The Monarch butterfly (Danaus plexippus) is an iconic migratory insect known for its incredible annual journey across North America. This species is divided into two major populations: the Eastern population, which migrates to central Mexico, and the Western population, which travels to the California coast. Over the last few decades, both populations have experienced dramatic declines, establishing a conservation crisis. The Eastern migratory population dropped by an estimated 88% between 1996 and 2020, while the Western overwintering population saw a decline of over 99% since the 1980s, highlighting the severe threats facing this species.
Reduction of Milkweed Availability
Milkweed plants (Asclepias species) are the singular host plant for Monarch eggs and the exclusive food source for their larvae, making their presence necessary for reproduction. The widespread adoption of modern agricultural practices, particularly in the U.S. Midwest, has been a major driver in the loss of this breeding habitat. This region, often referred to as the Corn Belt, is a crucial breeding ground where multiple generations of the Eastern Monarch are produced each summer.
The introduction of herbicide-tolerant crops, such as corn and soybeans modified to resist the herbicide glyphosate, fundamentally changed farming practices. Farmers can now spray entire fields with non-selective herbicides to eliminate weeds without harming the crop. This practice has effectively eradicated milkweed from vast agricultural landscapes and adjacent areas like field margins and roadsides.
The loss of milkweed in these agricultural fields has been significant; abundance in croplands in Iowa, for instance, declined by 81% between 1999 and 2010. This reduction in available host plants directly translates to a lower capacity for the Monarch population to reproduce. The sheer scale of this habitat removal across the Monarch’s primary breeding range severely limits the number of butterflies that can successfully begin the southward migration each fall.
Habitat Degradation at Overwintering Sites
The Eastern Monarch population congregates in specific, high-altitude oyamel fir forests in central Mexico each winter to survive the cold. These forests provide a unique microclimate that shields the clustered butterflies from freezing temperatures, heavy rain, and desiccation. The forest canopy is paramount for the adult butterflies to conserve the lipid reserves they need to survive dormancy and begin their journey north in the spring.
The integrity of this specialized habitat is threatened by illegal logging, forest degradation, and resource extraction. Even though the area is protected within the Monarch Butterfly Biosphere Reserve, unauthorized tree removal reduces the density of the oyamel fir stands. When the protective canopy is thinned, the microclimate is altered, making the Monarch colonies vulnerable to moisture and cold, which can lead to catastrophic mortality events.
The Western Monarch population faces similar habitat threats along the California coast, where they overwinter in groves of eucalyptus, cypress, and pine trees. Coastal development and habitat removal or trimming reduce the number of sites where these butterflies can aggregate and find shelter. Loss of these geographically limited overwintering sites places the adult generation at high risk during the most vulnerable stage of their annual cycle.
Chemical Exposure and Toxicity
Beyond the physical removal of their host plant, Monarchs face direct toxic exposure from various agricultural chemicals, affecting them at multiple life stages. Systemic insecticides, particularly neonicotinoids, pose a significant risk. They are often applied as seed treatments in major crops like corn and soybeans, where the chemicals are absorbed by the plant and circulate throughout its tissues, making the plant toxic to feeding insects.
Milkweed plants growing near agricultural fields can absorb these systemic insecticides from soil leaching or through spray drift, making them poisonous to Monarch caterpillars. Studies have shown that exposure to high doses of neonicotinoids, such as clothianidin, can negatively affect larval survival, pupation, and the adult butterfly’s body size. While adult Monarchs are more tolerant of low doses, the vulnerable larval stage, which feeds exclusively on the contaminated milkweed, is highly susceptible.
Herbicide drift, the movement of spray away from the target area, also contributes to toxicity by contaminating nectar sources that adult Monarchs rely on for fuel during migration. General pesticide use in residential gardens and municipal areas can poison adult butterflies foraging for nectar on wildflowers. This chemical contamination adds a layer of mortality risk throughout the entire Monarch life cycle.
Climate Change and Weather Extremes
Broad climatic shifts and the increasing frequency of extreme weather events present complex challenges to the Monarch’s multi-generational life cycle and migration. Rising temperatures can cause phenological mismatch, which disrupts the synchronized timing between the Monarch’s arrival and the availability of its food sources. Warmer spring temperatures can cause milkweed to emerge or senesce (die off) earlier than usual in the southern U.S.
If the overwintering generation arrives to lay eggs after the milkweed has already begun to senesce, the resulting caterpillars lack sufficient, nutritious food to develop properly. Conversely, unseasonably warm fall temperatures can delay the migratory instinct, causing Monarchs to arrive late at their wintering grounds. This delay increases the risk of the butterflies encountering early cold snaps or late freezes that can decimate the migrating population.
Extreme weather events pose an immediate threat to Monarchs across their range. Severe droughts, which are becoming more common, limit the growth of milkweed and reduce the nectar content of wildflowers, weakening both larvae and migrating adults. Massive storms, such as hurricanes or intense winter storms, can destroy large overwintering clusters. For example, a severe storm in 2002 killed an estimated 80% of the overwintering Monarch population in Mexico, demonstrating the devastating impact these events can have on concentrated groups.