Monarch Butterflies Added to the Endangered Species Red List

Ecology Florida August 2022 

by Monica Starr

The monarch butterfly embarks on an annual migration which can reach up to 2,800 miles from southeast Canada to central Mexico [1]. These beautiful insects have two pairs of orange-red wings with black veins and distinguishing white spots along the edges. The males have black dots along the veins of their wings and are slightly larger than females. Interestingly, the adult butterflies only live four to five weeks, leaving the population vulnerable to habitat loss, climate change and pesticides [2]. This migration phenomenon has been studied for generations, making this one of the continent’s most widely recognized species [2]. That being said, some disappointing news came last month when the International Union for the Conservation of Nature placed the monarch on its Red List of threatened species and classified it as endangered [2]. 

The IUCN Red List was developed back in 1964 and consists of information from public, private and nonprofit organizations to conserve species worldwide [2]. This list offers a standardized method for assessing the extinction risk of species like the monarch butterfly [2]. There are two migratory populations of monarch that travel on the east and west sides of the Rocky Mountains [2]. The eastern population travels from their overwintering site in central Mexico to their breeding grounds in southern Canada [2]. The western population has overwintering sites along the Pacific Coast in California and Mexico to their breeding grounds west of the Rocky Mountains [2]. This is the longest migration of any insect species known to science. There are monarch butterflies located throughout the world including Europe, but these non-migratory populations are not part of the recent IUCN listing [2]. 

There have been estimates explaining that the population of migrating monarch butterflies in North America has declined between 22% and 72% over the past decade [3]. Another estimate from the eastern U.S. population estimates that the decline is between 85% and 95% since the 1990s [3]. There are various factors that can contribute to this drastic population decline. One of the most common threats that all species endure is habitat fragmentation and loss. When urban development and expansion breaks up large areas of habitat, there are fewer places for monarchs to feed and lay eggs. Since millions of monarchs cluster together during their migration cycles, they are susceptible to things like pesticides, disease, climate change and threats from invasive species [2]. For example, a winter storm back in March 2016 killed 31%-40% of the butterflies in central Mexico [2]. Related to climate, temperature and precipitation can also influence the success of the butterfly’s spring and fall migrations. The spring weather influences the size of the summer monarch population and warmer springs will produce more butterflies on the summer breeding grounds. 

With a complex and long migratory pattern, this population is almost constantly at risk throughout their lifetime. Because of the long migratory route including many types of habitats, it can often be challenging to implement and enforce regulatory actions [2]. A petition back in 2014 to list monarchs under the U.S. Endangered Species Act did initiate conservation efforts which has slowed their population decline [2]. Planting native milkweed plants is a way that the community can easily participate in these conservation efforts. That being said, tropical milkweed is not a good alternative to the traditional milkweed plant that monarch caterpillars need to survive. Tropical milkweed is a non-native plant that has been introduced as the demand for milkweed increases. It becomes a problem when it is planted in temperate areas like Florida  where it does not die back in the winter. There is a monarch butterfly parasite that can travel with the butterflies and end up deposited on the leaves of the milkweed plant [4]. When the monarch caterpillar hatches, they eat milkweed plants along with the parasite. High levels of this parasite in the adult monarchs have been linked to a reduction in body mass, mating success and overall lifespan [4]. When the native milkweed plants die during the winter months, the parasite dies along with them, but the tropical milkweed has the ability to remain evergreen through winter [4]. This allows for a buildup of the parasite. In order to address this issue, gardeners can cut back their milkweed plants to the ground at the beginning of the migration season (late summer) in order to limit the spread and buildup of the parasite. It suggested to cut back the tropical milkweed plants throughout the summer as well, but make sure that there are no monarch eggs or caterpillars attached before doing so. Ideally, it is important to avoid the tropical milkweed all together. Planting the native milkweed when available is the best way to avoid this problem.

Monica Starr is the Public Communications Facilitator and Associate Editor of Ecology Florida News. Monica is a graduate student at the University of South Florida studying Global Sustainability.






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