The arrival of spring in Mexico and the southern United States usually means the mass arrival of Monarch butterflies.  But this year a hundred-million of them are missing.  Their disappearance is causing an international dispute over who or what is to blame. 

Michoacan state in Mexico is one of the few places on Earth where you can actually hear butterflies fly.

But this year, despite the way it looks, biologists who study the Monarch say it is in trouble.  American biologist Lincoln Brower works with organizations that are trying to save the butterfly.

"We've been studying it (the butterfly) for about 14 years and this is down about 75 percent below the average number that come here. There's no question the Achilles heel of the Monarch butterfly is deforestation in Mexico," says Lincoln Brower.

This is an enhanced image of what a butterfly sanctuary in Mexico is supposed to look like, but this denuded mountain shows the impact of illegal logging, which the Mexican government has been unable to stop.  The butterflies need the fir trees for protection from the winter cold and rain.

In the summer, Monarchs live in the soy and cornfields of the United States and Canada. They are finding less food there because powerful new herbicides are killing the milkweed plants that also grow in these fields, and milkweed happens to be the only thing Monarchs eat.

That's something that Ernesto Enkerlin of Mexico's National Commission for Protected Areas doesn't hesitate to point out.

"We need to make sure that our agricultural practices allow for certain areas to maintain milkweed populations," says Dr. Enkerlin.

The butterflies have less to eat and fewer places to live. Monarch butterflies have survived natural disasters like this killer frost three years ago.  But Professor Brower doubts they'll be able to withstand the man-made threats.

"We're close to where the straw breaks the camel's back.  How much can you load on this butterfly before its capacity to bounce back is wiped out ," says Lincoln Brower.

That's an issue that may have to be solved through international cooperation instead of finger-pointing.