Monarchs: Are the Numbers Up or Down?
By Kathryn Hokamp, Lepidopterist at Butterfly Pavilion
Monarch Butterflies (Danaus plexippus) have been in the news frequently in the past several months, and the information can be confusing and seem conflicting at times. There are reports of record low numbers of overwintering monarchs in California alongside apparent record high numbers of butterflies migrating through the Midwest. This can seem paradoxical, but it is important to remember that the monarch migration in the United States is split. Some Monarchs overwinter in California, some overwinter in Michoacán, Mexico, and some are part of non-migratory or semi-non-migratory populations in Florida and the Gulf States. As a rule, butterflies that migrate east of the Rocky Mountains overwinter in Michoacán, and butterflies west of the Rockies overwinter on the coast of California. See the map below from the Xerces Society for details, and note the mixing of populations as well as the uncertainty on the map (indicated with question marks and dashed lines). We know more about D. plexippus than we do about many butterflies, but our knowledge is far from complete.
What is happening in California?
The Xerces Society has been conducting a Thanksgiving count of overwintering monarchs in the state since 1997 with the help of citizen scientists. The count is conducted every year at various monarch overwintering sites in the weeks around the Thanksgiving holiday. This year’s count was extremely low with only 28,429 monarchs counted compared to 192,668 butterflies counted at the same sites in the previous year. This drop-off is even more concerning when compared to the estimated 4.5 million monarchs overwintering in California in the 1980s, showing a 99.4% decrease. A paper by Schultz et al. published in 2017 suggested that 30,000 butterflies is the probable threshold for the collapse of the western population of monarch butterflies, which means the population can likely still be saved, but it is important for conservation plans to go into effect immediately.
The Xerces Society has published an action plan to save the western monarch population which includes the following action items:
- Protect and manage California overwintering sites.
- Restore breeding and migratory habitat in California.
- Protect monarchs and their habitat from pesticides
- Protect, manage, and restore summer breeding and fall migration monarch habitat outside of California.
- Answer key research questions about how to best aid western monarch recovery.
With this plan, the Xerces Society hopes to conserve the western monarch population and to restore monarchs to previous numbers in the west.
Are the eastern migratory monarchs still imperiled? Preliminary data suggests that the eastern migratory population is having a good year while the western migratory monarchs are having a catastrophically bad year, but both populations of monarchs are still under threat from human activity. The monarch population in Mexico declined 84% between 1996 and 2015, and a good year is not necessarily indicative of an upward trend in the population and may fall into normal fluctuation within the greater downward trend. Nonetheless, the apparent high numbers, while preliminary, are a good sign for the population and we need to continue to foster the growth of the migratory monarch population.
Why are the monarch overwintering numbers high in Michoacán and low in California? There are many possible factors contributing to the low numbers of western D. plexippus versus the apparent high numbers of eastern D. plexippus, but it is safe to suggest that the difference in success is related, at least in part, to the weather patterns in the United States during 2018. The fall saw record highs in amount of precipitation in much of the eastern United States, while the west saw widespread drought (see map below from NOAA). Drought affects monarchs in many ways including decreases in the number of flowering plants that meet adult nutritional needs, decreases in milkweed and milkweed reproduction (Asclepias sp. are the larval foodplant for D. plexippus), and increases in the number and size of forest fires creating hostile air conditions and decreasing viable habitat for the butterflies.
On the other hand, weather conditions in most of the east were very good for the monarchs in 2018. The conditions were good for supporting the growth of milkweed in the spring while D. plexippus was breeding, and the conditions remained good throughout 2018. We are likely to see more monarchs overwintering in Michoacán than have been seen in many years, but it is important to remember that we do not have definitive numbers from overwintering sites in Michoacán yet, and we are unlikely to have conclusive numbers before March, so the monarchs in Mexico may have lower or higher numbers than predicted. There are many factors beyond climate that will affect the size of the monarch population including, but not limited to:
- Pesticide Use
- Disease (most notably Ophryocystis elektroscirrha or OE)
- Habitat destruction
- Poor management of overwintering sites
What is the impact of Colorado? Colorado is in an excellent position to foster the survival of both the Eastern and Western Monarch populations for various reasons.
- The eastern and western monarch populations mix along the Rocky Mountains, so Colorado is home to both populations (see below map from Monarch Watch)
- While most monarchs migrating through the front range are likely heading to Mexico, we are very near the mixing point and are well positioned to support both the eastern and western populations
- While Colorado does not see the large numbers of monarchs that migrate through other states to the east and the west, unusual weather patterns may push monarchs into the area, and we can help conservation efforts by providing butterfly habitats and recording monarchs seen in the area for Monarch Watch or Journey North
Questions to Consider:
- Is it possible that there are monarch overwintering sites on the Pacific Coast that we do not know about? It is only 43 years ago that the scientific community in the United States and Canada even learned the location of the Monarch overwintering sites in Michoacán, and there is much we still do not know about the monarch migration.
- Is it possible the overwintering sites are moving? We know climate change is affecting the distribution of the Asclecpias sp. needed for monarch breeding, and we know monarchs shift their overwintering sites, particularly in California, but the concern for western monarchs began with the low numbers observed breeding, so shifting in overwintering sites could not account for the extremely low numbers counted in California.
- Could the western migratory monarchs be migrating to Mexico? Some monarchs migrate to Mexico from west of the Rocky Mountains, but it is not a large scale migration. We will know more about the numbers migrating to Mexico from the western United States once more tagged monarchs are recovered this winter, but it is unlikely to be a huge number.
- How much do we know about the monarch migration? We know more about plexippus than we do about many butterflies, but much of the mechanics of the monarch migration is still a mystery. There are unconfirmed migration routes and unverified overwintering sites. There is much more to learn about Monarch Butterflies.
What you can do to help the monarchs
- Make your garden a butterfly habitat! Plant native milkweed and nectar plants in your garden, and don’t treat your garden or lawn with pesticides. If the monarchs and other butterflies have good garden habitats for feeding and reproduction, they will have higher chances of survival.
- Join Monarch Watch at Butterfly Pavilion and help us to tag monarchs in the fall! We need accurate data on monarch movement, and there is not a large amount of data coming from Colorado. We can help by tagging monarchs in our area that will hopefully be recovered later in the season.
- Become a citizen scientist! We rely on data and observations from people around the country to understand monarchs and the things affecting their survival. This list includes many opportunities to get involved with monarch conservation as a citizen scientist, and Butterfly Pavilion can help as well! You can become a volunteer for the Colorado Butterfly Monitoring Network or contact us for more opportunities!
- Read information from trusted sources when learning about monarchs and other invertebrates. Monarch butterflies are very popular animals, and some of the information generated about them is not based on good science. Monarch Watch and Monarch Joint Venture are good resources for any monarch questions.
Photo credit - 2018 precipitation departures from average. Areas in the purple shadings picked up at least 12 inches more than average in 2018. (NOAA/NWS) via weather.com