From: Lacey Avery , MONGABAY.COM, More from this Affiliate
Published July 16, 2013 06:06 AM

What is causing drop in Monarch Butterfly population?

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.

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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 mongabay.com. Rendon is the monarch butterfly program director with WWF.

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.

Monarch butterfly photo via Shutterstock.

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