stained glass butterfly installation

Monarch Population Decline: Causes and Conservation

By Aurora Allen


The eastern and western monarch butterfly populations rarely interact and occupy different habitats. Decades of research has documented the decline of the eastern monarch, but only recently did scientists note a substantial decline in the western population. Therefore, research into causal factors of the western monarch’s population decline has largely been conducted in the last four years. Further, many of the variables attributed to the eastern population’s decline cannot be extrapolated to the western population due to their differing abiotic and biotic factors. Although the reason for the western monarch’s population decline is still speculative, conservation efforts to restore the monarch population and habitat are underway west of the Rocky Mountains utilizing citizen science research.  


monarch butterfly

Figure 1. Image of adult monarch butterfly. Image provided by Amanda F. Barth, M.S. Rare Insect Conservation Project Leader of the Wildland Resources Department. 

caterpillar on a leaf

Figure 2. Image of monarch butterfly larvae on milkweed. Image provided by Amanda F. Barth, M.S. Rare Insect Conservation Project Leader of the Wildland Resources Department. 

Literature Review

Although the reason for monarch decline is not fully understood, most researchers agree that there are gaps in knowledge that require further research and data collection (Sauders et al. 2019; Agrawal et al. 2019; Dilts et al. 2019; Pelton et al. 2019). This data collection can be performed through citizen science efforts where invested individuals can contribute to monarch sighting data (Saunders et al. 2019). Citizen science collaborations will be vital for continuing research, but they are not the only effort that needs to be made to ensure the survival of the western monarch population. Other efforts that must be made in tandem with further data collection are restoration of monarch breeding and migratory habitats, as well as ensuring that habitats and butterflies are protected from pesticides (Pelton et al. 2019). 


Monarch butterflies have an eastern and western migratory population divided by the Rocky Mountains in North and Central America that occupy a broad range of habitats (Fallon et al. 2015). The population of the western monarch has declined up to 97% since the 1980s (Pelton et al. 2019). Monarch butterflies rely on milkweed as a host plant for their larva as they migrate seasonally (Fallon et al. 2015). Decrease in milkweed availability is one potential causal factor for monarch population decline as found in studies performed on eastern monarch populations (Sauders et al. 2019; Agrawal et al. 2019). However, the causal relationship is disputed for western monarchs as various methods of testing correlation have yet to prove any association (Inamine et al. 2016; Marini et al. 2017). This is likely because the reason for population decline cannot be attributed to only a single factor. Alternative reasons for monarch population decline include logging practices, agricultural practices, herbicide usage, pesticide usage, and climate change (Inamine et al. 2016; Marini et al. 2017; Pelton et al. 2019; Dilts et al. 2019).
map of monarch butterfly migration

Figure 3. A diagram showing the different monarch populations and their migration patterns. The Fall migrations are indicated using orange arrows and the Spring migrations are indicated using green arrows. Image from the U.S. Fish and Wildlife Services (USFWS).

Argument and Evidence 

The most significant decline in western monarch numbers was documented in the winter of 2018-2019 and showed that <1% of the historic western monarch population successfully migrated to their overwintering site in California (Xerces Society Western Monarch Thanksgiving Count, 2019). Causal factors for population decline and conservation efforts have been studied in the western monarch’s cousin across the Rocky Mountains in the east more thoroughly than in the west (Pelton et al. 2019).  

Milkweed might be one limiting variable in western monarch migration and population development, but it is not the only one of import. To understand what factors contribute most to western monarch success in reproduction and migration, patterns of migration must be examined and understood. Seasonal windows of opportunity are biotic and abiotic factors that differ seasonally in a fluctuating environment that impact the fitness of a species (Yang et al. 2020). Common biotic factors studied as contributors to monarch fitness in eastern and western populations are milkweed availability (Dilts et al. 2019), milkweed species preference (Pocius et al. 2018), floral nectar limitation (Saunders et al. 2019), and parasite influence (De Roode et al. 2010). 

Although overwintering and breeding habitat loss due to herbicide use is frequently attributed to lack of migratory success in eastern monarchs, such correlations have not been made in the western population (Brower et al. 2012; Vidal et al. 2013; Pleasants and Oberhauser 2012). Studies have recently been performed to better understand how breeding habitat loss and overwintering habitat loss due to pesticide use affect western monarch butterflies (Espeset et al. 2016; Forister et al. 2016), but the data is not yet conclusive. Overall, the factors causing the decline in western monarch butterfly populations are not fully understood, which emphasizes the need for further research to be performed on the matter.  

Methods and Discussion 

The attempts to record monarch population decline and causal effects have varied between studies, but most have been based on data collected by citizen science organizations including the North American Butterfly Association (NABA), the Xerces Society, and the National Wildlife Refuge (Pleasents et al. 2013; Pelton et al. 2019; Marini et al. 2017; Fallon et al. 2015; Inamine et al. 2016; Dilts et al. 2019). Decades of this data have been stored in museum databases, thus thousands of data points are available for modeling and analysis (Dilts et al. 2019; Boyle et al. 2019). Studies to determine causal factors have modeled seasonal environmental effects of milkweed decline, floral nectar limitation, parasitism, and agricultural practices such as herbicide use and the use of genetically modified crops in eastern populations (Saunder et al. 2019; Inamine et al. 2016; Boyle et al. 2019) but little of this data can be extrapolated to the western population as of yet 

Besides being a vital pollinator in western North America, monarch butterflies have emerged as a model organism for studying the mechanisms and genetics of migration (Reppert et al. 2018). The mechanisms behind migration are often difficult to research (Liedvogel et al. 2011; Roff et al. 2007; Delmore et al. 2016), thus the extensive genetic and population data gathered for monarch butterflies are vital for better understanding how migration works in other species. Having both the western and eastern populations surviving for this research could potentially be one way to determine why different populations of the same species exhibit various migration patterns.  

Locally supporting our scientists studying the western monarchs will not only be vital for increasing the odds of this essential pollinator’s survival, but also for understanding migratory genetics and behaviors. In Utah, there are various ways to get involved with helping conservation efforts and data collection efforts. We can plant native milkweed, support local plant nurseries attempting to propagate more local milkweed, and support governmental conservation and research efforts such as those by the USDA Forest Service (Luna et al. 2013). We can also join in citizen science efforts such as the Utah Pollinator Pursuit and the Xerces Society by doing intermittent data collection on monarch habitat and numbers. 

Literature Cited  

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Boyle, John H., Harmony J. Dalgleish, and Joshua R. Puzey. "Monarch butterfly and milkweed  

declines substantially predate the use of genetically modified crops." Proceedings of the National Academy of Sciences 116.8 (2019): 3006-3011. 

Brower, Lincoln P., et al. "Decline of monarch butterflies overwintering in Mexico: is the  

migratory phenomenon at risk?." Insect Conservation and Diversity 5.2 (2012): 95-100. 

Delmore, Kira E., and Miriam Liedvogel. "Investigating factors that generate and maintain  

variation in migratory orientation: a primer for recent and future work." Frontiers in Behavioral Neuroscience 10 (2016): 3. 

De Roode, Jacobus C., and Sonia Altizer. "Host–parasite genetic interactions and  

virulence‐transmission relationships in natural populations of monarch butterflies." Evolution: International Journal of Organic Evolution 64.2 (2010): 502-514. 

Dilts, Thomas E., et al. "Host plants and climate structure habitat associations of the western  

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Fallon, Candace, et al. "Milkweeds and monarchs in the Western US." Xerces Society for  

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Forister, Matthew L., et al. "Increasing neonicotinoid use and the declining butterfly fauna of  

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Inamine, Hidetoshi, et al. "Linking the continental migratory cycle of the monarch butterfly to  

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move." Trends in ecology & evolution 26.11 (2011): 561-569. 

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Pelton, Emma M., et al. "Western monarch population plummets: status, probable causes, and  

recommended conservation actions." Frontiers in Ecology and Evolution 7 (2019): 258. 

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herbicide use: effect on the monarch butterfly population." Insect Conservation and Diversity 6.2 (2013): 135-144. 

Pocius, Victoria Marie, et al. "Monarch butterflies show differential utilization of nine Midwestern  

milkweed species." Frontiers in Ecology and Evolution (2018): 169. 

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Count Data from 1997–2018. 

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Aurora Allen headshot

Aurora Allen graduated from Westminster University in 2022 with a BS in Biology and is interested in research that promotes conservation/restoration of local ecosystems.