Mexican Monarch Numbers Down 75% From 2004 - Reuters

EL ROSARIO - "A plunge in the number of monarch butterflies migrating from the United States and Canada to Mexican winter colonies has experts worried logging and pesticides are endangering the fragile insects.

Although masses of sleeping butterflies still hang like clumps of dead leaves from branches in the El Rosario sanctuary in central Mexico, and the air is filled with fluttering monarchs woken by the sun, biologists say the population this year is the smallest ever and down three-quarters from 2004. "Their numbers always fluctuate, but if you look at a chart of the past 10 years, it appears the trend is going lower," said Eduardo Rendon, a local World Wildlife Fund coordinator.

In a mysterious migration that fascinates biologists and delights tourists, tens of millions of bright orange butterflies make the grueling annual trip south to sit out the winter months in central Mexico's temperate fir forests.

Tracking them is hard as those that arrive in November are the great-grandchildren of those that left the previous March. But Mexican and US biologists are studying different points along the migration route to work out what is hurting most -- US pesticides, bad weather, deforestation in Mexico, predatory birds or even climate change."

EDIT

http://www.planetark.com/dailynewsstory.cfm/newsid/29820/story.htm

Damp, cold in early summer, then drought, had brought down Monarch numbers in the upper Midwest in the past year. Birds have little interest in the Monarch as they accumulate toxins from their host milkweeds. I still have not gotten a definitive answer on whether the Gypsy Moth aerial spray with BT affects the butterfly.