Common Milkweed (Asclepias syriaca) is broadly distributed in eastern North America. Though the species is common in much of its range, the widespread application of glyphosate-based herbicides has resulted in widespread population declines. The species is declining in number in agricultural settings, but appears to remain relatively stable or increase in abundance in other contexts. Though this has resulted in significant past population declines, these declines may be partially offset be expansions in the species’ range at its southern and northeastern margins. The species is an important host plant for several insect species and is also used for horticultural, culinary, and medicinal purposes. Though not specifically protected in much of its range, the species incidentally occurs in many protected areas. Despite historic population declines, which may result in regional extirpations and which may come close to qualifying for threatened status, the species is very widespread and common. It is very unlikely to be driven to extinction and is therefore listed as Least Concern. However, as the species is an important host for numerous species of insects, additional monitoring is needed to better determine the magnitude of population declines and their potential impact on dependent species.

Common Milkweed (Asclepias syriaca) occurs in large sections of eastern North America ranging from Quebec to Saskatchewan (VASCAN 2020) and south to Georgia, Louisiana, and Oklahoma (Kartesz 2015). It has been widely introduced outside its native range including in Prince Edward Island and Newfoundland (NatureServe 2020, VASCAN 2020) as well as in Europe and Central Asia (Board of Trustees, RBG Kew 2020). Its range also appears to be expanding southward (Weakley 2015). It occurs at elevations ranging from 0 to 500 metres and occasionally up to 900 metres (GBIF 2020).

The species is very common throughout much of its range (Bhowmik and Bandeen 1976, Weakley 2005). As an agricultural weed, the species was formerly common in corn and soybean fields within its range. Modelling suggests the species’ population expanded through the early 20^th century as a result of farm consolidation which resulted in greater availability of suitable habitat. Reductions in the rate of consolidations, increasing application of herbicide, and changes to farming practices resulted in declines after the 1970s (Boyle et al. 2019).

The introduction of glyphosate resistant crops has resulted in sharp population declines in significant portions of the species’ range. Introduction of glyphosate resistant crops has resulted in an approximately 90% decline in occupied area within surveyed corn and soybean fields (Hartzler 2010). Abundance along roadsides during the same period of observation showed modest increases, but due to widespread application of herbicides in these areas, gains are likely to be temporally or geographically discontinuous.

Adoption of herbicide tolerant soybeans, corn, and canola within the species’ distribution was most rapid during the period studied (approximately from 1999 to 2009) (Hellerstein et al. 2019). Since this time, planted acreage of herbicide tolerant soybeans has stabilized. Adoption of herbicide tolerant corn lagged slightly behind and began to plateau around 2014. Glyphosate resistant sugar beets were not introduced until 2008 (Morishita 2018), and had a rapid increase in adoption. By 2009, over 90% of sugar beet acreage in the United States was planted with glyphosate resistant sugar beets (Hellerstein et al. 2019). Adoption of glyphosate resistant sugar beets is most likely to impact subpopulations in central Michigan, northwestern Ohio, western Minnesota, eastern North Dakota, and isolated portions of Ontario.

Though the acreage under herbicide tolerant crops has stabilized, it has done so at rates of 90% or more of planted acres for canola, soybeans, and sugar beets. Further declines in Common Milkweed population are not likely to occur as a result of increasing adoption, but the population is unlikely to recover unless herbicide tolerance develops in Common Milkweed. Recent development and commercialization of 2,4-D resistant crops is unlikely to result in further severe reductions in population as Common Milkweed is slightly less susceptible to 2,4-D than it is to glyphosate (Wyrill and Burnside 1976). However, the introduction of 2,4-D and dicamba resistant crops (commercialized between 2016 and 2018) (USDA, National Agricultural Statistics Service 2018) is likely to slow the development of herbicide resistance within the species resulting in a prolonged period of depleted population.

Widespread population declines attributable to the adoption of herbicide resistant crops have been documented within the state of Iowa, where land use patterns are quite similar to those observed in much of the Midwest. Weighted assessments of population across different land use types suggests overall population declines of 58% between 1999 and 2010 (Pleasants and Oberhauser 2013). The bulk of this decline is driven by losses within agricultural fields. 96% density declines have been observed in corn and soybean fields. A broader view suggests a 40% population reduction across the entirety of the Midwest driven by herbicide application, and land conversion for agricultural purposes (Pleasants 2017).

Though widespread population declines have been documented, there is considerable uncertainty as to the magnitude of these declines relative to historic population levels, and the expansion of the species northward and southward into disturbed habitats. Further, documented population declines reflect changes in portions of the species' range in specific contexts. In other portions of the species' range (e.g. those not intensively farmed using herbicide resistant crops) and in other contexts (e.g. outside of farm fields in preferred habitat along roadsides), the population may be stable or even increasing. It is also apparent that the majority of observed decline in the species occurred during the introduction of glyphosate resistant crops (roughly between 1999 and 2009). It is reasonable to conclude that the rate of decline is no longer as high as it was during the period of rapid glyphosate resistant crop adoption. At present, it is not possible to precisely determine the rate of population decline for the species over the relevant three generation (12 year) window required for listing under a threatened category following criterion A. At its peak, the population declines observed for the species weighted across different habitats reached perhaps 40% over an 11 year window. Following suspected sharp reductions in the rate of population decline since this time, it is unlikely that the current rate of reduction meets (or nearly meets) any necessary thresholds for listing under a threatened category following criterion A. Given the species' wide range, very large population, and presence in a variety of disturbed habitats which are unlikely to be subjected to further reductions as a result of herbicide application, an evidentiary approach to the available data suggests a listing of Least Concern is most appropriate for the species.

Common Milkweed is a perennial forb which reaches a height of 120 cm. It occurs in dry and mesic grasslands and disturbed environments including roadsides, agricultural fields, tallgrass prairies, and fallow fields. It may be limited by the absence of boron in some soils (Bhowmik and Bandeen 1976). It colonizes prairie and savanna habitats through rhizomes or seed germination following fire events (Mojzes and Kalapos 2015).

Common Milkweed differs from many other milkweed species in its tendency to form large clonal colonies via underground rhizomes. Seed output for the species is very high (Wilbur 1976) and persistent soil seedbanks may buffer against short-term population declines (Yenish et al. 1996). Anecdotal accounts of gardeners suggest the species first flowers in its first or more commonly second year of growth, and has a lifespan of around five years. A generation length of three to four years can be inferred.

Common Milkweed is an important food source for a number of insects, most notably the Monarch Butterfly (Danaus plexippus), but also Pyropyga minuta (Faust 2017), Liriomyza asclepiadis, Aphis gossypii, Aphis nerii, Aphis rumicis, Myzus persciae, Macrosiphum redbeckiae, Frankliniellia tritici, Lygaeus bicrusis, Brachyacantha ursina, and many others (Betz et al. 1997).

The primary threat to the species is adoption of herbicide tolerant soybeans, corn, and canola within the species’ distribution (Hellerstein et al. 2019).

Common Milkweed is widely cultivated in horticultural settings for its benefit to wildlife. Though widely known for its toxic cardenolides, portions of the plant, including the flower buds, flowers, young shoots, seed pods, and seeds are edible in certain contexts (Plants for a Future 2020). The species has also been used for a wide variety of medicinal purposes and as a source of fibre (Moerman 2020). The species has been investigated as a source of latex, though commercial production for this purpose is currently limited.

The species incidentally occurs in several protected areas (UNEP-WCMC and IUCN 2020). The species is not specifically protected within its range. Instead it is widely regarded as a weed and is targeted for eradication (USDA, NRCS 2020). Though the species itself is unlikely to face extinction despite recent, precipitous population declines, additional monitoring is needed to better assess potential declines in habitat suitability for Monarch Butterflies, and other species dependent on milkweeds. Such studies should be focused on both agricultural and non-agricultural settings. Population monitoring is also needed on a broader geographic scale. While excellent population data is available for the state of Iowa, far less is known about population trends in the northeastern and northwestern portions of the species’ range where land use differs considerably. Public awareness campaigns, and efforts to encourage the planting of several species of milkweed are ongoing and have been undertaken by a number of authorities (Borders and Lee-Mäder 2014).