Bring Back the Monarchs: 2024-2025 Season Update

Create, Conserve, Protect…

The Monarch butterfly migration is a phenomenon, mainly across North America, where millions of Monarchs (Danaus plexippus) migrate from the United States and Canada to mountains in central Mexico, where they wait out the winter until conditions favor a return flight in the spring. Eastern Monarchs spend summers east of the Rocky Mountain range and travel over 3,000 miles to overwinter in Oyamel forests in the mountains of Central and South West Mexico. The western Monarch population spends summers west of the Rocky Mountain range in North America and overwinters in California along the Pacific coast near Santa Cruz and San Diego. 

The monarch migration is truly one of the world’s greatest natural wonders yet it is threatened by habitat loss at overwintering grounds in Mexico and throughout breeding areas in the United States and Canada. Monarch butterflies have declined an average of 80% over the past 20 years. From the 1990’s to the 2020 winter count, the Eastern Monarch population has declined from 1 billion to 155 million butterflies.

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Monarch overwintering numbers in Mexico for the 2024 season – Update from Dr. Chip Taylor, Founding Director of Monarch Watch (monarchwatch.org)

Eastern Monarch population

The monarch numbers are in, and they are of interest again, but this time not for the fact that the population declined but for the rate of increase. Last year the overwintering population was extremely low, occupying only 0.90 hectares when all colonies were counted. That number was second only to 2013 when the total was 0.67 hectares. The number this year is 1.79* hectares and that is remarkable. It’s a doubling of the population. How many vertebrate species can double their numbers in one year? Surely few, except for some fishes and a few birds that brood large clutches.

Out of curiosity, I went through all of the records for increases to see whether there are other years with remarkable recoveries and there are 7, including this year. The rate of increase data, represented by the number for the following year divided by the previous year, are as follows:

YearsIncrease
2000-20013.3
2004-20052.7
2009-20102.1
2013-20141.7
2014-20153.5
2017-20182.4
2023-20242.0

As you can see, the 2.0 increase from 2023 to 2024 is in line with increases (1.7-2.7) that have occurred 5 times in the past.

Following that, I checked the spreadsheet I maintain of all the weather and other conditions that might moderate population growth. That revealed there were negative growth indicators for all the low years (droughts, low summer temperatures, etc.) and various degrees of positive growth conditions (near average temperatures and precipitation) in all the following years. In fact, the temperatures and precipitation were close to the long-term averages for the entire growing season for 5 of the years in which the populations increased. Negative years do follow negative years and that has been more common since 2010 than earlier in the record (none prior to 2010 and 5 since 2010). That may say something about greater instability in weather patterns during the growing season since 2010. However, for now, we should put that idea on the watch list.

A takeaway here is that monarchs demonstrate resilience over and over again. Weather knocks them down, but spectacular recoveries are the rule if negative conditions during one year are followed by favorable conditions for population growth. As some of you may recall, I wrote a text for the Monarch Watch Blog that summarized the data that speaks to the three r’s (resilience, redundancy and representation) that are at the core of the Species Status Assessment (SSA). That document is prepared and used as the basis for determining whether a species should be regarded as threatened or endangered. My comments were in reference to the SSA prepared in 2020 but seem relevant now that we are dealing with a new SSA.

*There is one more thing to tell you about the 1.79 hectare measurement. It is close to the three- and five-year running averages, which are 1.98 hectares and 2.18 hectares respectively.

Reference

Taylor, O. R., 2023. The species status assessment (SSA) and the three r’s. Monarch Watch Blog.
monarchwatch.org/blog/2023/10/13/species-status-assessment-and-the-three-rs

Addendum

If you go through the updates I wrote for the Monarch Watch Blog from March through June, you will see that I was really high on the possibility that the recovery this year could be the best ever. It wasn’t. Something happened during late May and early June that limited the reproductive success of the first generation in the Upper Midwest and therefore the size of the second generation that reached maturity in July. That, in turn, limited the size of the third/migratory generation and ultimately the size of the overwintering population.

So, what happened? Probably too much rain and too many days with unfavorable conditions for egg laying in the Upper Midwest, the region that produces the majority of the monarchs that arrive at the overwintering sites. The foundation for this interpretation was outlined by Myron Zalucki and Wayne Rochester in a chapter in The Monarch Butterfly: Biology and Conservation (2004). These authors pointed out that weather which limits egg laying results in a reduction of the total number of eggs laid during a lifetime. In other words, female monarchs don’t fulfill their full reproductive potential if egg laying is significantly delayed and that limits the number of offspring and the size of the next generation. This interpretation deserves a follow up, but the outcome is similar to a reduction in population growth that occurred following a long rainy period I tracked in Ontario a number of years ago. Demography isn’t easy and it becomes really difficult when events that occur over a few days, or a week or more, have a big impact on mortality or reproductive success.

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Reasons for the Decline

According to Dr. Chip Taylor, “We are losing habitat in this country at a rapid pace of 6,000 acres per day, or a loss of 2.2 million acres per year. Further, the overuse of herbicides along roadsides and elsewhere is turning diverse areas that support Monarchs, pollinators, and other wildlife into grass-filled landscapes that support very few species. The adoption of genetically modified soybean and corn have further reduced Monarch habitat. Climate change impacts Monarch Butterfly migration patterns and, in turn, their survival. Not only does climate change influence climate cues that Monarchs use to navigate and begin their migration, but it also impacts their food sources and overwintering habitats. If these trends continue, Monarchs are certain to decline, threatening the very existence of their magnificent migration.”

Whether monarchs rebound this year will depend on the number of returning monarch females, nectar and host plant availability, March and early April temperatures, and then the conditions from May to September.

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Create, Conserve, Protect Monarch habitats

To restore habitats for Monarchs, pollinators and wildlife in general, Monarch Watch has a nationwide landscape restoration program called “Bring Back the Monarchs”. The goals of this program are to restore 20 milkweed species to their native ranges throughout the United States and to encourage planting of nectar-producing native flowers that support adult Monarchs and other pollinators.

You can help!

Monarchs cannot survive without Milkweed plants, as it is the only host plant eaten by newly hatched Monarch eggs and caterpillars, but you can help! By planting Milkweed with some of the Monarch’s favorite nectar producing flowers every year in your yard, garden, or community, and reducing or eliminating the use of pesticides and herbicides, you will be providing food and shelter for Monarchs and other native butterflies and bees. I truly believe that by caring for native wildlife in our own backyard or in our community, each of us can make a difference and keep these beautiful butterflies alive for future generations!

Perennial Host plants for Monarchs – Butterfly Weed (Asclepias tuberosa), Common Milkweed (Asclepias syriaca), Swamp Milkweed (Asclepias incarnata)

Annual and perennial Nectar plants – Zinnia (Zinnia elegans), Mexican Sunflower (Tithonia), Joe Pye Weed (Eupatorium purpureum), Purple Coneflower (Echinacea purpurea), Indian Blanket (Gaillardia pulchella), Pincushion Flower (Scabiosa), Bee Balm (Monarda)

Planting and Growing Directions for Monarch Host and Nectar plant

Links and Resources

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