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Monarch
This unmistakable butterfly makes an incredible, multi-generation journey.
About This Species
The monarch butterfly is one of the most recognizable insects in North America, renowned for its long-distance migration spanning up to 3,000 miles to overwintering sites in Mexico. However, many people don't realize that on the western side of the Rockies, monarchs migrate and overwinter on the California coast. Colorado is unique in that we have monarchs from both populations in our state on either side of the continental divide. While the monarch has a vast range and is commonly observed during the summer, both overwintering populations have experienced significant declines. Volunteers counted 9,119 butterflies overwintering in California in 2024, which was the second lowest count since surveys started in 1997, and a 96% decline from the previous year. While the overwintering population in Mexico increased from 2.22 to 4.42 hectares of forest occupied from 2023 to 2024, this is still well below the estimated 6-hectare minimum population size that scientists believe is necessary for the long-term survival of this species.
The striking colors of monarch caterpillars and adults are a form of aposematism, when animals use bright colors to warn predators that an animal is toxic to eat. Other animals that use aposematism include poison-dart frogs and some species of sea slugs. Monarch caterpillars eat milkweed plants exclusively, which contain toxic cardenolide chemicals. The monarch is able to store these toxic chemicals in their bodies as a form of defense. This defense mechanism is so effective that other species, such as the viceroy butterfly, have evolved to mimic the monarch coloration.
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Physical Characteristics
Monarch caterpillars have black, white and yellow stripes. Adult wings are bright orange with black lines and white dots around the margins. The viceroy butterfly closely resembles the monarch; however, viceroys have a black band across the middle of their hind wings that is absent in monarchs.
Range
Monarchs can be found across North America, with their summer range extending all the way into the southern part of Canada, and the migration taking them down into the heart of Mexico.
Habitat
Monarchs require an abundance of their milkweed host plants across their entire summer breeding range. Adults also require abundant nectar sources throughout the breeding season. As long as these needs are met, monarchs will occupy a vast diversity of habitat types, from grasslands and wetlands, to roadsides, urban gardens and agricultural fields.
Diet
Monarch caterpillars exclusively eat milkweed plants. The plant gets its name from its milky sap exuded when the plant is damaged. Adult monarchs are generalist feeders, favoring abundant, nectar-rich species.
Reproduction
During the summer months, monarchs live two to six weeks as adults. They forage for nectar, mate and lay their eggs on milkweed plants. The caterpillars emerge after three to four days and begin consuming their milkweed host plants. After 10-14 days the caterpillars form a chrysalis, a hard outer coating around their bodies. Monarchs spend nine to 14 days in the chrysalis before emerging as an adult butterfly. There are several generations of monarchs across the summer, spreading northward in the east and eastward in the west until the lower temperatures and shorter day lengths of fall trigger the emergence of the migratory generation. Migratory monarchs emerge in reproductive diapause, a state in which they cannot reproduce so they can save energy for the strenuous migration ahead. The migratory monarch generation flies up to 3,000 miles to Mexico in the eastern part of Colorado and up to 800 miles to the California Coast in the western part of the state. This migratory generation lives six to nine months and will break reproductive diapause in the spring, and begin the migration back northward.
Threats to Species
The loss of summer breeding and migratory habitat is a major threat to the monarch butterfly. This habitat loss occurs in many ways, including urbanization, agriculture, logging and fire. Pesticides are another major threat to monarchs, with widespread contamination across the landscape that can have both lethal and nonlethal effects. Climate change is also a threat to the monarch. Higher temperatures cause monarchs to burn through their fat reserves quickly, which causes more monarchs to die during winter. Climate change also causes important habitats to change, leaving monarchs with less food and shelter.