Monarchs Butterflies and Roadsides
How does roadside vegetation support monarchs?
Milkweed on roadsides is readily used by adult monarchs who seek out milkweed stems and leaves to lay their eggs on and nectar on milkweed flowers. Monarch larvae, or caterpillars, eat the leaves of many different species of milkweeds that grow in roadside areas. Roadsides can also provide diverse nectar sources which fuel adult flight, breeding, migration, and overwintering. Furthermore, roadside habitats may serve as important habitat corridors, can provide millions of acres of monarch habitat nationwide, and can be especially important in areas devoid of natural habitat (e.g., agriculture and cities). Sources: Kasten et al., 2016; Pitman et al., 2018; Cariveau et al., 2019; Kaul & Wilsey, 2019; Schact & Wu-Smart, 2019; Grant et al., 2020
Which types of roadside vegetation support monarchs?
Adult monarchs feed on nectar from a variety of blooming plants, including wildflowers and shrubs, while caterpillars feed on milkweed plants. Roadsides with diverse, flowering vegetation provide habitat for monarchs during their breeding and migration periods. For example, fall-blooming flowers can be especially important to migrating monarchs, which need large quantities of nectar to generate the fat reserves that enable them to complete their long-distance migration to overwintering grounds and survive winter. Sources: Cariveau et al., 2019; Western Monarch Milkweed Mapper, 2019; Schact & Wu-Smart, 2019
Do monarchs reproduce on roadsides?
Yes. Monarchs use milkweed on roadsides for reproduction—eggs, caterpillars, chrysalises, and adults have all been observed utilizing roadside habitat. Studies and monitoring efforts in the US Midwest, Southern Plains, West, and other areas have documented monarchs’ use of roadsides for reproduction. Sources: Mueller & Baum, 2014; Kasten et al., 2016; Pitman et al., 2018; Western Monarch Milkweed Mapper, 2019; Cariveau et al., 2019; Grant et al., 2020
Are there tools available for roadside managers to monitor milkweeds and/or monarchs on roadsides?
Yes, a rapid field assessment for milkweeds, nectar plants, and monarchs can be found on the Roadside Habitat for Monarchs website. Another option is the Rights-of-Way as Habitat Working Group’s Pollinator Habitat Scorecard. The Scorecard can be used to monitor pollinators (including monarchs) and vegetation on transportation and energy lands. Lastly, the National Integrated Monarch Monitoring Program compares habitat and use by monarchs across different land-use types, including roadsides. This program is more quantitative and may be more rigorous when comparing different habitat types or regions across time.
Does roadside vegetation that supports monarch butterflies also support pollinators and other beneficial insects?
Yes! Roadside vegetation that supports monarchs also supports a wide range of insects, including butterflies, bees, flies, wasps, beetles, and more. Roadsides with diverse, flowering vegetation can serve as foraging habitat, provide a place to breed, nest, or overwinter, and help pollinators and other insects to move through landscapes by linking fragmented habitats. Sources: Dirig and Cryan, 1991; Munguira and Thomas, 1992; Ries et al., 2001; Saarinen et al., 2005; Hopwood, 2008; Schaffers et al., 2012
Does monarch-friendly roadside vegetation provide any advantages to adjacent landowners?
Yes, habitat on roadsides can help maintain healthy ecosystems and provide ecological services, such as crop pollination or crop pest suppression. Habitat increases the diversity and stability of the pollinator community, and when on or near farms, habitat can improve pollination and increase crop yields. Roadside habitat also supports the beneficial insects that are predators of crop pests and contribute to natural pest control. In addition, roadside habitat can provide other ecological benefits that can translate to advantages to adjacent landowners, including reducing soil loss and water runoff, increasing water filtration and carbon sequestration, and supporting grassland birds. Sources: Mader et al., 2014; Morandin and Winston, 2006; Morandin and Kremen, 2013; Varchola and Dunn, 1999; Losey and Vaughan, 2003; Harrison, 2014
Threats to Monarchs Associated with Roads
Are monarchs killed by collisions with vehicles?
Yes, like many other animals and pollinators, monarch butterflies are killed by vehicles on roads. Existing research suggests that the greatest risk for vehicle-related monarch mortality occurs during the fall migration. For instance, researchers estimate that up to 20,000,000 monarchs were killed in Illinois during the 1998 fall migration. Another study estimates that 1-3 million monarchs are killed during the fall migration each year in the southern migration corridor (Oklahoma to Mexico), as the population’s migration concentrates closer to the overwintering sites. A study in northeastern Mexico estimated 196,500 individuals killed at two highway crossing spots during October 15-November 11, 2018 (approximately 2 million per year). Despite this mortality, roadsides serve as quality breeding and foraging habitat for monarchs and in many locations, it is likely that there are more monarchs produced on roadsides than killed by vehicles (though further studies are needed on this topic). Roadside habitats not only have the potential to provide millions of acres of habitat nationwide but also play a role in connecting existing pieces of high-quality habitat. Additional surveys will improve our understanding of the variation in monarch road mortality within and among seasons, and to identify roadkill hotspots. Sources: McKenna et al., 2001; Munoz et al., 2015; Kantola et al., 2019; Alvarez et al., 2019; Phillips et al., 2020; Campioni et al., 2022
If roadsides have higher quality monarch habitat, will that increase collisions of monarchs with vehicles?
No studies have examined this with monarch butterflies specifically, but research involving other butterfly species suggests that more diverse roadside habitat, and roadsides with less frequent mowing are associated with reduced butterfly mortality, perhaps because butterflies are better able to find resources within the roadside habitat and are less pressured to cross the road in search of additional habitat. This research suggests that, rather than luring butterflies to areas where they are killed by vehicles more frequently, roadsides with high quality habitat reduces butterfly mortality compared with grassy, low diversity roadsides. Sources: Munguira and Thomas, 1992; Ries et al., 2001; Skórka et al., 2013; Halbritter et al., 2015; Phillips et al., 2020
Do vehicle collisions with monarch butterflies increase during migration, and is there any way to prevent roadkill?
Data are limited, but it appears that more monarchs are killed due to vehicle collisions during fall migration compared to other parts of their migratory cycle. In Illinois, monarch mortality due to vehicles peaked during fall migration. In Texas, when monarchs funnel through the state on their way to overwintering grounds in Mexico, researchers found hotspots of mortality due to vehicle collisions. Roadkill hotspots occurred in less densely populated areas and sites with a more arid climate. The researchers suggest that migrating monarchs may spend more time flying lower to the ground during the afternoon in desert areas to seek shelter from the heat and may need to search more for nectar sources. Studies in northeastern Mexico have identified similar mortality results at two highway crossings during fall migration. Potential mitigation strategies for reducing monarch roadkill on recurring hotspots in Texas and Mexico are under investigation but may include reduced vehicle speeds, deflection structures to raise the height of crossing monarchs, and manipulation of habitat to lower the potential for monarchs descending to roost near key crossing points. Sources: McKenna et al., 2001; Kantola et al., 2019; Alvarez et al., 2019
Does roadside runoff, including heavy metal deposition and road salt deposition, affect monarchs and milkweeds?
Studies have shown that roadsides can suffer from heavy metal accumulation from car wear-and-tear and residual leaded gasoline emissions. In northern states, sodium from road salt application can accumulate along roadsides, and exhaust emissions can elevate levels of nitrogen. While these chemicals can make their way into the leaves and nectar of plants growing next to the road, research suggests that the concentrations of plant toxins are typically not lethal to pollinators. Studies to date suggest that toxic levels of metals, sodium, and other roadside pollutants are most worrisome along very high traffic volume roads and just adjacent to the roadside. Thus, prioritizing habitat enhancement and restoration along low- or medium-traffic volume roads, and keeping a mowed buffer adjacent to the roadside, will likely avoid negative effects of roadside toxins on milkweed and monarchs. Sources: Snell-Rood et al., 2014; Mitchell et al., 2020; Phillips et al., 2020; Shephard et al., 2020; Phillips et al., 2021; Shephard et al., 2021
How does road noise affect monarch caterpillars?
Recent studies suggest that highway noise may negatively affect developing larvae. One study found that exposure to wasp buzzing decreased larval development time and pupal weight. The authors argue that because wasp wing noise is similar to road noise, the long-term fitness and survival of monarchs may be reduced in airports or roadside environments. A second study found that monarch larvae experienced significant increases in heart rate when exposed to simulated highway noise for a two-hour period. However, those that were exposed continuously for 7-12 days did not experience elevated heart rates at the end of larval development. Thus, the researchers suggest that larvae may become desensitized or accustomed to the noise with constant exposure. Sources: Davis et al. 2018; Lee et al. 2021
Milkweeds on Roadsides
What milkweeds are most common on roadsides?
The answer to this question depends on what part of the country you are in. There are over 70 species of milkweeds native to the United States, but none of them occur in every state. To help roadside managers and others recognize milkweeds in their regions, we have developed milkweed recognition guides for 16 regions of the lower 48 states, found here.
How do I know if there is milkweed on my roadsides? / How can roadside managers recognize milkweeds?
We have created recognition guides to help you to recognize the milkweed species that are most common on roadsides in your area, found here. Most milkweeds have milky sap, so if you see a plant that looks like one of those in the guide to your region and milky sap oozes from the plant after breaking a leaf or stem, this is an indicator that you might be looking at a milkweed (though there are a few other types of plants, such as dogbane and spurges, which also have milky sap). Most milkweeds also have distinctive star shaped flowers that cluster together at the top of the plants. Then, when flowering is done, most milkweeds produce seed pods that open to release brown seeds with white fluff attached (the fluff helps those seeds travel on the wind). There are a variety of factors (height, leaf shape, leaf arrangement, flower color, etc.) to help one distinguish among the different milkweed species—or even to recognize dried milkweed stems in the fall and winter. Additionally, some milkweed species are only found in certain soils. If you are seeking information beyond what is found in the recognition fact sheets, see Milkweeds: A Conservation Practitioner’s Guide for more about milkweeds. Sources: Borders and Lee-Mader 2014
Are milkweeds in roadsides likely to spread to adjacent land and become weeds?
Although milkweed, the common name for plants in the genus Asclepias, implies that the plants are indeed weeds, milkweeds are a diverse group of native wildflowers that are not listed as noxious weeds at either the state or the federal level in the United States. Milkweeds may have been perceived as weeds historically because a few species (out of the 70+ species in the U.S.) will readily colonize disturbed areas. These species tend to reproduce vegetatively (in addition to reproduction by seed), sending up new shoots from roots that spread outward from the parent plant. This clonal reproduction allows their populations to expand over time, and plants may spread out of their original area. Common milkweed (Asclepias syriaca) exhibits the highest degree of clonal reproduction, and vegetative growth also occurs to a lesser degree in horsetail milkweed (A. subverticillata), narrowleaf milkweed (A. fascicularis), plains milkweed (A. pumila), prairie milkweed (A. sullivantii), showy milkweed (A. speciosa), and whorled milkweed (A. verticillata). Despite the vegetative growth, many of these species are unlikely to create an ongoing and unmanageable weed problem for roadside managers (or adjacent landowners, other land managers, homeowners, etc.). Sources: Borders and Lee-Mader 2014
Are milkweeds in roadsides a concern for grazing animals on adjacent land?
Milkweed species present in roadsides are unlikely to be a threat to livestock on adjacent property. Very few milkweed species will spread from their planting site. If milkweeds are present in pastures or rangelands, most livestock take care to avoid them. Although milkweeds are toxic, livestock generally find them highly unpalatable. Poisoning events are rare—but not unheard of— possibly because livestock must consume a large amount of milkweed to become sick or die. An average cow weighing roughly 1,200 lbs would need to eat 12 lbs or more (1-2% of their body weight) of dried milkweed on average to die. Milkweed poisoning typically only occurs when livestock are confined to a barren paddock with no alternate food sources or when hungry animals are released into milkweed patches. However, there are two species, western whorled milkweed (A. subverticillata) and narrowleaf milkweed (A. fascicularis) which have been reported as especially problematic for cattle and sheep, likely because of their growth forms and thin stems and leaves which are easily tangled in grasses and thus difficult for grazing animals to separate out. It is also important to note that the palatability of milkweed increases when it is dry. If adjacent landowners are haying the roadside, it is best to avoid haying areas where concentrations of milkweed are high. Sources: Panter et al. 2011; Burrows and Tyrl 2007; DiTomaso and Healy 2007; Schultz 2003; Malcolm 1991; Kingsbury 1964; Fleming 1920
Are milkweeds in roadsides that are hayed by adjacent landowners a risk to livestock?
The palatability of milkweed increases when it is dry and so it is more likely that livestock will be sickened or even die from consuming dried milkweed in great enough quantities than fresh milkweed. If adjacent landowners are haying the roadside, it is best to avoid haying areas where concentrations of milkweed are high. However, large quantities of milkweed need to be ingested in order to cause harm. For example, an average cow weighing roughly 1,200 lbs would need to eat 12 lbs or more (1-2% of their body weight) of dried milkweed on average to die. The toxicity of milkweed varies by species and cardenolide concentrations (as well as local growing conditions); some species are generally of very low risk to livestock (e.g., butterfly milkweed, Asclepias tuberosa) while others are consistently quite high (e.g., woollypod milkweed, A. eriocarpa). Sources: Burrows and Tyrl 2007; DiTomaso and Healy 2007; Schultz 2003; Malcolm 1991; Kingsbury 1964; Fleming 1920
How do milkweeds support pollinators or beneficial insects?
Besides providing food for monarch caterpillars and adults, milkweeds support a wide range of pollinators and beneficial insect species. Milkweed flowers are a high-quality nectar source for pollinators such as bees, butterflies, wasps, beetles, flies, and more. Milkweeds also attract a wide range of insects that contribute to crop pest control, and some producers (e.g., vineyards in the Pacific Northwest) have begun to integrate milkweeds into their agricultural system to attract these important insects and support biological control. Milkweed leaves, stems, and roots support insect herbivores such as other lepidoptera (butterfly and moth) species, wasps, flies, beetles, true bugs, and more. Milkweed “silk” (the fibers attached to the seeds and help the seeds catch the wind) can be used for nesting materials by vertebrates such as birds and small mammals. Sources: Tilman and Carpenter 2014; Borders and Lee-Mader 2014; James et al. 2016
What times of the year are milkweeds most readily observed in roadsides?
In all of the lower 48 states, most milkweed species will be observable during the growing season. In all of the lower 48 states, most milkweed species will be observable during the growing season. The months during which you can most readily observe milkweeds depend somewhat on where you are. In much of the northern half of the country, the best months to see milkweeds on roadsides are from July to August. However, in Texas and southern Oklahoma, green antelopehorn and spider milkweed (Asclepias viridis and A. asperula) emerge in March and are extremely important in that they help sustain the first generation of monarchs produced in the U.S. each year. Interestingly, in those two states, it can be difficult to find milkweeds in midsummer, as some of the most abundant milkweed species go dormant then; milkweeds can become abundant again in late summer or fall when conditions are more favorable. In the Desert Southwest, there are some native milkweeds that remain green during the late fall and winter. The same is true in the southeastern coastal plain with regards to a wetland species, aquatic milkweed (A. perennis), which can be found along roadsides in winter in eastern TX (and presumably in other places along the Gulf Coast that have warm winters).
How can roadside managers share information about the occurrence of milkweeds and monarchs in roadsides to improve understanding of habitat conservation?
- Set up a survey of monarch habitat and monarchs in your road system using the Roadside Monarch Habitat Evaluator or the Pollinator Habitat Scorecard. Share findings at statewide and regional meetings, such as via the Rights-of-Way as Habitat Working Group.
- Designate a staff member as a point person for milkweeds and monarchs who can answer questions and be a liaison between roadside managers, administration, state monarch efforts, and conservation organizations. In your local, regional, or state jurisdiction, include milkweed and/or monarch sightings and location maps to the list of communications and reports that are shared among staff.
- Add milkweed and monarch sightings to community science portals such as Journey North (North America) and/or the Western Monarch Milkweed Mapper (western US). Monitoring data can also be uploaded to the Rights-of-Way as Habitat Geospatial Database, a tool used to store and track information about habitat restoration projects and conservation measures on rights-of-way lands.
Roadside Management for Monarchs
How can roadside vegetation inventories benefit monarchs?
A roadside vegetation inventory involves mapping the composition and condition of current roadside vegetation, including native, invasive, and noxious weeds. Such inventories can inform management plans that can benefit monarchs in a number of ways:
- Identification of remnant habitat can allow roadside managers to make informed decisions about how to manage remnant habitat to maintain and improve it, to help sensitive plant species survive and sustain habitat for monarchs and other wildlife.
- Roadside inventories can also be used to map out existing weed issues and identify emerging weed problems.Those data can then be used to help target management operations and to evaluate the effectiveness of weed management techniques.
- Inventories may be used to learn about the effects of management strategies across different management areas.
- Finally, inventories can help identify opportunities for future monarch-friendly revegetation efforts.
How can a roadside manager assess the value of roadside vegetation for monarchs?
There are multiple tools available for assessing monarch habitat in roadsides. These tools allow users to evaluate monarch or pollinator habitat quality, document the presence of monarch butterflies and/or pollinators, and understand outcomes of management practices. The Monarch Joint Venture’s Roadside Habitat Evaluator pairs a 15-minute rapid assessment survey with a habitat calculator to provide users with information about the quality of roadside habitat locations for monarch butterflies. The survey runs in ESRI’s Survey123 and can be customized according to information needs (a paper datasheet is also available).
The Rights-of-Way as Habitat Working Group’s Pollinator Habitat Scorecard is a similar tool, designed for the evaluation of pollinator habitat on a range of energy and transportation lands. Organizations may choose one of three tiers, from simple to advanced monitoring, and can record data on paper or in ESRI’s Survey123 (as well as on paper).
The Integrated Monarch Monitoring Program’s (IMMP) Milkweed & Blooming Plant Survey evaluates monarch habitat in a variety of landscape types, including rights-of-way. The IMMP’s Data Portal provides data summaries and visualization, such as the density of milkweed plants at a site, flowering plant composition, diversity, and frequency, and more.
How does roadside mowing impact monarch caterpillars and adults?
Mowing during the growing season affects monarchs by removing nectar sources and reducing milkweed availability, and can result in direct mortality of butterfly eggs, larvae, and sometimes adults. For these reasons, mowing can set back monarch breeding temporarily and can remove nectar sources needed during monarch migration. (In some regions, mowing can stimulate milkweed regrowth, and monarchs prefer to oviposit on mown milkweed when both mowed and unmowed plants are available; see “Could roadside mowing stimulate milkweed” question below.) In general, it is preferable to mow when monarchs are not present; however, there may be circumstances when mowing when monarchs are present is more beneficial to the long-term quality of habitat for monarchs. See the Monarch Joint Venture handout “Mowing and Management: Best Practices for Monarchs” for more information, including recommended management windows. Sources: Morris 2000; Johst et al. 2006; Noordijk et al. 2009; Kayser 2014, Thomas 1984; Wynhoff 1998; Humbert et al. 2010; Kayser 2014; Phillips et al. 2020
When should roadsides be mown to reduce impacts to monarchs?
It is best to mow when monarchs are not present (see management window map below). Based on the best available data for when and where monarchs breed, Monarch Joint Venture and Xerces Society have developed regionally-appropriate monarch breeding habitat management windows. These windows are periods when management activities are least likely to have negative effects on monarchs—especially immature monarchs.
The exact timing of monarch breeding may vary from year to year and site to site (but try consulting a website such as Journey North or Western Monarch Milkweed Mapper to see when monarchs are reported in your area)—and these windows may be revised in the future as we learn more. This is especially true for areas where few data are currently available on the timing of monarch breeding, such as the states that straddle the continental divide. Also, if milkweed is present on the landscape during the breeding season, there is a chance that monarchs are also there and that management actions could result in monarch mortality. As every year and site are slightly different, it is useful to survey milkweed plants for immature stages of monarchs prior to mowing. This is time consuming but is especially helpful if the management timing falls on the cusp of the recommended window for your region or if it has been an early spring/late fall year (see Figure 43).
Could roadside mowing stimulate milkweed growth and support monarch breeding?
Research in eastern North America has shown that spring or summer mowing can promote new growth and extend the availability of milkweed plants for monarch breeding. Mowing may stimulate growth of some milkweed species, particularly those that spread through rhizomes like common milkweed (Asclepias syriaca) and showy milkweed (A. speciosa). Summer (June or July) mowing in Michigan and southern Ontario resulted in more monarch eggs on regenerated stems than unmowed stems, and higher first instar survival. Summer (July) mowing and burning can increase green antelopehorn milkweed (A. viridis) availability in the late summer and early fall in the Southern Great Plains, whereas in areas without mowing, the milkweed has senesced by August. In the West, showy milkweed will regrow after summer mowing and continue to support monarch breeding (Stephanie McKnight, personal observation). However, more research is needed in other areas to determine the optimal timing and frequency of mowing that promotes not only milkweed but also nectar plants. It is also unknown if the benefit of additional milkweed availability in the fall outweighs the costs of the larval mortality caused by summer mowing. The benefits are likely greater in areas that primarily have breeding monarchs in the spring and fall and where the dominant species of milkweed spread by rhizomes. Sources: Alcock et al. 2016; Baum and Mueller, 2015; Bhowick 1994; Haan and Landis 2019; Fischer et al. 2014; Haan and Landis 2019; Knight et al. 2019; Haan and Landis 2020
Figure 43. Map depicting best timing for management actions that may affect monarch breeding habitat (from Mowing & Management: Best Practices for Monarchs handout).