Abstract
The eastern migratory monarch butterfly (Danaus plexippus) population has declined by ∼84% between 1993 and 2024. Population recovery in the Midwestern United States is limited by the availability of the monarch's main host plant for egg laying—common milkweed (Asclepias syriaca). The extent to which common milkweed availability is limiting in other breeding regions is unknown. Our objective was to determine whether Canada has enough common milkweed to support its share of the trinational eastern migratory monarch population recovery target, given ∼29 stems of common milkweed are needed to contribute one adult monarch into the fall migratory population. To meet this objective, we estimated the number of common milkweed stems in Canada using published common milkweed availability estimates by land cover type. We also estimated the size of the Canadian monarch population if the recovery target was achieved using published estimates of wintering monarch density in Mexico, fall migration survival rates, and the relative proportion of monarchs entering fall migration from Canada. We estimate that Canada currently has 484 million common milkweed stems (range: 111 million–1 billion stems) and increasing this amount by 1.61 times (i.e., by ∼295 million stems), or equivalently, by 61%, would support the recovery target.
Introduction
Each year millions of eastern monarch butterflies (Danaus plexippus) capture people's imaginations across North America as they migrate thousands of kilometers in a single generation from their breeding grounds in the United States and Canada to their wintering grounds in central Mexico, and then back again via a multi-generational migration (Diffendorfer et al. 2014; Gustafsson et al. 2015). However, long-term monitoring on the wintering grounds indicates that this population has declined by 84% from 1993/1994 to 2024/2025 (32-year period; Rendón-Salinas et al. 2025; Fig. 1). Many different stressors acting concurrently or at different times of their annual cycle may be contributing to the observed decline, for example, decreased nectar flower availability—particularly during fall migration, toxic effects of pesticides, climate change, parasites, and forest loss on the wintering grounds (Brower et al. 2012; Pleasants and Oberhauser 2013; Flockhart et al. 2015; Inamine et al. 2016; Thogmartin et al. 2017c; Saunders et al. 2019; Wilcox et al. 2019; Van Deynze et al. 2024). One often cited hypothesis is that loss of common milkweed (Asclepias syriaca), particularly from agro-ecosystems, is largely responsible (Pleasants and Oberhauser 2013; Flockhart et al. 2015; Oberhauser et al. 2017; Pleasants 2017; Thogmartin et al. 2017c; Pleasants et al. 2023). Milkweed species are the primary host plants used by monarchs for egg laying, although they can successfully breed on other plant genera (Greenstein et al. 2022). Importantly, latex sap (containing triterpenes and cardenolides) from milkweed ingested by larvae provides monarch caterpillars and adults with a chemical defense against vertebrate predators and parasites (Emon and Seiber 1985; Malcolm and Brower 1989; Malcolm 1994; Gowler et al. 2015; Agrawal 2017; Züst et al. 2019).
Approximately 130 species of native milkweed occur in North America, with 76 species occurring in the United States and Canada (Woodson 1954; Luna and Dumroese 2013). Twenty-three and 12 species are used by monarchs for breeding in the eastern United States and Canada, respectively (Woodson 1954; White 1996; Luna and Dumroese 2013; for common native species of milkweed used by monarchs in different geographies, refer to The Xerces Society 2021). Currently, the most broadly distributed and abundant milkweed species east of the Rockies in both countries is common milkweed (Woodson 1954; Malcolm et al. 1989; White 1996). For example, ∼72% of milkweed stems found along an urban to rural gradient in Chicago, Illinois, United States of America, were common milkweed (Johnston et al. 2019). However, the broad distribution and abundance of common milkweed is a relatively recent phenomenon. Genetic evidence indicates common milkweed rapidly expanded in range and abundance with clearing of forests in North America for agriculture in the 19th century (Boyle et al. 2023). Regardless, today, common milkweed attracts and supports some of the highest rates of monarch egg deposition and larval survival, respectively, relative to other milkweed species (Pocius et al. 2017, 2018a, 2018b), highlighting its current importance in the eastern migratory monarch butterfly's life-cycle.
Within the eastern monarch's current core breeding range in the Midwestern United States (i.e., Kansas, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Wisconsin), common milkweed availability is estimated to be below the amount that would support monarch population recovery (Flockhart et al. 2017; Pleasants 2017; Thogmartin et al. 2017b). However, the degree to which other breeding regions can support monarch population recovery is unknown. In the Midwestern United States, common milkweed availability is estimated to have declined by 40% between 1999 and 2014, with 98% of this loss occurring in soy and cornfields, which represent 67% of agricultural land covers in the region (Pleasants 2017; Crossley et al. 2021). This loss coincides with the introduction of the herbicide glyphosate and genetically modified glyphosate-resistant soy and corn in 1996 and 1998, with 94% and 89% adoption rates by 2015, respectively (Pleasants 2017; United States Department of Agriculture (USDA) 2023). Glyphosate-resistant soy and corn were introduced in Canada in 1997 and 1998, respectively (Beckie et al. 2014; Soltani et al. 2014a, 2014b). However, information on its use is not publicly available (Bacon et al. 2023) and long-term monitoring of common milkweed before and after the introduction of glyphosate did not occur. Moreover, different from the Midwestern United States, soy and corn comprise only 33% of all agricultural land covers in the breeding region of the monarch in eastern Canada (Supplementary Material 2: Milkweed Calculator). This difference in agricultural land cover composition and potential differences in glyphosate adoption rates suggests current and historical common milkweed availability may differ between the Midwestern United States and Canada.
