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)


24 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.

    • Thistles, which are an important nectar source for monarchs, are considered “noxious” in some states. Monarchs don’t seem to mind large-flowered thistles that are not native to the US (along with swallowtails, finches, bees, skippers, the Painted Lady, as infinitum) but some people seem to believe that having spiniest leaves makes a thistle worthy of eradication. I’d say the spininess is an asset since human activity (e.g. extinction of the Passenger Pigeon) has resulted in an overabundance of deer and rodents. The only thistle I consider noxious is “Canada” thistle, as it spreads underground and doesn’t have near the wildlife value of “Bull” thistle (for instance), a plant that continues to be added to planting ban lists — by people with no plans to plant native thistles in their place.

      • Pardon the typos. I don’t type well on my phone, especially due to its overzealous auto-change design (erroneously referred to as “correct”). It should be “spinier” not spiniest. There are native thistles that are very spiny. Also, it was “ad infinitum”. I have, indeed, noticed a pattern where people who advocate the elimination of valuable plants like “Bull” thistle cite the greater spininess as if it’s a substantive justification, along with them being non-native. The reality is that human activity has created more non-native landscapes, including the plants, than native — in much of the Midwest and elsewhere. So, everything has to conform to a mostly non-native paradigm. What really matters is how much wildlife value a plant offers in a given space. Plants like “Bull” thistle have no business being banned, since they’re practically identical to native species like discolor — and I have read that Iowa politicians classify native and non-native thistles identically in terms of weed control policy.

        • The weevil that was approved for release is apparently rapidly killing native thistles that haven’t been destroyed by agriculture and general “development”.

          The USDA, by contrast, sat on its hands when it came to the monophagous weevil that feeds on garlic mustard. I don’t think it’s still even available, even though it has been studied since 1999. The difference seems to be that the agricultural lobby doesn’t like thistles and doesn’t care about garlic mustard damage so much since it mainly happens in woodland and at woodland edges. Also, chemical companies get money to spray forests to “control” it.

          Even if C. scrobicollis finally is made available for purchase for those of us who have suffered so long from garlic mustard eradication efforts, who knows if they’ll ever bother to approve C. constrictus. The two together yield something like 90% control as I recall.

  6. Are there any current sources of the wasp that was approved as a biological control agent for control of the non-native oleander aphid? I did a Google search and couldn’t find any.

    The aphid is very bad for leaf health (turning shoots yellow and weak) from what I’ve seen and they also like to hide in flower bud clusters which get damaged if you try to remove them (e.g. A. perennis). It’s also far too labor-intensive to deal with the infestations. So, why not release a lot of the wasps? It seems that more emphasis should be placed on that.

    “Little things” like aphid control might help significantly to improve the numbers of monarchs. At the very least it would make it a lot easier to have healthy plants.

    • I would like to formally apologize to all of you wonderful Monarch enthusiasts for the selfish moment, 51 years ago, when I captured a Monarch and hid it in my home. (now hoping it was a male, or better yet, a Viceroy)

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