Monarch population size over winter 2018-2019 announced, and it’s good news!

The estimates of the monarch butterfly overwintering population were announced today (Wed. Jan. 30th 2019) by WWF Mexico.  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.  Butterflies have been declining over the past three decades, and the annual announcement is a welcome addition to our understanding of the long-term dynamics of our beloved monarch.

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 (2018-2019), there were approximately 6.05 hectares (nearly 15 acres) of forest occupied with dense monarchs in the Mexican highlands (somewhere in the neighborhood of 300,000 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 well over double compared to last year, great news for monarchs!

Where does this leave us?  The good news is that this year’s population was huge in the summer months throughout the USA and Canada, and the resulting migration and overwintering population in Mexico was the highest in 12 years, higher than predicted by many.  The season started with a very early spring and a far reaching northern migration.  As I have previously argued, there is often a disconnect between summer breeding populations of monarchs and the overwintering population — that seems to not be the case this past year.

With 26 years of data, there are various ways to plot and assess the trends.  Below I have plotted four year averages for seven periods working backwards (so the first average on the left is only for 2 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 two and five hectares?  How dangerously low are these numbers? And what can be done to continue to reverse the trend and buffer the population?  I have recently written more about this issue in a scientific article as a well as my book.

For now, let’s celebrate. The government is open, and thus the Fish and Wildlife Service will be deciding on the petition to list monarchs as threatened under the Endangered Species Act this summer. Looking forward to seeing butterflies and their caterpillars once Ithaca, NY thaws in spring.

monarch6 (woods)


18 Replies to “Monarch population size over winter 2018-2019 announced, and it’s good news!”

  1. It was a fantastic year for Monarchs!!!
    Several weather events seem to have converged to create the abundance: a warm wet spring in Texas with headwinds from the north which kept them in Texas to feast on healthy milkweed. Conditions across the rest of their breeding range continued to be great with adequate moisture and good temps.

    All that to say a confluence of events proved fantastic. However, change one or another and we’d be back to disaster.

    For example, long-term trends in moisture in the Texas re-migrant flyway is projected to decline (climate change) meaning more frequent drought, poor quality milkweed, and fewer gen 1 cats.

    Celebrate, but do not give-up trying to change the use of weed killers in the midwest.

  2. When I heard that the population of monarchs were on the decline, my concerned pushed me me to make my own contribution in a small way. I bought milkweed plants and also got some mail order caterpillars. Turns out that I hadn’t needed the extra caterpillars since there were already eggs on the milkweed.

    I’ve since been self educated on the lifecycle of Monarchs and have watched our habitat in the back yard create dozens and dozens of new caterpillars. I’ve also become aware that only 10 to 15 percent of those caterpillars make it to flying Monarchs.

    I do feel a pride that I’ve helped add to the Monarch population. I get a charge out of checking out butterfly crops in the back yard.

  3. Last summer (2018) I witnessed the most Monarchs flying around that I can remember since I was a kid, Circa 1970’s. There were cats all over the milkweed in our garden. Was very encouraging!

  4. If the Monarch is added to the list of Endangered Wildlife as threatened what will that mean to the thousands of butterfly enthusiasts that ware currently raising the wild eggs to adults in their efforts to increase the population. If the monarch is listed will that make it a crime for these citizens to continue to raise and release these beautiful creatures? After all many of these enthusiasts have been working tirelessly for decades to help educate and increase the population and it seems that with a single stroke of a pen they could end up being in violation of the law and could be fined for trying to continue to help preserve these butterflies.

      • Not quite. Everyone above is talking about raising them from eggs on their own milkweed. Those raised monarchs do fine. The ones the study discussed are ‘farmed’ monarch caterpillars which are sold and shipped to people. Wild sourced do fine, mail order ones seem to have deficits.

      • One of my friends raised and tagged monarchs arrived in Rosario, Mexico last Winter. Therefore they do know how to migrate, it is in their genes.


  5. We have an amazing density here in central Wisconsin; I was out photographing today in the prairie; where they more abundant than I have ever seen.

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