The Plight of the Monarch Butterfly

A monarch butterfly feeding milkweed at one of the 7,450 Monarch Watch way

stations spread along migratory routes.

The monarch migration is the largest insect migration in the world.  Every year, millions of monarch butterflies travel up to 2,500 miles to the Oyael fir forests in central Mexico to overwinter.  This year is different: monarch populations have plummeted.  Monarch butterfly colonies in Mexico are now covering only .67 hectares of forest, and the population is down by almost 44% from last year’s record low of 1.19 hectares.  There are approximately 50 million butterflies per hectare, so this year Mexico could be down to about 30 million total butterflies. The monarch population has been declining for the past 15 years, by as much as 81% between 1999 and 2010, and a recent study indicates that the long-term survival of the species may be in doubt. 

The total annual area occupied by overwintering monarch butterflies from 1994 through 2014 has declined significantly. The all-time smallest area was reported during the 2013–2014 overwintering season.

Life History

      The monarchs begin their migration in Canada and the Pacific Northwest, where they feed on milkweed until metamorphosing into adults.  The migration involves a unique genetic directional imprint that is still not understood by scientists.  At the Rocky Mountains, the monarchs divide into eastern and western migratory populations.  The eastern group overwinters in Mexico, while the western group overwinters in California.  In March, before beginning their journey back, the males die.  The females travel north, each only living for a couple hundred miles, and their final task is to lay their eggs on milkweed.  The trip back north involves four generations of adult butterflies; each feeds on flowers throughout their trip before breeding and dying.  The monarch that returns to California or Mexico the following year is actually five generations removed from their last ancestor that wintered there.    

Why are the monarchs disappearing?

There are three major factors involved in the butterfly decline: severe weather, deforestation in Mexico, and most importantly, the growth of herbicide-based agriculture, which destroys milkweed. 

Milkweed is the only food of the monarch caterpillar, so if milkweed disappears, then it only makes sense that the monarchs will too.  According to the director of Monarch Watch, an organization that monitors U.S. populations, we’ve lost approximately 100 million acres of monarch habitat due to corn and soybean fields since 2000, while millions more are being lost to development.  These fields of crops are genetically modified to resist herbicides.  When the fields are sprayed with herbicides, the milkweed present in the fields is killed.  It’s been estimated that 60% of milkweed has been eliminated from the grassland ecosystem.  Herbicides also kill wildflowers, the monarchs’ source of nectar.  The nectar is essential to build up fat that the monarchs need for their migration. 

 In addition to herbicides and genetically modified crops, severe weather plays a role in the declining monarch populations.  The cold spring in 2013 held up the migration last year, which affected the monarchs breeding schedule.  Also, in 2002, the overwintering grounds in Mexico had the worst storm ever recorded.  It was estimated that approximately 75% of the monarch population was killed that year due to freezing rain and snow. In Texas, in 2012, there was a major drought.  Texas is along the path of migration, and the monarchs need to feed on nectar during their journey.  The drought caused an insufficient supply of nectar due to the death of wildflowers, and an insufficient supply of nectar can’t support the monarchs while they over winter.

Deforestation and illegal logging in the monarchs overwintering site in Mexico was a major threat to the Monarch, and aided in their decline.  The logging leads to a deterioration of the forest.  In 2007 the Mexican government began to enforce the laws and provide economic alternatives to communities.    

Other threats to the monarch include the misguided attempt at planting the wrong variety of milkweed, which could assist in the decline of monarchs.  People are planting a species of tropical milkweed, which doesn’t die during the winter.  This would allow the monarch to have year-around food and have no need for migration.  Also, the monarchs are susceptible to a deadly parasite, Ophryocystis elektroscirra, due to staying in one place for many generations.  This disease spreads when the infected butterflies drop spores on milkweed plants, which are eaten by caterpillars. 

Conservation Efforts: A Brighter Future?

A major project was taken on by Monarch Watch in hopes to increase the monarchs’ numbers.  Nearly 7,450 “way stations,” milkweed-rich areas, have been planted along migration routes on the East and West Coasts to increase feeding and breeding spots. At the University of Minnesota, a coalition of nonprofits and government agencies called Monarch Joint Venture is funding research and conservation efforts.  Other ideas are on the table, such as to push for federal legislation to stop state highway departments from mowing roadsides and to plant wildflowers and milkweed in those areas.

Although the butterflies in Mexico are the same species as the butterflies in California, California has seen an increase in wintering butterflies over the past five years.  According to experts, this increase is a result of conservation efforts.  With conservation efforts in place, there is hope for the monarch.