In the U.S., monarchs need places to reproduce and feed. However, herbicide use is decreasing the availability of their primary food source, the milkweed plant (Asclepias). Conservationists attribute the disappearance of milkweed species to agricultural practices in the Midwest, where genetically-modified seeds are bred to resist herbicides that eliminate milkweed nearby. Growers eliminate milkweed that previously grew between the rows of food crops. Corn and soybeans are resistant to the effect of the herbicide glyphosate. The increased use of these crop strains is correlated with the decline in Monarch populations between 1999 and 2010.
Chip Taylor, director of Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone" with 120-150 million acres lost.
The monarch butterfly is in trouble. Monarch and milkweed population have decreased by 90% of what they were in 1992.
Milkweed plants are the only source of food for the monarch caterpillar. But these plants are rapidly disappearing, due to the loss of habitat stemming from land development and the widespread spraying of weed killer on the fields where they live.
Monarchs need mountain forests in Mexico for their winter habitat, however nearby human communities also rely on them and create pressure on forests through agriculture and tourism activities.
Many migrating monarchs rely on overwintering grounds in Mexico, but illegal logging operations are wiping out critical butterfly habitat. While the Mexican government has set aside 217 square miles for the Monarch Butterfly Biosphere Reserve, there are things you can do, too, to avoid supporting illegally harvested wood from this butterfly habitat. When buying wooden furniture or flooring, look for the Forest Stewardship Certified seal. This means the lumber was taken in an ecologically responsible way. Better yet, look for used wood products whenever you can.
America’s native grasslands are critically important for monarchs. They offer both milkweed for monarch caterpillars as well as nectar plants for adult butterflies (and many other pollinators too). Today, more than 90% of native grasslands have been converted to cropland and development. Grasslands are disappearing faster than any other ecosystem in North America, and that’s a big problem for monarchs.
According to Monarch Watch, housing developments, factories, and shopping centers are swallowing up habitats for monarchs and other wildlife at a rate of 6,000 acres a day-that's 2.2 million acres a year, which is the size of Delaware and Rhode Island combined. Instead of supporting businesses that promote sprawl, look to cities and your local downtown businesses before heading for big-box stores in suburbia.
Climate change threatens to disrupt the monarch butterfly’s annual migration pattern by affecting weather conditions in both wintering grounds and summer breeding grounds. Colder, wetter winters could be lethal to these creatures and hotter, drier summers could shift suitable habitats north. WWF’s 2013 report from Mexico showed that the number of monarch butterflies wintering there was at its lowest in 20 years. The number is measured by the amount of forest they occupy, and in 2013 the number of butterfly acres decreased from approximately seven to three. Abnormal patterns of drought and rainfall in the U.S. and Canada breeding sites may have caused adult butterfly deaths and less plant food for caterpillars. Fewer butterflies up north means fewer will then migrate south to Mexico for the winter.
Climate variations during the fall and summer affect butterfly reproduction. Rainfall, and freezing temperatures affect milkweed growth. Omar Vidal, director general of WWF-Mexico, said
"The monarch’s life cycle depends on the climatic conditions in the places where they breed. Eggs, larvae and pupae develop more quickly in milder conditions. Temperatures above 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate."