"It's mind-numbing to realize that virtually the entire population is now existing in an area about the size of my yard," wrote Journey North participant Pat Swerkstrom.

Monarch experts reacted to the news with concern:

Dr. Lincoln Brower
''The main culprit is genetically modified herbicide-resistant corn and soybean crops and herbicides in the USA, which lead to the wholesale killing of the monarch's principal food plant, common milkweed.''

Dr. Chip Taylor
"We may never see large numbers we had in the 1990's but monarchs won't be gone. The monarch butterfly is not threatened with extinction. It's the migratory phenomenon that's in jeopardy."

Dr. Karen Oberhauser
"Individuals can help by planting locally sourced, native milkweed and nectar plants, and by spreading the word. It's important to use local plants because some varieties, especially those planted year-round in some southern states, have led to disturbances in the migratory patterns. Individuals also can help by asking property managers and local governments to avoid mowing milkweed plants when monarch eggs and caterpillars might be present, and by avoiding insecticides."

• Journal: Brainstorm ways you can help monarchs at home, at school, and in your community.