Monarch Butterflies In Decline

Updated: Apr 12

Guest Blogger: Dan Bantz, Silent Generation, Caledonia, Wisconsin



Ten years ago, my wife Pat and I took a trip to Mexico City to see two of the twelve known sites where monarch (Danaus plexippus) butterflies overwinter from October to March. It was amazing! There were an estimated population of 200,000,000 monarchs in each of the two sites we visited and our jaws dropped when there was there was a sudden burst of 50,000 butterflies just over our heads. It made an unmistakable roar of butterfly wings………really!


All of the Monarchs that migrate to Mexico are from the East side of the Rocky Mountain range all the way to the East coast of the U.S and north into Canada as far north as milkweed grows. There are 11-12 known sites where monarchs will overwinter in the states of Mexico and Michoacan. There are over seventy varieties of milkweed and all of them are toxic to most herbivores. Monarch caterpillars (larvae) ingest the milkweed and use this toxicity as a defense. The larvae also have a warning for predators; the larvae have bright black, white, and orange rings that warn birds to STAY AWAY! The adult monarch butterflies are orange and black as a warning also. Viceroy butterflies (Limenitis archippus) are smaller than the monarch but have a similar black and orange pattern and mimic the monarch in color and toxicity. If a robin eats a monarch larva or butterfly it becomes ill and throws up. It will avoid anything orange and black. Lesson learned!


When we went to the second roosting site the next day, there was a riot of activity at an elevation of eight thousand feet where there was a spring with fresh water. There were butterflies everywhere and they were coming out of their long resting period (October – March) and getting ready to mate and head north to Texas. They lay eggs on milkweed plants along the way. The water is needed to metabolize the monarch’s stored fat into energy for the long trip. It was an incredible sight and we had to watch every step so we didn’t step on any monarchs. The latest generation was at risk and every monarch is important. If there were problems, the vigilantes would escort violators out of the park; there were no second chances.


That was ten years ago and things changed drastically. The sites now didn’t have two hundred million butterflies; they had fifty to seventy-five million or less. One of the problems was a few huge pine trees the monarchs rest on were illegally cut in the middle of the night and sold. The park rangers do the best they can but there are eleven or twelve known sites where the Monarchs overwinter and that is a lot of territory for a very few rangers to protect.

There is a second smaller monarch migration on the West side of the Rocky Mountain Range and it reaches to Oregon in the summer and Southern California in the winter. Along the California coast, there are over 300 overwintering sites from Ensenada and Baha, California north to San Francisco in Sonoma County. Some of the sites have 50,000 monarchs or more and thousands of people visit theses sites to see the wintering butterflies and marvel at such a site.


For Further Reading:


Butterflies of Florida by Jaret C. Daniels 2003

Field Guide to Butterflies by The National Audubon Society

Butterflies of North America by Jim P. Brock & Kenn Kaufmred, 2005




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