Monarch population size over winter 2019-2020 announced by WWF Mexico: not great news!

mon2The last 10 years have been a wild ride for the monarch butterfly population. Between 2010 and 2014 was the all-time population low, as recorded during overwintering in Mexico and reported by the WWF.  This terrible low point, something I termed in the “red zone” corresponded to the 100-year drought in Northern Mexico & Texas, and the two are most probably linked.  Since then, the population increased, doubled, then tripled, and then stayed more or less comfortably even.  Although this bounce back was a relief, there have been questions about whether the monarch butterfly has truly rebounded.  I note here that even if there was a true rebound, that does not mean the monarch population is healthy and stable, just that it is above the red zone (even still, the population is more than 60% lower than the highs observed in the mid-1990s, see below).

A new statistical study of the monarch population (not yet peer-reviewed, but available here) authored by Thogmartin et al. stated: “If the population estimate for winter 2019 is ≥4.0 ha, we will then be able to credibly assert the population has been increasing since 2014.”  Unfortunately, given that the butterflies have been declining over the past three decades, this year’s annual announcement was not welcome news: the monarch population has not been on a sustained increase over the past 6 years.


The estimates of the monarch butterfly overwintering population were announced today (March. 13th 2020) by WWF Mexico: unfortunately, they are about half the density of last year.  The butterflies are so dense at their dozen or so mountain-top clustering sites that overwintering butterflies cannot be counted individually. Instead, the area of forest that is densely coated with butterflies (at about 5,000 butterflies per square meter looking up into the canopy) is estimated as a measure of monarch abundance. Butterflies arrive to Mexico around the day of dead in November and stay until March each year as part of their annual migratory cycle.  Indeed, the monarchs of 2020 have already been observed migrating into Texas and laying eggs!

The annual multi-generational migratory cycle of the monarch butterfly. The southernmost red dot indicates the high elevation overwintering grounds in central Mexico where populations are censused. North pointing arrows indicate the spring and summer generations that migrate, breed, and eat milkweed. Learn more in my book Monarchs and Milkweed.

This winter season (2019-2020), there were approximately 2.83 hectares (nearly 7 acres) of forest occupied with dense monarchs in the Mexican highlands (somewhere in the neighborhood of 150,000 million overwintering butterflies).  The monarchs end up congregating in a tiny area, with the bulk of the butterflies concentrated among twelve mountain massifs (clusters of peaks) within three hundred square miles (eight hundred square kilometers), an area smaller than New York City. In other words, most of the monarchs from eastern North America, from Maine to Saskatchewan, and south to Texas, probably covering two million square miles, funnel down and overwinter in a location 0.015 percent the area that they occupy in the summer!  Unbelievable. This year’s estimate is about half of last year’s, which is NOT great news for monarchs!

Where does this leave us?  A year ago, we reported that the population was huge and the overwintering population in Mexico was the highest in 12 years, higher than predicted by many.  Last summer (2019) the number of monarchs in the USA was also huge, leading many of us to predict a great migration and population making it to Mexico. Nonetheless, this points to the critical issue of the migration itself, what happens after the caterpillars feed on milkweed. Chip Taylor has noted that “the two biggest factors that appear to account for the lower numbers this winter are the lateness of the migration and the drought in Texas.”   As I have previously argued, there is often a disconnect between summer breeding populations of monarchs and the overwintering population.  Indeed, this means that limitation of milkweed host plant was not the driver of this year’s halving of the population.  Annual fluctuations in spring and summer rains and temperature, coupled with the autumn climate, which dictates the trajectory of the southern migration, have apparently been leading to migratory failure. Drought, in particular, reduces milkweed quality during the breeding season, and perhaps more importantly, reduces availability of flowers and nectar (of other plant species) during the autumn migration.


With 27 years of data, there are various ways to plot and assess the trends.  Above I have plotted four year averages for seven periods working backwards (so the first average on the left is only for 3 years).  Any way you slice it, the trend has been negative, and the population is not what it was.  Nonetheless, the extreme downward trend seems to have bumped up in the last period of four years.  Is this the new norm, a winter population hovering between 2 and 4 hectares?  How dangerously low are these numbers? And what can be done to continue to reverse the trend and buffer the population?  To what extent is limited milkweed the driver of the momarch population’s precipitous decline? I have recently written about these issues in a scientific commentary as a well as my popular science book, which covers much more (read chapter 1 here).32 IO

I, for one, needed a distraction from the craziness surrounding Covid19… unfortunately, I wish there was better news for our beloved butterflies. We continue to wait for the Fish and Wildlife Service’s decision, which is evaluating the petition to list monarchs as threatened under the Endangered Species Act (they are compelled to deliver their decision by December 2020). For now, I am looking forward to seeing milkweed, butterflies, and their caterpillars once they arrive to Ithaca, NY in a few weeks this spring.

10 Replies to “Monarch population size over winter 2019-2020 announced by WWF Mexico: not great news!”

  1. II think two problems with the “drought reduces availability of flowers and nectar in Texas/Oklahoma causing migration failures” hypothesis are: 1) during west Texas droughts, nectar really isn’t all that scarce, just hard to see from a speeding car since alot of the flowers that monarchs (and other butterflies in the region) find attractive are only an inch or two high 2) where nectar occurs the monarchs are not flocking to it in large numbers as one would expect, even when the flowers are located next to cluster trees containing 100’s or 1000’s of butterflies. One example from the ultra severe Oct. 2011 drought in Texas:

    • Hi All,   We all know there are many many variables that contribute to the Mexican monarch overwintering population. One fact is the 2018-19 eastern count demonstrated when weather cooperates there’s at least enough drought and cold tolerate native milkweed and nectar sources on the landscape on the majority of the eastern range to get 6 hectares of monarchs to Mexico to be counted. 6 hectares is above the average of the counts of the last 25 years linked here We know, since that same drought and cold tolerant habitat didn’t just disappear last season, weather conditions had to affect how much of it was above ground leafing and flowering. Unless predators doubled last year to cut the population in half, extremely dry weather conditions must have contributed to bad habitat health and thus monarch health. I too read all the reports last year from the summer breeding grounds of lots of monarchs. Paul, you know from all the posts on our 5000+ member Facebook group “How to Raise Monarch Butterflies”  linked here  Everyone was seeing lots of monarchs. Of course all that is anecdotal and means nothing scientifically. I traveled from New Haven, Ct to my home just outside Austin, Texas Oct 1 and spent 10 weeks throughout the migration route all the way from the Red River in the North to the Rio Grande River in Mission, Tx, and followed the river to the gulf and South Padre Island,Texas. It was mostly dry and nectar sites I normally monitor weren’t showing many blooms.  If I was forced to answer what was the biggest contributer to the numbers being cut in half from last year I would say this map of the entire U S for the month of October 2019 archived here    demonstrates there was some level of drought October 15th all through the funnel through Texas and the entire eastern range that probably extended all the way to the sanctuaries. During October in 2018 when there was 6 hectares in Mexico there was plenty of rain and no drought in Texas or the entire eastern range archived here  .  Estella the writer for Journey North said a few weeks ago there were monarchs overwintering outside the Sanctuaries that are counted, that wouldn’t be counted. Of course, I don’t know how much auditing of the count there is, and how many people are witnessing the computation of the count although the lower numbers the better for those who want the monarch listed so tax dollars will be thrown at the monarch butterfly through  government agencies and NGOs to pay salaries and plant a bunch of milkweed and nectar sources that naturally propagate and spread on it’s own during good weather in the summer breeding grounds. The people involved in the count are from these same organizations that want more donations and tax dollars. They hope there will be a new director of Fish and Wildlife who will list the monarch in December.  Of course, as citizen scientists, we suggest to our group and others to protect monarch eggs indoors until they are adults and then release them and plant pollinator gardens with a supplemental water source available to keep some habitat viable during the next catastrophic drought like we had 2010-13 linked here. As you know, precipitation and temperature is the key to almost everything in nature.

      • Here in Edna, South Texas, we had an bunch of Monarchs wintering here – mild. At one time I had 35 larva on my 4 small milk weed. Was able to get branches of a giant weed from an educational garden to supplement their food. They went through their cycle, more larva, more Monarchs. Right now I estimate our educational garden has over 50 larva, I have 14 and Monarchs flying around enjoying orange tree flowers. Some just did not go south.

        • That’s great Jayne. Nothing wrong with local overwintering populations. I was on South Padre Island in December and there were monarchs and queens everywhere. I then went to Florida but I’m sure there were monarchs on South Padre all winter. It would be great to know if your local monarchs or their offspring moved north as it warmed up. I guess you could tag some and maybe someone would find them in the summer breeding grounds. Of course, by now, you could maybe be seeing some Mexico monarch. I’m back in Connecticut now but I’m sure the antelope horn milkweed is coming up in Central Texas. Did you have a period of drought during the migration in October ?

  2. Last year my first butterfly arrived June 20.This year I saw one on July 5th. I live outside of Pittsburgh Pa in Irwin Pa. I am so worried not seeing more by now! Will more come to our area soon?

  3. Have not seen any in Nashville. Hardly any other butterflies either. I have huge Passion flower plants and have not seen one orange butterfly…no caterpillars.

  4. The killing of the two advocates for the Monarch butterfly does help either. I feel like I’m reliving the rubber tapper situation all over again.

  5. I live in Maryville, Tn and grow several types of milkweed. I have only seen one Monarch so far this spring. I save my seeds and give them to a local nursery which shares them with customers. Hopefully more will be planted in the area.

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