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2021 Thanksgiving Count Shows Almost 250,000 Overwintering Western Monarchs: Let’s Celebrate, and then Get to Work

Jan 25, 2022


  • Community Science
  • Migration
  • Population Trends
  • Important Monarch News

The excitement about the increase in 2022 western monarch overwintering numbers has grown since the butterflies began congregating on the California coast last October, and so have the questions and speculation about the possible reasons for this drastic population increase. In conversations with monarch researchers and other conservationists, open dialogue is ongoing trying to deduce the most likely hypothesis for why this large increase may have happened. As we watched the preliminary western monarch count numbers grow, we pondered even the feasibility of that growth by trying to calculate how such a  2021 overwintering population of under 2,000 could produce tens of thousands, and then hundreds of thousands, of butterflies. 

With today's announcement of 247,237 overwintering western butterflies, mixed in with relief and celebration is a lot of uncertainty about "what happened" and what might happen next; we may never be able to pinpoint the exact combination of factors that led to it.

The population this year rivals the overwintering numbers that we saw consistently from 2013-2017, which relative to last year’s 1,914 butterflies is a huge reason to celebrate. But we weren’t celebrating five years ago; 200-300,000 butterflies still represents a 95% decrease in the population from historic numbers. Researchers, along with biologists from the Xerces Society put together this blog titled “The Bounciness of Butterflies,” which helps to outline some of the reasons we expect monarchs, like many other insects, to have large population fluctuations. 

Most insects have high reproductive potential if given the right conditions and reduced threats, but they may also experience drastic declines, especially when faced with many compounding threats. With monarchs' complex annual life cycle, producing multiple generations and migrating across and occupying a large geographic area, there are many variables worthy of further study. Researchers and community scientists across the West are contributing to trying to answer several remaining questions about the western monarch population. Some include: 

  • To what extent is the timing and availability of early season milkweed a primary limiting factor? The MJV has funded the 2022 Western Monarch Mystery Challenge - check it out for an opportunity to contribute to our understanding of early season monarch breeding activity. 
  • How has (and is) winter breeding and the presence of non-native tropical milkweed affected (affecting) migratory population dynamics? Professional research and community science efforts all aim to try to better understand breeding population dynamics in the West, including resident populations. Easy, opportunistic sightings of monarchs and milkweeds are reported to the Western Monarch Milkweed Mapper and more volunteers are being trained and recruited for more regular surveys of monarch breeding activity through the Monarch Larva Monitoring Project (MLMP). You can register for upcoming MLMP training here. The Integrated Monarch Monitoring Program is also increasing its efforts for broader landscape scale surveys. 
  • To what extent is there interchange between the eastern and western populations and what are the origins of the overwintering monarchs? One program to help understand this question is the Southwest Monarch Study, which conducts ongoing research and engages volunteers who tag butterflies.
  • How is the use of pesticides impacting monarch production and survival? The Xerces Society recently released a publication discussing their worrisome findings in the Central Valley.
  • How is climate change affecting monarchs in the West? Chip Taylor of Monarch Watch discusses some of the climate change variables in a recent blog article.
  • The list goes on - from questions about the conditions and protection of overwintering habitat to impacts of wildfire, drought, and natural enemies. 

As we work together to collect data and attempt to better understand western monarch population dynamics and threats, we must also scale-up landscape conservation efforts. One year of increased numbers does not represent population recovery, but it gives us an opportunity to help sustain and grow this population further by providing more widespread and higher quality habitat across the western landscape. Here are a few ways you can participate: 

  1. Find more information on where to find native milkweeds and other native wildflowers to support monarchs and other pollinators on our Milkweed Vendor Map
  2. Get involved in any of the community science programs mentioned above. Please note that to handle monarchs in California you must have a scientific collection permit per regulation by the California Department of Fish and Wildlife
  3. Educate others about monarchs and pollinators and how they can be a part of the solution. A great way to become a monarch ambassador is by taking one of our upcoming Monarch Essentials courses.
  4. Limit insecticide use that may cause unintended harm for monarchs and other beneficial insects.
The Xerces Society's graph of the total abundance estimates of western monarch butterflies with number of sites monitored from 1997-2021 by the Western Monarch Thanksgiving Count.