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Heat Wave Cooks Washington's Monarchs

Record heat decimates fall migrating population

Entry 13:  August 19, 2015.  

In mid-August, a group of volunteers (including myself) planned to work with Professor David James of Washington State University to tag monarch butterflies at a site near the town of Vantage and monitor their fall migration.  The work was cancelled due to poor emergence of the fall migrating population of butterflies.  According to Dr. James, only about three potential migrant monarchs per hour were observed around August 17th and 18th.  It's a devastating count compared to the same time in 2014, when around 22 per hour were observed.  

It's the first year since he's been monitoring the site that the population has declined in August.  He believes this is due to a heat wave that happened at the same time that the first generation of butterflies was due to emerge as adults at the site in late June and early July.  

A WSU climate recording station 2 miles from the Vantage site recorded an average maximum temperature from June 26-July 10 of 103.1F.  Every day during this period exceeded 100F with a maximum of 109 on June 28.  

Data from The Weather Channel show the hottest temperatures ever observed in the month of June happened within a couple days of the 28th at the sites shown on their map: 


Map from The Weather Channel showing the hottest temperatures ever recorded in the month of June

According to Dr. James, "These temperatures exceed optimal temperatures for Monarchs by a significant margin and appear to have had a detrimental effect on population development and survival." 

Monarchs face many threats to their future as a species:  habitat loss, loss of milkweed from rapidly expanding herbicide use on farms, deforestation at their overwintering sites in Mexico, and now it appears that climate change may pose yet another threat.  

I will follow this story and write more as additional information becomes available.  

 

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