Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies (2020)

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47 Background The Roadside Monarch Habitat Calculator uses data from the Rapid Assessment of Road- side Habitat for Monarchs to generate a Monarch Habitat Quality Score for each assessed ROW site. Managers may wish to use monarch habitat quality scores to compare sites under different management schemes (e.g., reduced mowing, weed control), in different bioregions (e.g., prairie, woodland) or for different road types. The Habitat Calculator groups the data measures under four functional components of habitat: breeding, adult foraging, landscape context and threats, and management (see Table 6). Methods The Habitat Calculator is based on the Environmental Defense Fund’s Monarch Habitat Quantification Tool (HQT) (Environmental Defense Fund et al. 2017, Anderson et al. 2017). However, in interviews with transportation managers and background research, the team identi- fied a number of important factors in roadside ROWs that were not incorporated in the HQT. In particular, the HQT does not include measures related to road characteristics, threats by invasive non-native plant species (weeds), or mowing and herbicide application practices that are commonly applied in the roadside corridor. Therefore, the researchers designed a data col- lection protocol, the RA, that would provide data for all of these categories (see Table 6), and then designed the Monarch Habitat Calculator to incorporate these data inputs. In designing the RA and Habitat Calculator, the team reviewed other pollinator rating systems in collaboration with the Rights-of-Way as Habitat Working Group (Energy Resources Center, University of Illinois-Chicago; http://www.erc.uic.edu/biofuels-bioenergy/pollinator-habitat/ rights-of-way-as-habitat/), Metrics and Targets Taskforce (A. Cariveau, co-chair; see listing in Table 1, Chapter 2). Function and measure weights were determined by the project work team through meetings and discussions regarding the relative importance of various habitat components. For each measure represented in the Habitat Calculator, the researchers developed point distributions (0–100 for each measure) to correspond with various levels in the scores. The development of scoring for each functional area is explained in the following sections. Each of the functional components is weighted within the full score (function weight), and measures are then weighted within each functional component (weight within function). Multi- plying the function weight by the weight within function determines the weight of the measure in the total score (measure weight). For example, within the functional component of breeding C H A P T E R 5 Product C: Roadside Monarch Habitat Calculator

48 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies (function weight 30%), the measure of milkweed density (weight within function 80%) repre- sents 24% (30% × 80%) of the overall quality score (when scored maximally, generates 24 points for the quality score). Monarch Breeding Habitat Milkweed Abundance Monarchs require milkweed plants (primarily in the genus Asclepias; 76 species in the United States) for their larvae to develop. Sites with more milkweed plants provide more habitat for reproduction and therefore receive a higher score. Smaller sites with fewer milkweed plants may support higher densities of eggs per plant (Stenoien et al. 2015). Nail et al. (2015) found that monarch egg survival was higher in sites that had more milkweed plants, but that there was a negative effect of per-plant larvae density. Also, it is thought that higher rates of predation and parasitism are encountered in higher density patches of milkweed (Zalucki and Kitching 1982, Bartel et al. 2011, Stenoien et al. 2016, Pitman et al. 2018). Based on modelling that takes into account behaviors of monarchs looking to lay eggs, it appears that lower-density milkweed spread out across the landscape will benefit monarch reproduction more than fewer higher- density patches of milkweed (Grant et al. 2018). Taking all of these studies together, one would expect an increase in breeding habitat quality with an increase in milkweed plant densities, but only to a point, after which the effect declines or plateaus. There are a few studies explicitly relating monarch reproduction to the density of milkweed (milkweed plants or stems per unit area). Studies such as the Monarch Larva Monitoring Project have data regarding monarchs per milkweed plant, but only recently began collect- ing data on site level milkweed density. A study by Kasten et al. (2016) in Minnesota, South Dakota, Iowa, Wisconsin, and Illinois found that milkweed density was the strongest predictor of density of monarch eggs and larvae, with an increase in immature monarchs leveling off above 0.6 milkweeds per square meter (2,428 plants or stems/acre). However, it should be noted that milkweed abundance was rarely observed at this level, with the median count of Functional Component Measure Function Weight Weight within Function Measure Weight Breeding Milkweed Abundance (density; plants/acre) 30% 80% 24% Milkweed # Species 20% 6% Adult Foraging Potential Nectar Plants: % Cover 25% 50% 12.5% Potential Nectar Plants: # Species 30% 7.5% Potential Nectar Plants: # Native Species 20% 5% Landscape Context /Threats Adjacent Land-Use Type 25% 30% 7.5% Road Type 35% 8.75% Weeds: % Cover 20% 5% Weeds: # Species 15% 3.75% Management Herbicide Application Practices 20% 40% 8% Frequency of Full Width Mow 30% 6% Width of Mow: Proportion of ROW Width 30% 6% Total 100% 100% 100% Table 6. Roadside Monarch Habitat Calculator functional components, measures, and weights.

Product C: Roadside Monarch Habitat Calculator 49 0.0036 plants/m2 (14 plants/acre) and mean of 0.0508 plants/m2 (206 plants/acre). Thogmartin et al. (2017b), based on their review of the literature, suggested that mean milkweed density for primary (larger) roadside ROWs is 57.15 plants/acre, with an amended (when habitat is enhanced) density of 100.02 plants/acre. Points assigned for milkweed densities followed Kasten et al. 2016 and the Milkweed Density Suitability Index of the HQT (Anderson et al. 2017). In the HQT, 2,000 stems/acre was determined to be the highest desirable level and the curve was generated with this equation: 1 – (2/(1 + EXP(6 × (I2/2,000)) (see Figure 9 in Chapter 3). In the RA, surveyors either count the number of milkweed plants within the survey area or use abundance categories of 0, 1–5, 6–10, 11–25, 26–50, 51–250, >250. The Habitat Calcu- lator then divided the number of plants (or used the midpoint of the abundance category; 300 for the highest category) by the size of the survey area to generate milkweed density in plants/acre. Next, points are assigned according to the midpoint of these milkweed densities (0, 1–50, 51–100, 101–200, 201–500, 501–1,000, 1,001–2,000, >2,000) as in Figure 24. Milkweed Species Composition The number of milkweed species present at a site is a factor that can enhance habitat quality. Having more than one milkweed species available at a site may increase the period of the year when milkweed is available to monarchs for oviposition and for nectar, as milkweed species found together often are offset in the timing of their peak blooms (personal communication, Bill Handel, retired, Illinois Natural History Survey). Also, in trials, monarchs lay more eggs when they encounter a mix of milkweed species rather than the same number of plants of one species on which to lay their eggs (Pocius et al. 2018), although they reproduce successfully on all of the milkweed species tested (Pocius et al. 2017). Therefore, for providing blooms throughout a longer period and for providing more options to ovipositing monarchs, sites with more than one species of milkweed are ranked higher in quality. Points were assigned such that surveys with one milkweed species received 2 points, two milkweed species received 4 points, and three or more milkweed species received the maximum of 6 points. Adult Monarch Foraging Habitat Nectar Plant Abundance Adult monarchs require nectar from flowering forbs and shrubs for feeding during their reproductive and migratory phases. It is widely recognized that nectar sources are important and may be limiting for monarchs, with particular concern about availability of nectar in early 0 10 20 30 40 50 60 70 80 90 100 0 500 1000 1500 2000 2500 Pe rc en ta ge p oi nt s Milkweed plants/acre Figure 24. Milkweed abundance point curve based on values calculated from data from the RA.

50 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies spring and during the fall migration period (see Malcolm 2018). In addition, a variety of other pollinators (including wild bees), many of which are only in flight for a short time of the year, rely on floral resources (nectar and pollen). Best management practices for providing nectar resources for monarchs and other pollinators typically recommend providing blooming native plants during each season of spring, summer, and fall, to ensure availability of floral resources when needed by these various species and life stages. Roadside vegetation managers are typically limited to visiting sites once per monarch season (i.e., once a year in the northern part of the breeding range, twice a year in the southern parts of the breeding range). Therefore, the research team defined a term that might characterize the potential for a site to provide flowers—potentially blooming nectar plants (hereafter, nectar plants)—as species of forbs (e.g., herbaceous plants that are not grasses) and blooming shrubs that may provide nectar resources, whether or not they are blooming at the time of the survey. For these plants their aerial cover was estimated on the site in the categories of none, <5%, 6–9%, 10–25%, 26–50%, 51–75%, and >75%. These cover classes were derived from classic Daubenmire cover classes (<5%, 6–25%, 26–50%, 51–75%, 76–95%, and >95%; Daubenmire 1959) with the following modifications. A zero category was added for places where no nectar plants were found; the highest two categories were consolidated into one >75% category for simplification and because the team did not encounter any instances of >95% in the field in 2017–2018. Also, a category from 6–9% was added to better distinguish among sites and to correspond with a minimum threshold value of 10% cover by nectar plants included in the draft CCAA for the monarch as developed by the Rights-of-Way as Habitat Working Group of the Energy Resources Center at the University of Illinois-Chicago. The researchers considered assigning points according to the midpoints of the cover class values (Figure 25). However, because these values are small for the lower cover classes and because this factor is only a small weight among other factors (12.5% of total score), we found that this did not distinguish well among sites with different cover classes of nectar plants. To better reflect the scoring levels for this component, points were assigned with greater spread among the lower cover classes such that each increase among cover classes added at least 1 point when considering the final point contributions (Figure 26). Also, based on expert opinion, the team chose to score the highest two cover categories the same, as forbs are not expected to grow in such high relative abundance in grassland areas and the team of experts felt that anything above 50% should be given full points. 0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 100 Pe rc en ta ge P oi nt s Percent Cover (in Classes) Figure 25. Rejected point curve where points were given in proportion to midpoint of cover class for nectar plants.

Product C: Roadside Monarch Habitat Calculator 51 Nectar Plants: Number of Species Having a variety of blooming species with different blooming periods such that a site pro- vides nectar through the season when monarchs may be present is optimal. This follows the recommendations set forth in many management guidelines for pollinator habitat. Thus, greater points are awarded to sites where there are greater numbers of species of nectar plants (Fig- ure 27). Surveyors recording the number of distinct types of nectar plants they see create a rela- tive index across all sites surveyed in a similar fashion with regard to blooming plant richness. Nectar Plants: Number of Native Species A roadside manager may prefer to grow plant species native to their area because of the benefits they have in providing habitat for other species, such as wild bees or butterflies that may rely on particular species as a host plant or for nectar or pollen during limited flight times. Many native plants are perennials with strong root systems effective at soil stabilization, and 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Pe rc en ta ge P oi nt s Percent Cover Class Figure 26. Adopted set of percentage points assigned to various cover class midpoints for nectar plants, providing better distinctions among sites with different cover classes in the lower part of the range. 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 40 Pe rc en ta ge P oi nt s Number of Nectar Plant Species Counted Figure 27. Points awarded for various numbers of nectar species.

52 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies in the right combinations, these native plantings can be resilient against invasion by weed spe- cies and require less maintenance than traditional plantings of ornamental flowers or turfgrass. As a matter of background, only native plants are counted in EDF’s HQT nectar plant scoring system; a site with only non-native species would receive a zero score for nectar sources. Experts agree that there are great benefits provided by native species; however, they acknowledge that non-native flowers also may provide nectaring resources. Therefore, the research team decided to make points for native species additive to those determined by the full count of nectar species. In this way, the team acknowledged that some non-native plants can provide nectaring habitat, while also giving additional credit to sites where native plants are provided. Figure 28 depicts the scoring curve developed for number of native nectar species. The team based the levels from the numbers of plants reported in the statewide field-testing in Minnesota and Oklahoma. Context/Threats In the Context/Threats component, three factors were considered: adjacent land use, road characteristics, and presence of noxious weeds. Adjacent Land Use Adjacent land use may affect the suitability of monarch habitat in two primary ways noted below. The researchers noted adjacent land uses in the field when conducting the RA. While this could have been brought in from the Landscape Prioritization Model, the researchers found that the scale of the national datasets was often inaccurate at the local scale, so the on-the- ground notation of adjacent land use was more accurate. First, adjacent land use may bring the risk of pesticide (herbicide, fungicide, or insecticide) drift from agricultural or developed areas. The extent of drift is highly variable according to application practices and weather conditions during application (primarily wind speed and direction). The HQT and USDA Natural Resources Conservation Services (NRCS) recommend a monarch habitat buffer distance of at least 100 feet from the edge of fields where insecticides are applied or for there to be a woody barrier (such as a hedgerow) in place. Xerces Society suggests a buffer distance of 60 ft for aerial insecticidal spray applications, or 125 ft for neo- nicotinoid applications, including treated seeds (Code et al. 2016), or for there to be a barrier, such as woody vegetation. It was assumed that the risk or likelihood of pesticide exposure is 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 Pe rc en ta ge P oi nt s Number of Native Nectar Plant Species Counted Figure 28. Points awarded for numbers of native nectar plant species counted in each RA.

Product C: Roadside Monarch Habitat Calculator 53 highest near agricultural fields and developed areas with no buffer, and therefore these loca- tions receive the lowest ranking in the Habitat Calculator. Sites bordered by woody barriers and neighboring agricultural and developed areas are ranked higher, and the highest ranking is attributed to those that neighbor wooded or grassland areas (Table 7). Secondly, adjacent land use may affect the suitability of the habitat by attracting monarchs to the area. Areas that are adjacent to natural vegetation containing nectar resources, such as nature reserves, are predicted to have the highest suitability, followed by less diverse grass- lands and developed areas. Woodlands and agricultural fields are thought to have the lowest value as adjacent habitat. In one study, butterfly abundance was lowest in roadsides adjacent to woodlands and greatest near grasslands (Kasten et al. 2016). In another study, insect mortality was lower when adjacent to woodlands and increased along roadways with vegetated medians (Keilsohn et al. 2018). Roads as Sources of Chemical Inputs and Vehicular Collisions Roads pose two main threats to monarchs: an influx of chemicals from vehicles and road main- tenance (e.g., application of road salts) into the plant material consumed by larval monarchs, and adult butterfly collisions with vehicles. As a proxy for speed limit and traffic volume for each road, the researchers categorized roads into two lane, four lane, and greater than four lane. Then, low, higher, and highest in terms of both speed limit and traffic volume were ascribed to these road types. Roads bring an influx of chemicals from vehicle wear-and-tear, exhaust emissions, and from the application of road salts (e.g., Lagerwerff and Specht 1970, Jaradat and Momani 1999). These chemicals can be consumed by monarchs when they are taken up by milkweeds (heavy metals, sodium) or deposited on the plant surface (residues from exhaust). High levels of sodium, metals such as zinc and lead, or exhaust emissions can have lethal or sub-lethal effects on road- side monarchs. For this study, it was assumed that larger roads with more traffic lanes bring greater chemical inputs. Therefore, relative to chemical inputs, the researchers gave a higher habitat quality rank to roads with two lanes, followed by four lanes, followed by roads with greater than four lanes. Preliminary results from the Snell-Rood Lab support the idea that sodium and zinc content of roadside milkweeds scales with traffic intensity (Mitchell et al. 2020 in revision). Risk of collision with moving vehicles is a threat that has been studied for butterflies in general as well as recently, specifically with regard to monarchs. Limited research on traffic volume indicates that butterfly mortality increases with traffic volume to a point, but there Adjacent Land-use Category Points Cropland 10 Cropland with hedgerow or other woody barrier 50 Developed 25 Developed with hedgerow or other woody barrier 50 Woodland or forest 75 Diverse grassland (contains native species including forbs) 100 Not diverse grassland/open space/idle ground 50 Wetland 75 Table 7. Adjacent land-use categories and associated point scores.

54 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies appears to be an avoidance effect on roadways with the highest traffic volume. Therefore, rela- tive to potential vehicular collisions, smaller, slower roads are best, but moderate traffic levels would rank worse than the highest traffic volumes. The team coded two-lane roads as best (100 points) and ranked moderate and high-speed and volume roads the same at 50 points due to the conflicting predictions for the two factors above. Noxious Weeds Invasive plants, non-native species, or noxious weeds have not been documented as having a direct effect on the quality of monarch habitat, but, due to their propensity for dominating areas and becoming monocultures, it is known that the potential exists for them to displace a diversity of native nectar plants and milkweeds. The threat of noxious weeds is viewed in two ways. First, noxious weeds may directly replace native vegetation that is more beneficial to monarchs, particularly over the long term. This would be true for an area that is invaded by a species that does not supply nectar, such as any number of invasive grasses. However, some invasive species are utilized as nectar sources by monarchs. Although this would seem to benefit adult monarchs, the tendency to grow as a monoculture results in a highly synchronized bloom time, reducing the ability for the site to provide nectar throughout the season of monarch activity and potentially reducing the availability of milkweed. The second way in which noxious weeds could threaten monarch habitat is through manage- ment actions that are performed to reduce weed infestation, such as repeated mowing and/or treatment with herbicide. Because the goal of this management is to improve habitat, those negative effects are short term for one to several growing seasons while the weeds are actively managed. In many cases the habitat quality will be greatly improved after treatment, particularly when enhanced by seeding or other restoration measures. To rank weeds in the model, the research team faced a difficulty in that managers in different locations, particularly in different states, had distinct lists of weeds they were trying to control. Some DOTs aggressively track all species on their noxious weed lists, while in other states this is not practical. Because the effect of weeds is driven largely by the management actions needed to control them, the team decided to have the list of weeds be self-defined by each management entity. Therefore, the team enabled managers when setting up the RA to specify the species of plants that they are concerned about and are actively working to control. Then these are noted if present or not during RAs. The measures included in the Habitat Calculator are percent cover estimated across the survey area and the number of species present (Table 8). Metric Level Points Weeds – Percent Cover 0% 100 1–5% 80 6–10% 60 11–25% 40 26–50% 20 51–75% 20 >75% 0 Weeds – Number of Species *0 100 *1 75 *2 50 *3–4 25 5+ 0 Table 8. Weed metric levels and points.

Product C: Roadside Monarch Habitat Calculator 55 Management Vegetation management in roadsides is essential to operations for safety, aesthetics, and steward ship. The research team wanted to include management within the Habitat Calculator to illustrate how changes in management can be expected to affect habitat quality for monarchs. The team included two common management actions that may have great impacts on monarch butterfly habitat: herbicide use and mowing. Herbicide Application Herbicide is applied in many ROW contexts, primarily for the control of invasive species, which may be grasses, forbs, or woody species. A variety of herbicide types are used and are applied in different ways for different target vegetation, different management objectives, at dif- ferent times of year, differently by bioregion, etc. These sources of variation make it difficult to score in a national protocol. However, it is an important factor, so the team created a descriptive scale that can be applied in many situations (Table 9) and weighted it fairly low (8% of total). Also, a best management practices support document has been developed, Monarch Butterflies, Weeds, and Herbicides (see Appendix E). Mowing Mowing is a necessary tool in ROW management for providing safe sightlines along road- ways and also for managing undesirable woody or weedy vegetation. Mowing can provide both negative and positive effects on monarch habitat depending on timing, condition of the habitat, and time scale of consideration. Please see Mowing and Management: Best Practices for Monarchs (Monarch Joint Venture 2019; https://monarchjointventure.org/images/uploads/documents/ MowingForMonarchs.pdf ). Great variation was noted in a survey of roadside managers when asked about the frequency of mowing the safety strip, as well as the full ROW width including the backslope. There has been more interest building on the topic of reducing mowing, and it is being implemented in the mowing policy for some DOTs (such as the state of Illinois). Metric Level Points Herbicide application None 100 Against noxious weeds only; applied only as needed (with plan, spot treatment) 100 Against weedy grasses to stimulate forbs 100 Broadleaf applied in clear zone 1 x /yr 75 Broadleaf applied in clear zone at > 1 x /yr 50 Broadleaf applied throughout ROW 1 x /yr 0 Frequency of full width mow Never 100 Every few years 90 1 x /yr 90 2 x /yr 50 3 x /yr 25 >3 x /yr 0 Width of mow as proportion of ROW width No mowing 100 Less than 1/3 of the ROW width is mowed 75 Between 1/3 and 2/3 of the ROW width is mowed 50 Greater than 2/3 of the ROW width is mowed 25 ROW entirely mowed 0 Table 9. Management factors and point assignments.

56 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies Mowing can have negative effects, including: • Removes flowers that provide nectar (Halbritter et al. 2015). • Cuts down milkweed used by larval monarchs (and assumed to destroy most larvae and eggs). • Can spread weed seeds (e.g., wild parsnip, personal communication, Christa Schaefer, Wisconsin DOT). Mowing can have positive effects, such as: • Provides good visibility and safe areas for emergency exit from the roadway. • Can stimulate the ability for nectar-producing forbs to compete with grasses. • Stimulates regrowth of milkweed (if done early enough in the growing season), providing more tender (and likely nitrogen-rich) leaves farther into the season (Fischer et al. 2015, Baum and Mueller 2015, Alcock et al. 2016, Haan and Landis 2019). • Can reduce predators of monarchs in milkweed by a single mowing in early or peak growing season (Haan and Landis 2019). • Removes woody vegetation which often provides fewer nectar resources that non-woody- dominated areas (but not always). • Can be used to control some weeds that may reduce habitat quality due to encroachment, benefits more likely to be seen particularly over the long term. • Removes plants adjacent to roadsides that may have higher levels of chemicals from road runoff (e.g., for both sodium and zinc, Mitchell et al. in preparation). To create a simple index of mowing intensity, the researchers combined one temporal factor and one spatial factor. For the temporal component, managers were asked to report the fre- quency with which they typically mowed the full width of the ROW. For the spatial component, the team used the observed width of the mowed area during our field visit (typically this is the backslope but sometimes the entire area or sometimes the entire area has been mowed (to less than 10 inches) (see RA protocol, Appendix C). Together, these two factors serve as a surrogate for mowing pressure (see points allocated, Table 7). Running the Habitat Calculator Information about how to install the proper software, collect data, and retrieve data is pro- vided in the User Guide for the Roadside Monarch Habitat Evaluator (Appendix D). Field Testing the Habitat Calculator To validate the Habitat Calculator, the team processed data from field surveys in Minnesota and Oklahoma in 2018. The data collected were used to make small changes to the calculations. In Minnesota, the team ran the Habitat Calculator on 298 sites. In Oklahoma the sample size was 282 sites. Results Minnesota In Minnesota, the overall mean Monarch Habitat Quality Score was 52.36 (standard deviation = 13.57, minimum = 20, maximum = 88 for 298 random sites (Figure 29)). The research team also depicted the distribution of values for the breeding component score (Figure 30) and the foraging component score (Figure 31). See Figure 32 for a geographic representation of the site locations and overall habitat quality scores. Summary statistics for all of the component sub-scores were as follows (Table 10).

Figure 29. Histogram depicting the distribution of monarch habitat quality scores for 298 randomly located roadside sites in Minnesota, sampled once during the summer of 2018. Figure 30. Histogram depicting the distribution of Monarch Breeding Habitat Component Scores for 298 randomly located roadside sites in Minnesota, sampled once during the summer of 2018. The left-biased curve is indicative of many sites that did not contain milkweed (n = 74). Figure 31. Histogram depicting the distribution of monarch foraging habitat component sub-scores for 298 randomly located roadside sites in Minnesota, sampled once during the summer of 2018.

58 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies Figure 32. Monarch habitat quality scores for 298 sites visited in Minnesota in 2018. Breeding Foraging Threats Weeds Management mean 11.546 11.295 10.941 6.921 4.339 SD 9.012 4.906 3.002 1.164 1.452 minimum 0 0 4.375 2.125 0 model maximum 30 25 16.250 8.750 20 field maximum* 30 22.938 16.250 8.750 6 *The maximum for management that was recorded in the field was 6 points, based upon the proportion of the area that was mowed. The team was lacking data regarding mowing frequency and herbicide application practices because it was not within the managing transportation department. Table 10. Summary statistics for component scores within the monarch habitat quality scores for 298 random roadside locations in Minnesota, surveyed in 2018.

Product C: Roadside Monarch Habitat Calculator 59 Oklahoma In Oklahoma, the mean overall habitat quality score was 37.64 (SD = 10.38; min 14.75, max 72.75) (Figure 33). See Figure 34 for the histogram of breeding habitat component scores and Figure 35 for nectar plant component scores. A summary of all of the primary habitat component scores is presented in Table 11. Discussion Additional research regarding preference, use by, and survival on different species of milk- weed in roadside ROWs is needed to improve the milkweed component of the Habitat Calculator. Since monarchs successfully reproduce on many different milkweed species (Pocius et al. 2018), Figure 33. Monarch habitat quality scores for all site visits in Oklahoma, first visits (May 28 through June 18, 2018; n = 282). Figure 34. Histogram depicting the distribution of monarch breeding habitat component scores for 282 sites in Oklahoma, sampled once during the summer of 2018. The left-biased curve is indicative of many sites that did not contain milkweed (n = 150).

60 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies Breeding Foraging Threats Weeds Management mean 4.162 8.321 9.945 6.512 3.431 SD 5.580 4.091 2.970 2.053 0.866 minimum 0 2.25 4.375 1.875 1.5 model maximum 24 21 16.25 8.75 4.5 field maximum* 30 25 16.25 8.75 20 *The maximum for management that was recorded in the field was 4.5 points, based upon the proportion of the area that was mowed. The team was lacking data regarding mowing frequency and herbicide application practices because it was not within the managing transportation department. Table 11. Summary statistics for component scores within the monarch habitat quality scores for 282 first visits to Oklahoma sites surveyed in 2018. Figure 35. Histogram depicting the distribution of monarch foraging habitat component sub-scores for 282 roadside sites in Oklahoma, sampled once early in the summer of 2018. but milkweed species vary significantly in stature and biomass, it is likely that the quantity of milkweed consumed by larvae for each species may vary as well. For example, more stems of smaller-statured milkweeds, like Asclepias verticillata (whorled milkweed), may be necessary for monarchs to develop to maturity. The Thogmartin et al. (2017b) “all hands on deck” model that drives national, state, and sector based goals was restricted to common milkweed (A. syriaca), and therefore, there is growing interest among researchers to further define optimal densities for other species of milkweed that vary greatly in size, number of stems, and biomass. In addition, because of this focus on common milkweed, many recent articles define “plants” synonymously with “stems,” due to the rhizomatous nature of A. syriaca (which makes them impossible to distinguish above ground). For species that grow in clumps, like A. tuberosa, the number of stems per plant can vary widely, and there is more to be learned about how this variation affects monarch density or use of that plant. The Habitat Calculator could be updated as new species- specific information becomes available. Knowledge of the availability and species-specific value of nectar resources for monarchs is also limited. In future versions of the Habitat Calculator, the scoring for nectar plant spe- cies could be weighted toward plants considered to be particularly good nectar sources for monarchs as new research becomes available. Some efforts to list preferred or particularly

Product C: Roadside Monarch Habitat Calculator 61 beneficial plants have been made, which could serve as a starting point for developing more robust research studies and updating tools like the Habitat Calculator. The NRCS published an in-depth set of regionally specific guides, entitled Important Plants and Plant Lists of the Monarch Butterfly (USDA NRCS 2018); https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/ plantsanimals/pollinate/?cid=nrcseprd402207). The NRCS list of plants used by monarchs gives a rating of “very high” for plant species “reported to be of superlative use by the monarch” or for “plants mentioned in multiple sources as providing nectar to monarchs’” (USDA NRCS 2018). Monarchs appear to prefer large flowers in the aster family or those with upright floral displays such as the Gayfeathers (Liatris sp.) (USDA NRCS 2018). A second set of quick guides was developed, also regionally, called Monarch Nectar Plants, by the Xerces Society, Monarch Joint Venture, and the National Wildlife Federation (https://xerces.org/monarch-nectar-plants/) with funding from the NRCS and Monarch Joint Venture. Landscape context, while an important part of valuing a potential habitat area, is often not within the land manager’s control. While it could be considered low hanging fruit to always prioritize areas that are in areas that pose less potential risk, it is also important to make efforts to mitigate risk and in other areas to distribute more habitat across the landscape. Through education and outreach to surrounding landowners and land managers, the Habitat Calculator and other associated tools serve an important role in engaging the surrounding community to improve practices for monarchs, pollinators, and greater environmental benefit. Our field trials identified a number of sites with high-quality habitat for monarchs. Interest- ingly, the overall scores are distributed more as a bell-shaped curve, suggesting that most sites are at least decent habitat and have potential to be high-quality habitat (rather than a skew towards lower scores). While this method clearly identifies the top roadside sites with current quality habitat for monarchs, it is less clear what the minimum score is for monarch success. Additional field research that considers factors such as survival of monarch larvae on roadsides can help to determine the range of scores that need minimal investment apart from protection versus the range of scores that would benefit from restoration. This may also depend on the resources available to invest in restoration. For instance, roadside managers with moderate resources may choose to protect sites with scores of 80–100 and invest in restoration of sites with scores of 60–80. Future research may also consider interactions between the bottom-up approach that com- bines assessment of existing roadside quality with the top-down approach of the Landscape Prioritization Model that uses higher-level data to predict potential roadsides. For instance, regions with high potential identified through the Landscape Prioritization Model and median scores from the Habitat Calculator may represent the best potential sites for restoration.