Guest Essay by Kip Hansen — 8 February 2024 —1900 words/10 minutes

The science on Monarch Butterflies has been getting a shake-up over the last little while. I have taken a deeper dive in the topic and share what I have found.  Part 1 started out with this:

In the worlds of entomology, ecological conservation and threatened and endangered species, there is general agreement on some points about monarchs:

1)  The number of monarch butterflies that have been migrating to overwintering grounds have sharply decreased from the very high numbers seen in the 1980s and 1990s.

2)  Agricultural practices on the great plains of North America and elsewhere have changed to include the use of weed killing agents (Roundup) that have eliminated the once plentiful milkweeds that in the past co-inhabited fields of corn and soy beans.  Milkweed is important as monarchs lay eggs only on milkweeds and monarch larva (caterpillars) only eat milkweed leaves. 

3)  There are a great many advocacy groups oriented to “saving the monarchs” – some are science-oriented and do monarch tracking, counting and tagging.  Some encourage rearing monarchs or gathering and protecting monarch chrysalises until the butterfly emerges. Some encourage planting milkweeds and “butterfly gardens” and supply milkweed plants or seeds.

4)  Commercial enterprises have arisen that mass breed monarchs (and other butterflies) either for scientific purposes (biology or nature classes) or for sale to be released at celebrations such as weddings. [examples, not recommendations:  here and here ]

I covered the first item on the list, population numbers as counted at overwintering roosts in Mexico and California, in Part 1.

This essay will cover list item #2,  the broader issue of milkweeds, Roundup, and their effect on both the populations of Monarch Butterflies and Milkweed species.

If you search in Google for “Why have monarch populations declined?”

Google will give you this answer, with a link to the Xerces Society:

“Loss of milkweed breeding habitat due to the widespread use of herbicide-resistant crops. Pesticide use, which kills non-target insects and degrades habitat. Climate change is affecting monarch populations in a number of ways. Logging and development have shrunk monarch overwintering sites.”

The above appears to be an AI response, taking the text of the captions of icons in the middle of the linked page.  The actual full text of the page linked contains this text:

“Monarchs require suitable habitat that provides host plants for breeding and flowering plants to provide nectar for adults. In the case of monarchs, the loss of milkweed means the loss of breeding habitat. Loss of milkweed from prime migration routes is primarily due to the dramatic increase in the use of the herbicide resistant crops Commonly known as Roundup™ Ready Crops, these corn and soy crops are genetically modified to be resistant to glyphosate, a broad-spectrum herbicide that kills everything other than the resistant crop, including milkweed. Harsher winters in monarch overwintering sites have caused larger than usual die-offs. Erratic weather may also delay the emergence of milkweed in spring and change the bloom time of flowering plants that provide resources to migrating monarchs. Legal and illegal logging in the oyamel fir forests of Mexico where eastern monarchs overwinter has removed important winter cover for the species and impacted microclimates that protect the butterflies from extreme cold and precipitation. In California, many sites where western monarchs overwinter have been lost due to development.”

The Center for Biological Diversity uses this language:

“Monarchs are threatened by pesticides — including toxic neonicotinoids and herbicides, which are killing off the milkweed plants they need to survive — as well as urban development and climate change.”

Thogmartin et al. (2017) “Monarch butterfly population decline in North America: identifying the threatening processes“ concluded:

“A structural equation model implicates the loss of milkweed as the mechanism by which glyphosate application influences monarch butterfly population size.”

The World Wildlife Fund says:

“Milkweed is the only plant on which monarchs will lay their eggs and the only source of food for baby caterpillars. But urban planning and agricultural expansion have paved and plowed over millions of acres of milkweed.”

A podcast piece on national PBS had this to say, spoken by Scott Hoffman Black, executive director of the Xerces Society for Invertebrate Conservation:

“But on an individual level, whether you’re a gardener, a farmer, manage a park or even a National Wildlife Preserve, you can make sure that we’ve got milkweed in the landscape, native milkweed are the best, as you mentioned, it’s the only host plant for monarchs are these native milkweeds, but also a buffet of flowers. So these butterflies, as you mentioned, migrate long distances, and they need lots of fuel.”

Good enough for our purposes, the general overall point of agreement seems to be that monarch populations, numbers of monarch butterflies, has been greatly reduced due to loss of milkweeds in the general environment, and particularly in the vast fields of corn and soybeans that have shifted to Roundup™ Ready Crops which are no longer co-inhabited by wild native milkweeds, often coupled with the idea that insecticide use in agriculture and home lawns and gardens must be killing monarchs as well as targeted species.

So, if everyone is so certain that the problem, or a great part of it anyway, is the lack of milkweed which is necessary for monarchs to breed and raise each new generation:

What’s the controversy?

To have a scientific finding, it is necessary to actually do some science.  If your hypothesis is that fewer monarchs today has been caused by fewer milkweed plants than there were before, then it follows that someone, somehow, must accurately count monarch butterflies and someone must accurately count milkweed plants.  Since the hypothesis is about fewer – a comparative measure of quantity – then one needs to have past counts and present counts to compare.

First, do we have these counts for monarch butterflies in general?  In the public eye, in the eyes of the Xerces Society, in the eyes of the World Wildlife Fund, and others, the count that counts is the number of monarch butterflies at overwintering sites – estimated by the size of the roosts in California and by the acreage of roosts in Mexico.

But, as we learned in Part 1 of this series, something is going on here, and we don’t know what it is.   In 2020-2021, the Western Monarch roosts were almost nonexistent – fewer than 2,000 monarchs were found – few enough at some roosts to count individually.  Many called this an extinction event.  But, then — a miracle!  The next year, when all were expecting even worse news, the overwintering sites were full of monarchs, well over 200,000. 

According to Chip Taylor, easily the most knowledgeable of all monarch experts, that was simply impossible: “ .. it’s impossible for a cohort of 1849 overwintering monarchs to initiate a cascade of reproduction over 3-4 generations that would result in a large fall migratory population” the next year of over 200,000. 

I mention this because it means, undeniably, that something is not right, something is wrong, something is inaccurate about the Western Monarch counts as they are currently carried out.  They do not, could not, represent the actual number of free-living, free breeding monarchs west of the Rocky Mountains.  This discovery, which was entirely accidental, as it was sprung on the monarch world by Nature itself, casts the same doubt now about the “counts” of monarchs in the Mexican reserves.

How about counts of milkweed plants?   John Pleasants  produced a paper in 2016  estimating the number of milkweed plants lost to Roundup.   The USGS made a big deal about a follow-up paper by one of its employees Wayne E Thogmartin published in 2017  on the number of milkweeds we needed to add to save the monarchs.  Pleasants and Thogmartin co-authored (with others) “Density estimates of monarch butterflies overwintering in central Mexico” (which was eventually published but had been harshly reviewed) in which they mention that: “There is, however, considerable variation in published estimates of overwintering density, ranging from 6.9–60.9 million ha−1.”  From that impossibly wide range on the number of monarchs overwintering in Mexico, for this winter the estimate would be from 15.25 million to 135 million monarchs, they extrapolate the number of milkweed stems needing planting in the U.S. to “save the monarchs”.

Summarizing:  Neither Monarchs themselves nor the Milkweed plants their breeding success depends on have been counted in any scientifically dependable way – though there have been some efforts and we may have to accept the results as “the best we’ve got for now.”

Now along comes a paper published in August 2023, in CellPress’s Current Biology titled: ”Temporal matches between monarch butterfly and milkweed population changes over the past 25,000 years“ by John H. Boyle et al. that announces these findings:

• Monarchs and milkweeds both had two concurrent population expansions in the past
• Neither species shows signs of recent declines in effective population size

Say what?  What exactly do they mean: effective population size?

“The number of individuals that effectively participates in producing the next generation is named effective population size.”

This is quickly followed by “Rethinking Monarchs: Does the Beloved Butterfly Need Our Help?” by Janet Marinelli on  January 15, 2024 in YaleEnvironment360. 

The lede to the article is:

“The Eastern monarch butterfly has long been thought to be in peril, but new studies indicate that its U.S. populations are not in decline. Scientists say the biggest threat the species faces is from well-meaning people who rear the butterflies at home and release them.”

Further, in “Opposing global change drivers counterbalance trends in breeding North American monarch butterflies” by Michael S. Crossley et al. (2022) in Global Change Biology:

“Here, we compile >135,000 monarch observations between 1993 and 2018 from the North American Butterfly Association’s annual butterfly count to examine spatiotemporal patterns and potential drivers of adult monarch relative abundance trends across the entire breeding range in eastern and western North America. While the data revealed declines at some sites, particularly the US Northeast and parts of the Midwest, numbers in other areas, notably the US Southeast and Northwest, were unchanged or increasing, yielding a slightly positive overall trend across the species range. ….  Overall, our results suggest that population growth in summer is compensating for losses during the winter and that changing environmental variables have offsetting effects on mortality and/or reproduction.”

Bottom Lines:

1.  Yes, monarch counts at overwintering sites, both in California and in Mexico, have been down.

2.  The very high counts of the 1980s and 1990s may well have been the boom years predictable by chaotic population dynamics (or mine here)

3.  The California “great rebound” shows that the counts may not be including all the overwintering monarchs – certainly not in California and, by extension, maybe not in Mexico.

4.  Using “effective population size”, it has been found the neither Monarchs or Milkweeds have declined overall.

5.  Monarchs do not need further protection and certainly do not need to be listed under the Endangered Species Act as “endangered”.

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Author’s Comment:

It takes a lot of nerve to do research and publish findings that are contrary to long-standing shared viewpoints in any field.  Kudos to those mentioned above who have done so.  We can only hope that the United States Fish and Wildlife Service (FWS) and the National Marine Fisheries Service (NMFS) follow the letter of the law and use all available science before acting on the constant drumbeat of demand to list the Monarchs as endangered.

One more part to this series, I think, on the last two points on the list.

Thanks for reading.

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