SOURCE: Xerces Society

DATE: November 29, 2018

SNIP: The California overwintering population has been reduced to less than 0.5% of its historical size, and has declined by 86% compared to 2017.

Going into the overwintering season, we were not expecting this to be a great year because we knew it had been a rough season in the breeding and migratory range. It’s worse than anyone had anticipated, however, with early count numbers showing that the population is down an order of magnitude from last year, which was a low population year itself. We currently have preliminary count results from 97 sites, which includes many of the most important overwintering sites. In 2017, these sites accounted for 77% of the total monarch overwintering population, hosting approximately 148,000 monarchs. In 2018, the same sites have only 20,456 monarchs. This represents an 86% decline since last year.

Some people have wondered if the monarchs are simply somewhere else, if they will arrive later. However, we do not think there is much evidence of a delayed migration. Monarchs are not being reported in large numbers elsewhere in their range.

These historically low numbers follow a long period in which monarchs have declined 97% since the 1980s. In my lifetime, the monarch population in California has gone from millions of butterflies to hundreds of thousands and now, possibly, mere tens of thousands.

It is not just monarchs that have had a tough year in California. According to Matt Forister, a professor at the University of Nevada-Reno, many butterfly populations at high elevations are still recovering from California’s historic drought. Art Shapiro, a professor at the University of California-Davis, who runs the long-term butterfly monitoring program in northern California, said by email that “MANY species on my transect are in trouble this year, with several no-shows at particular sites for the first time ever. Whether this can be attributed to a single cause remains to be seen. It’s far too early to say.”