Distribution and Habitat

Common milkweed is native to eastern North America (Fernald Reference Fernald1950; Georgia Reference Georgia1919; Vasey Reference Vasey1888). In the United States, it is widespread throughout the Great Plains region to the eastern seaboard, and it extends as far south as Texas, although it does not grow along the Gulf Coast (Bhowmik Reference Bhowmik1994; Stevens Reference Stevens2000). In Canada, common milkweed grows east of Saskatchewan, concentrated mainly in Ontario and Québec (Bhowmik Reference Bhowmik1994). The species was brought to Europe in the 17th century as an ornamental plant and has since escaped containment and become widespread (Bagi & Botta-Dukát, Reference Bagi, Botta-Dukát and Balogh2008; Dodge Reference Dodge1897; Follak et al. Reference Follak, Bakacsy, Essl, Hochfellner, Lapin, Schwarz, Tokarska-Guzik and Wołkowycki2021; Whiting Reference Whiting1943 and references therein) (Figure 1). Since 2017, it has been listed as an “Invasive Alien Species of Union Concern” in the European Union, and land managers are legally required to remove it (Tsiamis et al. Reference Tsiamis, Gervasini, Deriu, D’amico, Katsanevakis and Cardoso2019). It is now widespread in central Europe, and has spread along road networks, especially unpaved roads bordered by forests and grasslands (Follack et al. Reference Follak, Schleicher and Schwarz2018). In Slovakia, it is especially abundant in abandoned vineyards (Pauková et al. Reference Pauková, Káderová and Bakay2013).

Figure 1. Current distribution of common milkweed (figure generated by GBIF 2025).

Common milkweed is adapted to a wide range of habitats, and is often found in cropland, disturbed areas, pastures, and roadsides (Bhowmik Reference Bhowmik1994; Blatchley Reference Blatchley1912; Georgia Reference Georgia1919; Timmons Reference Timmons1946; Vasey Reference Vasey1888). It prefers warm, well-drained, calcareous soils with a loamy texture (Bhowmik Reference Bhowmik1994; Bhowmik and Bandeen Reference Bhowmik and Bandeen1976; Stevens Reference Stevens2000). Common milkweed requires a minimum of 50 cm of rainfall in the summer months but can withstand moisture stress (Bhowmik Reference Bhowmik1994; Bhowmik and Bandeen Reference Bhowmik and Bandeen1976). It is limited to regions with mean temperatures between 18 and 32 C in July, and it prefers partial shade to full sun (Bhowmik and Bandeen Reference Bhowmik and Bandeen1976).

Biology and Ecology

Common milkweed is a creeping perennial herb with large, waxy, opposite leaves that can reach 1 to 2 m in height at maturity and produces a milky latex in all parts of the plant (Blatchley Reference Blatchley1912; Britton and Brown Reference Britton and Brown1896; Georgia Reference Georgia1919; Kay and Lees Reference Kay and Lees1913; Vasey Reference Vasey1888) (Figure 2). The inflorescence is shaped like an umbel, with many small flowers with five purple-to-white petals (Bhowmik and Bandeen Reference Bhowmik and Bandeen1976; Stevens Reference Stevens2000) (Figure 3). Pollen dispersal occurs over short distances (Pleasants Reference Pleasants1991). In eastern North America, seedlings and vegetative shoots from adventitious root buds emerge in April or May, and flowering occurs from late June to early August (Bhowmik Reference Bhowmik1994; Sauer and Feir Reference Sauer and Feir1974). Plants reach reproductive maturity after 2 yr when produced from seeds and typically the first year when produced vegetatively (Bender et al. Reference Bender, Baskin and Baskin2000; Gerhardt Reference Gerhardt1929 as cited by Whiting Reference Whiting1943; Meitzen 1862 as cited by Whiting Reference Whiting1943). Most short-distance reproduction occurs from rootstocks, and most long distance reproduction occurs from seed dispersal (Bhowmik Reference Bhowmik1978). Viable seeds are produced as early as 5 to 6 wk following the onset of flowering (Evetts and Burnside Reference Evetts and Burnside1973a). Each plant produces an average of five pods, which contain an average of 226 seeds, although the actual number varies and can range between 100 and 425 (Sauer and Feir Reference Sauer and Feir1974; Bhowmik and Bandeen Reference Bhowmik and Bandeen1976; Willson and Rathcke Reference Willson and Rathcke1974). Common milkweed seeds are released synchronously when seed pods (follicles) dehisce in September or October and are dispersed by wind (Bhowmik Reference Bhowmik1994; Sauer and Feir Reference Sauer and Feir1974) (Figure 4). Seeds are initially dormant, with dormancy released by a period of cold moist stratification (Baskin and Baskin Reference Baskin and Baskin1977; Bhowmik Reference Bhowmik1978; Burnside et al. Reference Burnside, Wilson, Weisberg and Hubbard1996; Jeffery and Robison Reference Jeffery and Robison1971). Following this period of stratification, most germination occurs at day/night temperature cycles of 20/10 C to 35/20 C, and maximum seedling emergence occurs at a temperature of 27 C (Baskin and Baskin Reference Baskin and Baskin1977; Bhowmik Reference Bhowmik1978). The majority of common milkweed seeds readily germinate within 2 yr, provided environmental conditions are suitable, but around 5% can persist in the soil for up to 5 yr (Burnside et al. Reference Burnside, Wilson, Weisberg and Hubbard1996).

Figure 2. Flowering stage of mature common milkweed. Photo credit: Randy Prostak.

Figure 3. Common milkweed inflorescence. Photo credit: Scott Morris.

Figure 4. Dehiscing common milkweed follicle (pod). Photo credit: A. DiTommaso.

Vegetative reproduction occurs from root buds located either on lateral roots or near the crown, and parent roots can survive for multiple years under favorable soil conditions such as well-drained soils or infrequent tillage operations (Bhowmik and Bandeen Reference Bhowmik and Bandeen1976; Polowick and Raju Reference Polowick and Raju1982). Lateral root buds form within 25 d of plant establishment and are well developed by the time the plant is 3 mo old (Polowick and Raju Reference Polowick and Raju1982; Stamm-Katovich et al. Reference Stamm-Katovich, Wyse and Biesboer1988). The growth of root buds is inhibited by the parent shoot, but regrowth can occur when parent shoots are removed (Jeffery and Robison Reference Jeffery and Robison1971; Stamm-Katovitch et al. Reference Stamm-Katovich, Wyse and Biesboer1988).

Weediness

Common milkweed is known to reduce crop yields, notably sorghum [Sorghum bicolor (L.) Moench], soybean, and corn (Bhowmik Reference Bhowmik1994). It is not competitive with annual weeds during early developmental stages, so control of annual weeds may favor common milkweed within crop fields (Evetts and Burnside Reference Evetts and Burnside1975). Common milkweed can be especially problematic in spring wheat and early planted corn because the shoots emerge after tillage operations have been completed (Mohler et al. Reference Mohler, Teasdale and DiTommaso2021) (Figure 5). Seedling density was greater under conservation tillage than in conventionally tilled fields in Minnesota because conventional tillage causes soil to warm more quickly in spring, leading to greater emergence from seed and the destruction of seedlings during later tillage operations (Yenish et al. Reference Yenish, Fry, Durgan and Wyse1997a). In another study performed in Minnesota, high densities of 12 milkweed shoots per square meter led to a 47% reduction in wheat (Triticum aestivum L.) yield (Yenish et al. Reference Yenish, Durgan, Miller and Wyse1997b). Infestations in Nebraska led to yield losses of 2% to 10% of corn, 4% to 29% of sorghum, and 12% to 19% of soybean (Cramer and Burnside Reference Cramer and Burnside1982). Evetts and Burnside (Reference Evetts and Burnside1973b) found that infestations of common milkweed resulted in an average 21% decrease in sorghum, even at low population densities. Aqueous extracts from common milkweed can have allelopathic effects on sorghum, leading to reduced seed germination (Cramer and Burnside Reference Cramer and Burnside1982; Rasmussen Reference Rasmussen1975).

Figure 5. Common milkweed stands after corn silage harvest in central New York state. Photo credit: C. Pelzer.

Common milkweed is also problematic in pastures. It has been listed as a plant that can cause clinical emergencies in cattle due to its neurotoxic effects (McGuirk and Semrad Reference McGuirk and Semrad2005). It contains cardenolides such as asclepiadin, gomphoside, and afroside, which can be toxic to mammals, including humans, when consumed (Agrawal et al. Reference Agrawal, Petschenka, Bingham, Weber and Rasmann2012; Simpson et al. Reference Simpson, Cole and Ellsworth2013). Despite this, a study by Dickson et al. (Reference Dickson, Poynor and Helzer2023) found that cattle in Nebraska regularly grazed common milkweed in low amounts with no adverse effects. Out of 11 pasture weeds studied in Mississippi, Carlisle et al. (Reference Carlisle, Watson and Cole1980) found that common milkweed had the most digestible dry matter, the smallest canopy diameter, and the shortest height. These results indicate that low densities of common milkweed in pastures may be acceptable to livestock.

Common milkweed is tolerant to water stress, although water stress reduces growth and defensive traits (Couture et al. Reference Couture, Serbin and Townsend2015; Matzner et al. Reference Matzner, Konz, Marts, Eversman, Kasuske, Atkins, Acharya, Matuck, Derynck, Kreutzmann, Selberg, Glisar, Capers, Lind, Olimb and Olson-Manning2025). Plants exposed to increased day/night temperature cycles of 30/23 C displayed increased growth compared with reference temperatures of 25/18 C (Couture et al. Reference Couture, Serbin and Townsend2015). The species is sensitive to tissue loss, and photosynthesis can be substantially reduced in plants that have been severely damaged by insect herbivory (Delaney et al. Reference Delaney, Haile, Peterson and Higley2008). Competition with grasses has been shown to decrease common milkweed growth due to increased selective herbivory from insects (Agrawal Reference Agrawal2004). In its invasive range in Europe, common milkweed has detrimental effects on native plant communities in grasslands and natural areas. Much of this research has been conducted in grasslands of central Hungary. Tölgyesi et al. (Reference Tölgyesi, Tóth, Hábenczyus, Frei, Tóth, Erdős, Török and Bátori2024) found that the presence of common milkweed resulted in the depletion of soil moisture reserves. Another study found that plant communities invaded by common milkweed actually had higher Shannon or Simpson diversity than uninvaded communities (Meinhardt et al. 2024). The low-density milkweed stands allowed native plants to persist and may also have facilitated invasion by other weeds such as Indian blanket (Gaillardia pulchella Foug.), which would also have contributed to diversity (Meinhardt et al. 2024). Csiszár et al. (Reference Csiszár, Korda, Schmidt, Šporčić, Süle, Teleki, Tiborcz, Zagyvai and Bartha2013) reported that common milkweed suppressed native plant species due to its allelopathic effects. Common milkweed also reduced the cover of smaller native plant species due to shading effects (Kelemen et al. Reference Kelemen, Valkó, Kröel-Dulay, Deák, Török, Tóth, Miglécz and Tóthmérész2016).

Chemical Control

The herbicide most used to control common milkweed is glyphosate. Glyphosate applied at a rate of 2.24 or 3.36 kg ai ha^–1 has been found to kill top growth with no more than 10% regrowth in the following year (Bhowmik and Bandeen Reference Bhowmik and Bandeen1976) (Table 1). In a field study in Guelph, Ontario, Canada, glyphosate applied at 2.2 kg ai ha^–1 reduced stands of common milkweed for 2 to 3 yr (Bhowmik Reference Bhowmik1982). Satisfactory control was obtained with the same rate in a separate field trial in Lincoln, Nebraska (Cramer and Burnside Reference Cramer and Burnside1981). Bakacsy and Bagi (Reference Bakacsy and Bagi2020) found that a single glyphosate application at 0.72 kg ae ha^–1 resulted in some translocation of the herbicide to the dormant root buds, but that the stand was able to slowly regenerate over time. Waldecker and Wyse (Reference Waldecker and Wyse1985a) were able to increase glyphosate absorption by root buds in a controlled environment by applying a cytokinin, 1 mM 6-benzyl-aminopurine, prior to herbicide application.

Table 1. Examples of herbicides that have been successfully used to control common milkweed.

^a The sulcotrione treatment included an adjuvant.

Glyphosate is not the only postemergence-applied herbicide that has been studied for its potential to control common milkweed. Shoot control in this plant can be achieved with early applications of 2,4-D (Bhowmik Reference Bhowmik1994; Bhowmik and Bandeen Reference Bhowmik and Bandeen1976) (Table 1). However, common milkweed is generally less susceptible to 2,4-D than to glyphosate, due to the plant’s rapid metabolism of 2,4-D (Bhowmik Reference Bhowmik1985; Wyrill and Burnside Reference Wyrill and Burnside1976). Amitrole-T applied at a rate of 1.12 or 2.24 kg ai ha^–1 was mostly effective, with a regrowth rate of 5% to 10% occurring in the next growing season (Bhowmik and Bandeen Reference Bhowmik and Bandeen1976). Both 1.1 kg ai ha^–1 of amitrole and 0.6 kg ai ha^–1 of picloram reduced stands for 4 yr in a field experiment in Guelph, Ontario (Bhowmik Reference Bhowmik1982). In a greenhouse study, Cramer and Burnside (Reference Cramer and Burnside1981) found that common milkweed regrowth was reduced by the following treatments: amitrole, dicamba, and glyphosate at 0.2 kg ai ha^–1; amitrole + dicamba, amitrole + 2,4-D, amitrole + glyphosate, glyphosate + 2,4-D, and glyphosate + dicamba, all at 0.3 + 0.2 kg ai ha^–1.

Common milkweed has been reported to be tolerant of certain herbicides. This can be useful information for growers when they endeavor to control a different weed in an area where milkweed is not considered weedy. Common milkweed is more tolerant than other weeds of glufosinate applied at rates up to 0.25 kg ai ha^–1, although the activity of glufosinate was enhanced by adding ammonium sulfate (Pline et al. Reference Pline, Hatzios and Hagood2000). The relatively low absorption of glufosinate in common milkweed compared with other weeds might be a result of the plant’s waxy cuticle (Pline et al. Reference Pline, Wu and Hatzios1999). Umiljendic et al. (Reference Umiljendic, Saric-Krsmanovic, Santric and Radivojevic2017) found that common milkweed was moderately susceptible to sulcotrione at a rate of up to 0.90 kg ai ha^–1. Common milkweed was injured, but not killed, by fomesafen applied at rates up to 0.28 kg ai ha^–1 in a greenhouse study near Ames, Iowa (Lizotte-Hall and Hartler Reference Lizotte-Hall and Hartzler2019). Dunham lists common milkweed as being resistant to 2,4-D, MCPA, and simazine. Vangessel (Reference Vangessel1999) found that linuron applied at 0.37 kg ai ha^–1 only partially controlled common milkweed. At low rates of 1.1 kg ai ha^–1 or less, dicamba, fenac, alachlor, atrazine, and bentazon have been found to provide less than 50% control of common milkweed (Bhowmik Reference Bhowmik1994; Cramer and Burnside Reference Cramer and Burnside1981).

Common milkweed can be controlled in agronomic crops with soil-applied herbicides such as atrazine, EPTC, and metribuzin (Bhowmik Reference Bhowmik1994). Oxyfluorfen, terbuthylazine, and mesotrione have been shown to inhibit germination and reduce seedling size in a greenhouse study (Radivojevic et al. Reference Radivojevic, Saric-Krsmanovic, Gajic Umiljendic, Bozic and Santric2016). In another greenhouse study, Vangessel (Reference Vangessel1999) found that clomazone applied at 0.84 kg ai ha^–1, atrazine at 0.84 kg ai ha^–1, flumetsulam at 0.04 kg ai ha^–1, and cloransulam at 0.03 kg ai ha^–1 provided at least 80% control of common milkweed seedlings. Metribuzin applied at 0.28 kg ai ha^–1 and a combination of 0.18 kg ai ha^–1 metribuzin and 0.03 kg ai ha^–1 chlorimuron provided 97% and 99% control of common milkweed seedlings, respectively (Vangessel Reference Vangessel1999). As with any weed, the efficacy of herbicide applications to control common milkweed varies based on application timing, application methods, and environmental conditions. Most foliar-applied herbicides are most effective in June, during the early bud stage of development, rather than during early growth or after flowering (Bhowmik Reference Bhowmik1994, Reference Bhowmik1982; Bhowmik and Bandeen Reference Bhowmik and Bandeen1976; Zalai et al. Reference Zalai, Poczok, Dorner, Körösi, Pálinkás, Szalai and Pintér2017). Wyrill and Burnside (Reference Wyrill and Burnside1977) tested various surfactants and surfactant combinations to enhance glyphosate control of common milkweed. They reported that surfactant performance was variable, difficult to predict, and not related to the angle at which the herbicide mixture contacted the leaves. Glove, roller, and broadcast applications have all been found to be effective in controlling common milkweed (Cramer and Burnside Reference Cramer and Burnside1981). Water stress has been shown in greenhouse studies to decrease the efficacy of glyphosate due to reduced absorption and translocation (Waldecker and Wyse Reference Waldecker and Wyse1985b).

Cultural and Mechanical Control

Common milkweed emerges primarily in late spring to early summer, and competing summer annual crops limit growth (Evetts and Burnside Reference Evetts and Burnside1975; Mohler et al. Reference Mohler, Teasdale and DiTommaso2021). Timmons (Reference Timmons1946) found that crop rotations that included alfalfa (Medicago sativa L.) led to a decrease in the density of common milkweed populations. Rotations that include winter wheat (Triticum aestivum L.) have also been used to control common milkweed (Bhowmik Reference Bhowmik1994).

Due to the ability of the plant to reproduce from lateral root buds, tactics such as standard moldboard plowing or the removal of individual stalks may not be effective methods for controlling established common milkweed populations unless these tactics are repeated over time (Bhowmik and Bandeen Reference Bhowmik and Bandeen1976). Zalai et al. (Reference Zalai, Poczok, Dorner, Körösi, Pálinkás, Szalai and Pintér2017) found that mowing plants for two growing seasons was ineffective and that populations experienced vigorous regrowth. However, common milkweed cover and shoot number have been observed to decrease after two consecutive years of shoot removal, possibly due to the depletion of carbohydrate reserves (Berki et al. Reference Berki, Botta-Dukát, Csákvári, Gyalus, Halassy, Mártonffy, Rédei and Csecserits2023; Georgia Reference Georgia1919). Repeated tillage during a single fallow period starting in spring and ending in late summer, when the root buds become dormant, can also reduce populations (Mohler et al. Reference Mohler, Teasdale and DiTommaso2021). Inversion tillage has also been found to place seeds in soil that is too deep for seedlings to emerge (Yenish et al. Reference Yenish, Fry, Durgan and Wyse1996). Another method of control is to expose the roots to dry and freezing conditions by plowing at a depth greater than 20 cm in the fall (Bhowmik Reference Bhowmik1994).

Historical and Current Uses

Common milkweed is a culturally important food source to many Indigenous groups in the United States (Duke Reference Duke1992; Gaertner Reference Gaertner1979; Gonella and Kindscher Reference Gonella and Kindscher2024; Knudsen and Sayler Reference Knudson and Sayler1992; Medsger 1939; Moerman Reference Moerman1998; Stevens Reference Stevens2000). Typically, the parts of the plant that are eaten are early spring shoots, young pods, and unopened inflorescences (Gonella and Kindscher Reference Gonella and Kindscher2024). Common milkweed is safe to eat when properly prepared, which involves boiling it for 2 to 4 min and then discarding the water or parboiling it (Duke Reference Duke1992; Gaertner Reference Gaertner1979; Gonella and Kindscher Reference Gonella and Kindscher2024).

Common milkweed stems have been used in recent centuries as a source of fiber for making ropes and cloth (Dodge Reference Dodge1897; Knudson and Sayler Reference Knudson and Sayler1992; Porcher Reference Porcher1863; Stevens Reference Stevens2000; Vasey Reference Vasey1888; Whiting Reference Whiting1943). During World War II, floss from common milkweed seed pods was used to fill life jackets (Gartner Reference Gaertner1979; Knudson and Sayler Reference Knudson and Sayler1992; Whiting Reference Whiting1943). In recent years, there has been renewed interest in common milkweed as a sustainable fiber crop due to the unique structure of its fibers (Hassanzadeh and Hasani Reference Hassanzadeh and Hasani2017; Karthik and Murugan Reference Karthik, Murugan, Muthu and Gardetti2016). Fibers derived from common milkweed stems are lightweight, hollow, and hydrophobic, and can be used in applications such as sorption of oil and insulation (Hassanzadeh and Hasani Reference Hassanzadeh and Hasani2017; Karthik and Murugan Reference Karthik, Murugan, Muthu and Gardetti2016). The U.S. Department of Agriculture has also conducted research into the use of common milkweed oil as a sunscreen and as a potential source of latex for rubber production (Suszkiw Reference Suszkiw2009; Whiting Reference Whiting1943).

Conservation Efforts

Common milkweed has received significant interest from conservation groups (Harris et al. Reference Harris, Hall and Finke2023). Monarch butterfly larvae need to feed on common milkweed or other plants in the genus Asclepias to complete their life cycle; they also use the cardiac glycosides produced by the plants as a form of chemical defense (Pleasants Reference Pleasants2017; Stevens Reference Stevens2000) (Figure 6). Proper larval development most likely occurs in patches of two to four closely spaced milkweed plants, as opposed to single plants (Fisher et al. Reference Fisher, Hellmich and Bradbury2020). The widespread adoption of herbicide-tolerant field crops has dramatically reduced agricultural milkweed populations and thereby contributed to the decline of monarch butterfly in North America (Belsky and Joshi Reference Belsky and Joshi2018; Brower et al. Reference Brower, Taylor, Williams, Slayback, Zubieta and Ramírez2012; Malcolm Reference Malcolm2018; Saunders et al. Reference Saunders, Ries, Oberhauser, Thogmartin and Zipkin2018; Wilcox et al. Reference Wilcox, Flockhart, Newman and Norris2019). There is some controversy over the importance of this breeding habitat loss, relative to other threats to the monarch (Agrawal and Inamine Reference Agrawal and Inamine2018; Inamine et al. Reference Inamine, Ellner, Springer and Agrawal2016). However, many authors consider the loss of agricultural breeding habitat to be the primary driver of monarch population declines in recent decades (Flockhart et al. Reference Flockhart, Pichancourt, Norris and Martin2015; Stenoien et al. Reference Stenoien, Nail, Zalucki, Parry, Oberhauser and Zalucki2018; Thogmartin et al. Reference Thogmartin, Wiederholt, Oberhauser, Drum, Diffendorfer, Altizer, Taylor, Pleasants, Semmens, Semmens, Erickson, Libby and Lopez-Hoffman2017a). This argument implies that monarchs were dependent on agricultural breeding habitat before herbicide-tolerant crops were introduced. Indeed, it is likely that agricultural (primarily corn and soybean) habitats formerly produced more monarchs than any other habitat type in the midwestern United States (Oberhauser et al. Reference Oberhauser, Prysby, Mattila, Stanley-Horn, Sears, Dively, Olson, Pleasants, Lam and Hellmich2001; Pleasants Reference Pleasants2017). Corn and soybean fields hosted many monarchs because they cover large land areas and contained milkweeds that were highly suitable for monarch oviposition (see the discussion in Biology and Ecology). Most corn and soybean fields now contain few milkweeds because they are treated with nonselective, postemergence herbicides (Hartzler Reference Hartzler2010; Pleasants Reference Pleasants2017). In the Midwest, it is estimated that milkweed populations declined by approximately 40% between 1999 and 2014, leading to an estimated 70% loss in monarch support capacity (Pleasants Reference Pleasants2017). The difference between 40% and 70% reflects the fact that milkweed loss was concentrated in agricultural habitats, and more monarch eggs are laid per stem on milkweeds in an agricultural setting (Oberhauser et al. Reference Oberhauser, Prysby, Mattila, Stanley-Horn, Sears, Dively, Olson, Pleasants, Lam and Hellmich2001; Pitman et al. Reference Pitman, Flockhart and Norris2018; Pleasants & Oberhauser, 2015; Pleasants and Oberhauser Reference Pleasants and Oberhauser2013). Monarch population size has declined by an estimated 80% over a similar time frame (Pleasants Reference Pleasants2017; Pleasants and Oberhauser Reference Pleasants and Oberhauser2013). Based on these findings, milkweed populations need to increase by an estimated 1.6 billion plants to support a stable monarch population (Pleasants Reference Pleasants2017). In light of severe habitat loss and population declines, the U.S. Fish and Wildlife Service in 2024 proposed listing monarch butterflies as threatened under the Endangered Species Act (USFWS 2024).

Figure 6. A monarch butterfly larva feeding on common milkweed. Photo credit: R. Stup.

Many milkweed restoration efforts have focused on replacing the milkweeds lost from agricultural habitats with milkweeds in nonagricultural habitats, including roadsides, conservation areas, and gardens (Pleasants Reference Pleasants2017; Zaya et al. Reference Zaya, Pearse and Spyreas2017). One popular conservation method is the use of “monarch waystations,” which are pollinator gardens containing milkweed that are intended to provide patches of habitat for monarch butterflies (Landis Reference Landis2014). Management of existing patches of milkweed can also help conserve monarch butterfly populations. A study in upstate New York found that mowing twice in July promoted regrowth of milkweed, which led monarchs to lay more eggs and extended their breeding period (Fischer et al. Reference Fischer, Williams, Brower and Palmiotto2015).

While these programs are valuable, they sometimes struggle to overcome barriers such as 1) high cost, 2) limited available land area, or 3) low monarch support capacity. Consequently, some authors have concluded that successful milkweed and monarch conservation will require the use of agricultural habitats. For example, Thogmartin et al. (Reference Thogmartin, López-Hoffman, Rohweder, Diffendorfer, Drum, Semmens, Black, Caldwell, Cotter, Drobney, Jackson, Gale, Helmers, Hilburger, Howard, Oberhauser, Pleasants, Semmens, Taylor, Ward, Weltzin and Wiederholt2017b) tested 218 possible scenarios for restoring milkweed to the midwestern United States. In these scenarios, milkweeds would be restored in protected area grasslands, land designated as being part of the Conservation Reserve Program (CRP; a U.S. Department of Agriculture program that pays farmers to take environmentally sensitive land out of agricultural production), powerline, rail and roadside rights of way, urban/suburban lands, productive cropland, and/or marginal cropland. They found that it was not possible to reach 1.3 billion stems without converting some cropland to “CRP-like” areas that would support milkweed. However, it was possible to reach the goal by converting all marginal cropland in corn and soy agriculture (50,329 km²) to this CRP-like land use. Fifteen other successful scenarios involved converting half of the marginal cropland and also including contributions from three or more additional sectors. Landis (Reference Landis2017) agreed with the conclusion that changes in the agricultural sector will be crucial to monarch conservation, but noted that it might be difficult to encourage conversion of marginal croplands. Semmens and Ancona (Reference Semmens and Ancona2019) suggested that retiring cropland adjacent to rivers and streams could contribute to milkweed restoration in addition to providing other ecological benefits. Farmers may also want to consider allowing low densities of milkweed in their cropland to remain, because they provide habitat for beneficial insects in addition to the monarch butterfly. Common milkweeds often harbor aphids that attract parasitic wasps that control populations of the European corn borer (Ostrinia nubilalis Hübner) (DiTommaso et al. Reference DiTommaso, Averill, Hoffmann, Fuchsberg and Losey2016).