Monarch Butterflies wintering in California. The 2012-2013 winter survey indicated only 1.19 hectares (2.95 acres) were occupied, a dramatic 59 percent decrease from the previous year’s survey. Photo: Agunther

By Lacey Avery
15 July 2013

( – In the next few months, the beating of fragile fiery orange and black wings will transport the monarch butterfly south. But the number of monarch butterflies (Danaus plexippus) reaching their final destination has steadily declined, dropping to its lowest level in two decades last winter, according to a recent survey.

The insect’s journey begins in late summer and August, when monarchs fly from Canada and the Northeastern U.S. to highly selective overwintering sites in Mexico. Individually weighing less than a paperclip, monarch butterflies employ an inherited compass to make the longest insect migration in the world, flying up to 4000 kilometers (2,485 miles) to reach their final destination by November.

The monarchs gather in central Mexico’s mountainous forests in massive numbers, up to 50 million per hectare in only a dozen or so colonies. The monarchs create clusters on the trunks and branches of coniferous trees, filling the skies with orange. It is in this small monarch forests where scientists estimate population size. A survey conducted by the WWF-Telcel Alliance and Mexico’s National Commission of Protected Areas use the percentage of wintering forest inhabited by the monarch butterflies as an indicator, representing the number of butterflies that arrive in the country each winter to hibernate.

“As monarchs arrive to their hibernation forests in Mexico, we locate the sites where these butterflies group together to form colonies,” Eduardo Rendon told Rendon is the monarch butterfly program director with WWF.

The survey was conducted in five hibernation colonies inside Mexico’s Monarch Butterfly Biosphere Reserve and four colonies outside the reserve.

“We measure the colony’s fringes and calculate the occupied area through a geographical information system,” Rendon adds.

The most recent 2012-2013 winter survey indicated only 1.19 hectares (2.95 acres) were occupied, a dramatic 59 percent decrease from the previous year’s survey.

Rendon believes the decrease of monarch butterflies in Mexico may be related to the severe heat and drought of 2012 across much of the monarch’s summer range. The climate fluctuation may have reduced the monarch’s host plants and stressed the remaining insects.

Extreme temperature fluctuations during spring and summer can affect the survival and reproduction of butterflies, but climate is only one challenge. Monarch habitat across North America is disappearing, threatening the irreplaceable long-distance migration.

There is no single threat, said Craig Wilson, senior research associate within Texas A&M’s Center for Mathematics and Science Education and longtime butterfly enthusiast. “It is perhaps a deadly combination of climate change and human behavior.” [more]

Losing our monarchs: iconic monarch butterfly down to lowest numbers in 20 years


  1. Alexander Ac said...

    Hi Jim,

    here is the graph of the Monarch decline... terrible indeed!



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