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Western Monarch Numbers Expected to Be Low this Year

By Stephanie McKnight on 15. November 2018
Stephanie McKnight

What can you do to help the monarch? Protect habitat, avoid pesticide use, plant gardens, and contribute data to Xerces-led community science efforts.

Over the last three years, I have been lucky enough to do field research to better understand more precisely when and where monarch butterflies breed in the western states. Answering this question is critical to developing a meaningful conservation strategy for the western monarch population. My first season working with monarchs was in 2016, in Nevada. As a biologist with a background in botany, I admittedly knew very little about monarch biology or their life history when I started, but I was immediately intrigued by the diversity of milkweed species that monarch butterflies can use as host plants in the West—44 species to be exact. Of the 44 species, roughly half inhabit very arid desert habitats, with the greatest milkweed diversity occurring in Arizona. Incredibly, monarch butterflies have been documented using more than half of these arid species, and that number will likely continue to grow as more surveys are conducted and our understanding of their habits increases.


Monarch larvae crawl on the pink blooms of a showy milkweed, with an arid landscape in the background.
Monarch larvae on showy milkweed (Asclepias speciosa) in the Great Basin. (Photo: Xerces Society / Stephanie McKnight)


My favorite of the desert milkweeds is pallid milkweed (Asclepias cryptoceras), which inhabits steep, barren slopes in the Great Basin. Not only does this plant somehow survive in one of the driest and most exposed habitat types, but monarch butterflies also manage to find and use it as a larval host. Even as a trained botanist who has spent countless hours scouring challenging habitats for rare plants, finding some of these rare desert milkweeds proved to be fairly difficult—and yet this fragile-looking, brilliant-orange butterfly could fly through the desert and find 15 isolated milkweed plants on a barren hillside! The moment that I realized this was when I became fully enamored with the monarch butterfly.


A small milkweed plant is in bloom with yellow and pink petals in open, dry soil.
Pallid milkweed (Asclepias cryptoceras). (Photo: Xerces Society / Stephanie McKnight)


That first season, my goal was to document where monarchs were breeding in the Great Basin in order to fill a major data gap in our knowledge of western monarchs and inform the development of best management practices for monarchs in the West. I found monarch butterfly eggs, larvae, or adults at approximately 80% of the milkweed populations I surveyed in the Great Basin. To my surprise, it was relatively easy to find monarchs in the Great Basin: find milkweed, find monarchs.

In 2017, I started field work in support of a different research project with Cheryl Schultz at Washington State University–Vancouver and other partners titled “Western Monarch Breeding Phenology: Implications for Management” (a project supported by the Department of Defense Legacy Fund). This research project is investigating monarch breeding phenology (when and where they breed) in California, Idaho, Oregon, Nevada, and Washington to better inform land management for monarch butterflies. Finding monarch butterflies across these states in 2017 continued to be relatively easy: find milkweed, find monarchs.


An adult monarch rests on the pink-and-white blooms of a milkweed in a grassy landscape.
Monarch nectaring on swamp milkweed (A. incarnata), Idaho. (Photo: Xerces Society / Stephanie McKnight)


However, the second year of fieldwork on this project has been very different. In 2018, monarchs arrived almost a month late to most breeding areas. They didn’t even make to some areas of the northernmost breeding range (Washington state), and they were much harder to find in areas where they are normally abundant. At various research sites, the number of immature monarchs (eggs, larvae, pupae) was substantially lower than in the previous year. This downturn in numbers was not only documented by the research project I am working on, it was also noticed by community scientists, land managers, and other researchers across the West. One researcher, Art Shapiro, detected the lowest number of monarch butterflies in central California in 46 years. The consensus among all groups of monarch enthusiasts—researchers, community scientists, and land managers—is that numbers in 2018 were much lower than normal. Found milkweed, but no monarchs.

While it is hard to pinpoint the exact cause of this year’s very low monarch numbers, this is part of a much longer and more disturbing trend. Due to multiple factors, such as habitat loss, pesticides, and climate change, monarchs in the West have declined by 97% since the 1980s. And we know this because researchers used community-science data from Xerces’ Western Monarch Thanksgiving Count (WMTC), an annual count of monarch butterflies at their overwintering sites during a three-week window centered around the Thanksgiving holiday. This year, based on monarch breeding observations from the field and early overwintering observations, researchers, biologists, and community scientists are concerned that the annual WMTC count may be very low.


A monarch hovers above blooming pink milkweed flowers in a grassy field.
Monarch flying over showy milkweed (A. speciosa), Oregon. (Photo: Xerces Society / Stephanie McKnight)


You may be asking “What can I do to help the monarch?” Besides protecting habitat, avoiding pesticide use, and planting gardens, another way is to contribute monarch and milkweed data to Xerces-led citizen science efforts—namely, the Western Monarch Thanksgiving Count and the Western Monarch Milkweed Mapper.

If you live in or near coastal California, you can join the Western Monarch Thanksgiving Count to help estimate monarch populations at their overwintering sites.The 2018 WMTC started on Saturday, November 10, and runs through Sunday, December 2. We are also looking for volunteers to participate in the Western Monarch New Year’s Count. This second count will provide additional information on how overwintering monarchs are using these sites and for how long. The New Year’s Count starts on Saturday, December 29, and runs through Sunday, January 13. To learn more about these counts and to sign up, visit the Western Monarch Count Resource Center.

If you live anywhere in the West, you can contribute data to the Western Monarch Milkweed Mapper. All you need is a photograph of a monarch (adult, egg, larva, pupa), and/or a milkweed, the precise location, and the date of the observation. Providing this straightforward information can have a big impact on our understanding of the distribution of milkweed, monarchs, or both.


A pale green monarch pupa hangs from the underside of a milkweed leaf.
Monarch pupa on showy milkweed (A. speciosa), Nevada. (Photo: Xerces Society / Stephanie McKnight)


Every observation of monarch butterflies helps us to better understand what habitat types monarchs are using for breeding and migration, and when. We still know very little about important migratory pathways, roost trees and shrubs (otherwise known as aggregation stopover sites), and nectar resources used by monarchs both during migration and at their overwintering sites along the coast. By contributing your observations, you can assist with our understanding of this beloved butterfly species, and that information can ultimately help guide conservation efforts to preserve these stunning migrations for future generations.


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