(E) When we arrived yesterday close to the overwintering site of the monarch in Pacific Grove this Sunday, we could already see several monarchs above the trees canopy. Arriving at the site, we were able to find at least ten clusters of monarchs in the trees. One of the site volunteers told us that their last count of the population in the site was around 9,400 a week ago with the peak close to 14,500 during last year’s Thanksgiving.

So it is wonderful news that the population has increased in the overwintering site this year.

But the question is why? An excellent article “The Bounciness Of Butterflies” from the Xerces Society is exploring some possible answers:

“There could be many other causes of this year’s monarch butterfly population growth besides good weather and good luck. Quite likely, it is due to a combination of factors. A number of research groups (including authors of this blog) are studying the important unknowns for western monarch butterflies, including the role of pesticides in the Central Valley, the status of winter breeding monarch butterflies in the San Francisco Bay Area (maybe such as the Google campus), and the connections between migratory eastern, migratory western, and resident western monarch butterfly populations in North America.

Other factors sometimes discussed in western monarch circles that may have played a role in this year’s uptick, but currently lack sufficient evidence, including changes in the timing and location of wildfires in the West, winter breeding monarchs in the Bay area, an influx from the eastern population, changes in crop planting and pesticide use in the Central Valley due to drought, etc. However, it’s important to note that these are hypotheses only and, as of yet, lack strong evidence to support them.”

For more…

• Xerces Society: “The Bounciness Of Butterflies“
• Monarch Joint Venture: “Hope and Gratitude at Pacific Grove: Notes From a Western Monarch Count“
• Candy & Kasey: “Pacific Grove Monarch Butterfly Sanctuary“
• Elizabeth E. Crone and Cheryl B. Schultz: “Resilience or Catastrophe? A possible state change for monarch butterflies in western North America“

Note: The picture above is from Agunther, CC BY 3.0 https://creativecommons.org/licenses/by/3.0, via Wikimedia Commons.

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