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

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E-1 A P P E N D I X E Best Management Practices Resource Sheet: Monarch Butterflies, Weeds, and Herbicides

Best Management Practices Resource Sheet: Monarch Butterflies, Weeds, and Herbicides E-3 Monarch butteries are in decline in North America, and restoring monarch habitat, including roadsides, is important to the species’ recovery1. Monarch caterpillars require milkweed (primarily in the genus Asclepias) to complete their development. A diversity of milkweed species is found on roadsides2,3, and monarchs lay their eggs readily on milkweed plants in roadsides4 and consume nectar from milkweed owers. Roadsides provide more than just milkweed; they can also provide diverse nectar sources to feed adult monarchs and other pollinators. Nectar fuels adult monarchs in their breeding, migration, and overwintering. Adult monarchs feed on nectar from a variety of blooming plants, including wildowers and shrubs, throughout the growing season. Spring owers support monarchs as they leave their overwintering grounds to breed, and summer owers support several generations of breeding monarchs. Fall-blooming owers are also important, as monarchs migrating to overwintering grounds require lots of nectar to build fat reserves to support their long-distance ights and sustain them through the winter. Noxious and invasive weeds can degrade habitat for monarchs by displacing valuable nectar plants and milkweed. Herbicides are a tool employed by many transportation departments and other land managers to control noxious and invasive weeds or encroaching woody vegetation. However, some herbicide uses have nontarget eects that reduce the quality of roadside habitat for monarchs by removing owering plants and milkweed plants or reducing plant diversity over time. is guide highlights best management practices to reduce the impacts of herbicides on monarchs. Best Management Practices Roadside managers and other vegetation managers can reduce the impacts of herbicide use on monarch butteries by: 1. using herbicides within an integrated approach that incorporates a range of methods to prevent and manage weeds and non-compatible vegetation, 2. limiting nonselective broadcast applications, which can damage host or nectar plants, 3. using herbicides as eciently as possible to reduce the amount applied, 4. reducing o-site movement of herbicides, and 5. limiting direct exposure of monarchs to herbicides when possible. Specic management practices to reduce risk to monarchs from herbicide applications include: Applicator Training Train sta and contractors to distinguish noxious and invasive weeds and encroaching woody vegetation from similar species to reduce unintended damage to nontarget plants. For instance, training may help crews to distinguish the invasive Canada thistle (Cirsium arvense) from the native tall thistle (Cirsium altissimum), an important fall blooming native nectar plant for migrating monarchs in the central states. Train applicators in herbicide application techniques that reduce damage to nontarget plants. Create specications that would hold contractors accountable to using proper techniques. Assessment Inventory roadside vegetation regularly to identify emerging noxious and invasive weed issues or encroaching woody MONARCH BUTTERFLIES, WEEDS, AND HERBICIDES

E-4 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies vegetation. Early detection of weeds can result in improved control and may reduce the amount of herbicide needed overall. Document desirable plants that may be present, such as native nectar plants and milkweeds. Planning Use herbicides within an integrated vegetation management plan. Evaluate the range of management techniques (e.g., chemical, cultural, biological, physical, and mechanical) in order to select the most eective, feasible, and least harmful weed management method(s) that can increase or conserve the abundance and diversity of blooming plants. Prioritize selective herbicides—those formulated to control specic weeds or groups of weeds— whenever possible, to reduce damage to nontarget plants. If using nonselective herbicides—broad- spectrum products that kill or damage a wide range of plants—use direct or targeted application methods or apply when desirable plants are dormant. If possible, avoid applications during times when monarchs are present (Establish these times using on-site scouting as well as expected windows of monarch activity, found here: https://monarchjointventure.org/ images/uploads/documents/MowingForMonarchs.pdf.) Coordinate spray operations with mowing crews to enhance weed control. For example, it may improve control to treat mature weeds when they are actively growing, shortly aer mowing. Choose and calibrate equipment with dri management in mind. Use nozzles that produce larger droplets that are less likely to dri o target. Calibrate equipment regularly to avoid over-application. Select herbicides with low volatility, when feasible, to reduce the o-target movement of herbicide vapors. Do not apply herbicides when temperatures are high (see label for more information) or during temperature inversions, when herbicides are more likely to volatilize. Use appropriate dri control agents. Prioritize the use of formulations that are jointly terrestrial- and aquatic-approved, and that have lower residual activity and shorter half-life, when possible, in order to minimize potential impacts on the environment following application. Select adjuvants—products added to a spray solution to enhance performance of post- emergence herbicides—that are terrestrial- and aquatic-approved, and compatible with the selected herbicide formulation. Toxicity of herbicides to monarchs Although herbicides are formulated to kill plants and do not target insects, recent re- search indicates that some herbicides may be toxic to butteries, particularly when ingested by caterpillars eating treated plants. Often, the herbicides are not immediately lethal but still have negative eects such as reducing buttery size, weight, development rates, and survival5, 6, 7, 8. These sublethal eects may reduce buttery populations over time6. These studies did not focus on monarchs and further research into the eects of commonly used herbicides, tank mixes, surfactants and other inert ingredients in formulated products on monarchs is needed. Until more is known, we recommend a cau- tious approach when applying herbicides to milkweed where monarch caterpillars are pres- ent. Avoiding direct applications to milkweed plants when feasible, for example, can reduce direct herbicide exposure to monarchs. Swamp milkweed (Asclepias incarnata) growing along a roadside in Michigan. Identifying and recording the location of milkweed patches like this is a rst step in ensuring that they are considered during subsequent maintenance opera- tions. (Photo: Xerces Society / Jennifer Hopwood.)

Best Management Practices Resource Sheet: Monarch Butterflies, Weeds, and Herbicides E-5 Use of noxious or invasive weeds by monarchs Adult monarchs feed on nectar from a variety of blooming plants, including some noxious weeds or invasive nonnative plants (such as Canada thistle, Cirsium arvense). However, if invasive species become dominant, this can reduce the diversity of other plants available to provide nectar throughout the season. For example, if Canada thistle is the only owering plant pres- ent in a stretch of roadside, monarchs will only have nectar available to them from that single species which blooms during a small portion of the growing season, rather than a diverse patch of vegetation that could provide nectar from spring through fall. Hence, managing invasive plants will generally increase the abundance and diversity of plants that support monarchs and pollinators throughout the growing season. In highly degraded landscapes where na- tive nectar sources are scarce, the large-scale removal of the noxious or invasive species may cause a short-term reduction in nectar for mon- archs. In these circumstances, reseed with na- tive blooming plants that are attractive to mon- archs, known to compete well with weeds, and bloom within the rst few years of planting in your seed mix. In time, these species and other native perennial plants should deter recoloniza- tion of invasive plants and provide a haven for monarchs and pollinators. Herbicide Applications Always apply herbicides according to label directions and use the minimum application rate that will eectively control the weed. Apply herbicides at the stage of growth when the weed is most vulnerable and the application likely to be most successful. is will be the seedling or rosette stage for some weeds. Consider the mode of action of the herbicide and the application technique when determining timing of application. For example, when using a systemic herbicide, treat perennial weeds in the late summer and fall, when perennials begin to move sugars down to their roots, so that the herbicide will be translocated to vegetative reproductive structures where it will be most eective at controlling the plant. When possible, treat plants before they convert from vegetative phase to oral phase and bloom; this will reduce the weed seed bank (reservoir of weed seeds in the soil). If weeds are treated just before bloom or aer seed set, their populations may persist in future years. Treatment of weeds during their vegetative phase also reduce exposure of adult monarchs to herbicides and adjuvants. Apply herbicide sprays when weather conditions will minimize dri. Avoid applications when wind speeds are greater than 10 mph. Avoid applications during a temperature inversion (when warmer air above traps cooler air near the ground); these conditions cause herbicides and other pesticides to linger in the air, where they can move long distances o-site with any air movement. No wind or wind speed below 2 mph suggests a possible inversion. Make direct, selective applications to target plants to avoid weakening nontarget species. Target weeds or non-compatible species using spot treatment applications made with a backpack sprayer, weed wiper, or similar technology. Use highly targeted applications to cut stems, stumps, or underneath bark. Limit the use of broadcast treatments or pellet dispersal only for dense infestations of weeds or non-compatible vegetation, or for safety zone or guardrail treatments. Use an approved marker dye with spot treatments or cut stem/stump treatments to allow the applicator to know the target has already been treated and the extent of target coverage. Spray dyes reduce likelihood of an accidental retreatment or missing treatment of a target weed. Post-Treatment Keep records of locations where herbicides are applied. Records on the plants treated, application method, type and amount of herbicides used, and dates of application can help to evaluate the eectiveness of treatments over time and can be useful when adjusting management decisions. Your state agency charged with education or regulation of pesticide use will have example application record keeping forms that can be used. Multiple seasons of herbicide applications or other weed control methods may be needed to fully control an invasive species. Follow label directions and standard practices when rinsing or cleaning spray equipment in between work

E-6 Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies sessions; incomplete removal of a prior herbicide mix can have detrimental impacts to the next treatment area. Rinse o, or otherwise clean mower decks (upper and undersides), deectors, gear box housing, and mower blades and shas, between sites to avoid transferring weed seeds. is is especially important aer mowing an area known to contain noxious or invasive weed species. Aer treating a dense infestation, consider seeding or replanting the area, if necessary (e.g., if the seed bank was depleted of desirable species). Plant with desirable, competitive native species to reduce the need to re-treat the area. Always make sure that seed and vegetative planting stock is free of weed species. Aer treatment, monitor resulting conditions and outcomes to evaluate the eectiveness of management practices on target plants and any eects on nontarget plants. If desired conditions were not produced or if site conditions change, adapt management practices accordingly. Resources Monarch Joint Venture: Roadsides as Habitat for Monarchs https://monarchjointventure.org/roadsidehabitat Xerces Society: Regional guides to monarch nectar plants https://xerces.org/monarch-nectar-plants/ Federal Highway Administration: Environmental Toolkit Review: Pollinators https://www.environment.wa.dot.gov/env_topics/ecosystems/pollinators.aspx References 1. and L. L. Jackson. 2017. Restoring monarch buttery habitat in the Midwestern US: ‘all hands on deck’. Environmental Research Letters 12(7), p.074005. 2. Pleasants, J. M., and K. S. Oberhauser. 2013. Milkweed loss in agricultural elds because of herbicide use: eect on the monarch buttery population. Insect Conservation and Diversity 6:135–144. 3. Mueller, E. K., and K. A. Baum. 2014. Monarch–parasite interactions in managed and roadside prairies. Journal of Insect Conservation 18:847–853. 4. Kasten, K., C. Stenoien, W. Caldwell, and K. S. Oberhauser. 2016. Can roadside habitat lead monarchs on a route to recovery? Journal of Insect Conservation 20:1047–1057. 5. Russell, C., and C. B. Schultz. 2010. Eects of grass-specic herbicides on butteries: an experimental investigation to advance conservation eorts. Journal of Insect Conservation 14:53–63. 6. Stark, J. D., X. D. Chen, and C. S. Johnson. 2012. Eects of herbicides on Behr’s metalmark buttery, a surrogate species for the endangered buttery, Lange’s metalmark. Environmental Pollution 164:24–27. 7. Bohnenblust, E., J. F. Egan, D. Mortensen, and J. Tooker. 2013. Direct and indirect eects of the synthetic-auxin herbicide dicamba on two lepidopteran species. Environmental Entomology 42:586–594. 8. Schultz, C.B., J. L. Zemaitis, C. C. omas, M. D. Bowers, and E. E. Crone. 2016. Non-target eects of grass-specic herbicides dier among species, chemicals and host plants in Euphydryas butteries. Journal of Insect Conservation 20:867–877. AUTHORS: Jennifer Hopwood & Emma Pelton (Xerces Society for Invertebrate Conservation); Alison Cariveau (Monarch Joint Venture). ACKNOWLEDGMENTS: We thank Wendy Caldwell, Aimee Code, Aaron Feggestad, Dennis Martin, Dennis Markwardt, Stephanie McKnight, Ray Moranz, Lewis Payne, Cora Lund Preston, and Michael Retterer for their review of this document. This work was conducted in the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Academies of Sciences, Engineering, and Medicine. © 2019 by The Xerces Society for Invertebrate Conservation. Xerces® is a trademark registered in the U.S. Patent and Trademark Oce. 19-045_01 Selective herbicide applications such as this treatment for Johnson grass (Sorghum halepense) can control the undesire- able weeds while allowing milkweeds and other desireable plants to thrive. (Photograph courtesy Texas DOT.) ogmartin, W. E., L. López-Homan, J. Rohweder, J. Diendorfer, R. Drum, D. Semmens, S. Black, I. Caldwell, D. Cotter, P. Drobney,