10 Threats to Monarch Migration

How Human Activities May Put Migrating Monarch Butterflies in Danger

Monarchs gathering in Mexico for the winter.
Monarchs gathering in Mexico for the winter, from the National Geographic documentary Great Migrations. Photo courtesy of National Geographic. Used with permission.

Though monarch butterflies as a species are not in danger of extinction in the near future, their unique North American migration may cease without intervention. The International Union for Conservation of Nature (IUCN) calls the monarch migration an endangered biological phenomenon. Migrating monarchs face threats throughout their journey, from their overwintering sites to their breeding grounds.

Here are 10 threats to monarch migration, all of them the result of human activities. Until we change our ways, monarchs will likely continue to decline throughout their North American migration route.

1. Roundup-resistant crops.
American corn and soybean growers now plant mostly genetically-modified crops that are resistant to the herbicide Roundup. Rather than till the soil to control weeds in their fields, farmers can now plant their crops first, and then spray their fields with Roundup to kill the weeds. The weeds, including milkweed, die back, while the corn or soybeans continue to grow. Common milkweed (Asclepias syriaca), perhaps the most important monarch host plant of all the milkweeds, can still thrive in a tilled field. Ask any gardener who has planted a patch of it about how quickly it spreads, and how hard it is to keep from resprouting. But common milkweed (or any milkweed species, for that matter) cannot tolerate these repeated applications of Roundup on farm fields.

Milkweed in agricultural fields is believed to have been a food source for up to 70% of monarchs in the past; the loss of these plants could seriously impact the population. Roundup doesn't discriminate, either, so nectar plants that once bloomed between crops have disappeared in these areas, too.

2. Insecticide use.
This might seem like a no brainer (and perhaps it is), but monarch populations can be impacted by exposure to insecticides, even those intended for controlling other insects.

In some cases, the insecticide in question may be deemed safe to other, non-targeted wildlife, but often no studies exist to prove the product won't harm monarch butterflies. Fear of West Nile virus leads many communities to conduct aerial spraying programs of pesticides intended to kill mosquitoes, to the possible detriment of monarchs. Permethrin, for example, is used to control adult mosquitoes, but one study done by the Monarch Lab at the University of Minnesota showed that permethrin residue on milkweed foliage is highly lethal to monarch caterpillars, particularly in the early instars. Bt (Bacillus thuringiensis is a bacteria that specifically targets caterpillars. It is applied aerially to forests, to combat pests like the gypsy moth, and inserted into genetically modified corn, to help the plants repel pests like the corn borer. Studies show that windblown pollen from GM corn can kill monarch larvae, if the toxic pollen lands on milkweed foliage. Fortunately, recent research suggests Bt-laden corn pollen may not pose a serious threat to the overall monarch population.

3. Roadside maintenance activities.
Milkweed grows well in disturbed habitats like roadsides. In my experience, most monarch enthusiasts can spot a milkweed patch while driving 60 miles per hour down the highway!

One would think such an easy growing host plant would give monarchs an edge, but unfortunately the people who maintain our right-of-ways usually view milkweed as a weed, and nothing more. In many places, the roadside vegetation is mowed, often right when milkweed is at its peak and crawling with caterpillars. In some cases, roadside vegetation is treated with herbicides. As farmers eliminate milkweed from their fields with Roundup, roadside milkweed stands will be more important to migrating monarchs.

4. Ozone pollution.
Ozone, a major component of smog, is highly toxic to plants. Some plants are more sensitive to ozone pollution than others. Milkweed is highly sensitive to ozone at ground level, so much so that it is considered a reliable bio-indicator of ozone pollution. Milkweed plants affected by ozone develop dark lesions on their foliage, a symptom known as stippling.

While we know the quality of milkweed suffers in areas of high ground level ozone, we know little about how this may impact monarch larvae that feed on milkweed plants in smoggy areas.

5. Deforestation.
Overwintering monarchs need forests for protection from the elements, and they need very specific forests at that. The population that breeds east of the Rocky Mountains migrates to mountains in central Mexico, where they can roost in dense stands of oyamel fir trees. Unfortunately, those trees are a valuable resource, and even after the monarch wintering site was designated as a preserve, logging activities continued illegally. In the 20 years from 1986 to 2006, an estimated 10,500 hectares of forest were either lost entirely, or disturbed to a degree that they no longer provided suitable winter cover for the butterflies. Since 2006, the Mexican government has been more vigilant in enforcing the logging ban within the preserve, and thankfully, deforestation has decreased significantly in recent years.

6. Water diversion.
Since long before the monarchs were found clinging to trees by the millions in Mexico, Mexican families have subsisted off the land in and around the oyamel forests. Local residents need water, both for their homes and for their cattle and crops. In recent years, villagers have started diverting water from mountain streams, using plastic pipes to intercept and direct it to their homes and farms. Not only does this leave streambeds dry, but it also requires the overwintering monarchs to fly longer distances in search of water. And the farther they fly, the more energy the butterflies require to survive until spring.

7. Real estate development.
California boasts some of the country's highest property values, so it's no surprise that monarchs on the west coast might get squeezed out by land developers. Both breeding habitat and wintering sites are at risk. Remember, the monarch butterfly is not an endangered species, so it isn't afforded the protections of the Endangered Species Act.

So far, butterfly enthusiasts and monarch lovers have done a good job of pleading for the conservation of overwintering sites, which are scattered from San Diego County to Marin County along the California coastline. But vigilance must be maintained to make sure the monarchs keep this prime real estate.

8. Removal of non-native eucalyptus trees.
Why would the removal of non-native trees impact the monarch butterfly, a native species? In the middle to late 19th century, Californians imported and planted no less than 100 species of eucalyptus from Australia. These hardy trees grew like weeds along the California coast. Western monarch butterflies found the groves of eucalyptus trees provided ideal protection in the winter, even better than the stands of native pines where they roosted in the past. The western population of North American monarchs now relies heavily on these stands of introduced trees to see them through the winter. Unfortunately, eucalyptus is known for its propensity to fuel wildfires, so these forests are not so beloved by land managers. We may see a decline in monarch numbers where the non-native trees are removed.

9. Climate change.
Monarchs need very specific climate conditions to survive the winter, and this is why their overwintering sites are limited to just 12 mountains in Mexico and a handful of eucalyptus groves in California. It doesn't matter whether you believe climate change is caused by humans (it is) or not, climate change is real and it is happening now. So what will that mean for the migrating monarchs? Scientists used climate change models to predict what conditions at the overwintering sites will be in the near future, and the models paint a gloomy picture for the monarchs. By 2055, climate change models predict the oyamel forests of Mexico will see precipitation similar to what the area experienced in 2002, when an estimated 70-80% of the monarchs in the two largest overwintering sites died. Why is wet weather so detrimental to the monarchs? In a drier climate, the butterflies can adjust to the cold by a process known as supercooling. Wet butterflies freeze to death.

10. Tourism.
The very people who care the most about monarchs may be contributing to their demise. We didn't even know where the monarchs spent their winters until 1975, but in the decades since, millions of tourists have made the pilgrimage to central Mexico to see this mass gathering of butterflies. Each winter, up to 150,000 visitors travel to the remote oyamel forests. The impact of 300,000 feet on the steep mountain trails causes considerable soil erosion. Many tourists travel by horseback, kicking up dust that blocks spiracles and literally suffocates the butterflies. And each year, more businesses pop up to cater to butterfly tourists, requiring more resources and creating more waste. Even in the U.S., tourism has sometimes hurt more than help the monarchs. A motel built at one of the California overwintering locations degraded the forest and caused the butterflies to abandon the site.


  • North American Monarch Conservation Plan (PDF), prepared by the Secretariat of the Commission for Environmental Cooperation (CEC). Accessed online March 14, 2013.
  • Conservation Initiative in North America to Protect Monarch Butterfly, Convention on the Conservation of Migratory Species of Wild Animals (CMS). Accessed online March 14, 2013.
  • Monarch Butterfly Conservation in North America, U.S. Forest Service. Accessed March 14, 2013.
  • Migrating Monarch Butterflies in Monterey County, Ventana Wildlife Society. Accessed online March 14, 2013.
  • Species Profile (Monarch), Species at Risk Public Registry, Government of Canada. Accessed online March 14, 2013.
  • The Effects of Mosquito-Control Applications of Permethrin on Monarch Butterfly (Danaus plexippus) Larvae , Sara Brinda, 2004. Accessed online March 14, 2013.
  • Lethal and Sub-lethal Effects of Resmethrin on Nontargeted Species , Meredith Blank, 2006. Accessed online March 14, 2013.