1. Challenges on Breeding Grounds
The monarch cycles through 3-5 generations during the breeding season. Only the final generation migrates to Mexico. Monarch caterpillars feed exclusively on milkweed. Historically, the U.S. corn belt has produced half of the monarchs that migrate to Mexico. However, milkweed habitat has been greatly reduced in the region due to:
New agricultural practices
Milkweed is being eradicated from corn and soybean cropland by the increased use of herbicides on genetically modified, herbicide-tolerant crops (GMO's).
More acerage is being planted in herbicide-tolerant corn to meet increased demands for biofuels. High corn prices push more land into production.
Drought and excessive heat on the breeding grounds in summer 2012 is cited as a key cause of the record-low population in Mexico the following winter, and reinforces concerns about the potential effect of climate change.
2. Challenges on Wintering Grounds
Monarchs migrate to a unique forest habitat in central Mexico. The butterflies depend on the forest microclimate to survive the winter. The entire population overwinters together in a very small region.
Illegal logging continues to threaten/deplete the monarch's forest and disrupt its delicate microclimate.
Concentration of population
The fact that the monarchs concentrate in one place for the winter makes the entire population vulnerable to a single storm, drought, fire, or disease.
High-volume, unregulated ecotourism threatens the integrity of the monarch's winter refuge
Other socio-economic pressures
The needs of the people who live in the region must be balanced with the needs of the monarchs. Humans and monarchs have competing needs for food, water, shelter and space.
3. Challenges of Migration
Migration is inherently risky. Monarchs must find habitat to meet their needs every day of the journey. Like links in a chain, the loss of one habitat component could break the monarch's annual cycle.