good point about changes in our region over long periods of time.
20,000 years ago – milkweeds and monarchs at low latitudes, not here
1,000 years ago – how much here ???; extensive eastern forest cover
100 years ago – extensive deforestation; more extensive milkweed???
now – more forests than 100 years ago, but development has made the structure of the landscape complicated.
Ernest
The Explorer has posted a gallery of monarchs: https://www.adirondackexplorer.org/view_finder/monarchs-and-swallowtails
I live in NE Indiana. I have lots and lots of monarchs on my milkweed plants but hardly any caterpillars. Last couple summers I had several each day. Any idea why no caterpillars even though I have plenty of adult monarchs in the milkweed in my garden?