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Anderson, J.B. and L.P. Brower. 1996. Freeze-protection of overwintering monarch butterflies in Mexico: critical role of the forest as a blanket and an umbrella. Ecological Entomology 21:107-116.

Borders, B. 2012. A Guide to Common Milkweeds of Nevada. The Xerces Society for Invertebrate Conservation, Portland, OR, in collaboration with USDA-NRCS. Available online: Accessed 2 November 2016.

Borders, B. 2012. A Guide to Native Milkweeds of Oregon. The Xerces Society for Invertebrate Conservation, Portland, OR, in collaboration with USDA-NRCS. Available online: Accessed 2 November 2016.

Borders, B. 2012. A Guide to Native Milkweeds of Washington. The Xerces Society for Invertebrate Conservation, Portland, OR, in collaboration with USDA-NRCS. Available online: Accessed 2 November 2016.

Borders, B., E. Eldredge, E. Mader, and C. Burns. 2012. Great Basin Pollinator Plants: Native Milkweeds (Asclepias spp.). The Xerces Society for Invertebrate Conservation, Portland, OR, in collaboration with USDA-NRCS Great Basin Plant Materials Center, Fallon, NV. NVPMC Technical Note No. 56. Available online: Accessed 22 November 2016.

Borders, B. and E. Mader. 2011. California Pollinator Plants: Native Milkweeds (Asclepias spp.). The Xerces Society for Invertebrate Conservation, Portland, OR, in collaboration with USDA-NRCS. Available online: Accessed 22 November 2016.

Brower, L.P. and R.M. Pyle. 2004. The interchange of migratory monarchs between Mexico and the Western United States, and the importance of floral corridors in the fall and spring migrations. In Conserving Migratory Pollinators and Nectar Corridors in Western North America, edited by G. Nabhan, 144-166. Tucson: University of Arizona Press.

Calflora: Information on California plants for education, research and conservation. [web application]. 2014. Berkeley, California: The Calflora Database [a non-profit organization]. Available online: Accessed May 2016.

Calvert, W.H. and J.A. Cohen. 1983. The adaptive significance of crawling up onto foliage for the survival of grounded overwintering monarch butterflies (Danaus plexippus) in Mexico. Ecological Entomology 8:471-474.

Chaplin, S.B. and P.H. Wells. 1982. Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California. Ecological Entomology 7:249-256.

Cronquist, A., A.H. Holmgren, N.H. Holmgren, J.L. Reveal, and P.K. Holmgren. 1984. Intermountain flora, Vol. 4. Bronx: New York Botanical Garden. 573 pp.

Decker, K. 2006. Asclepias uncialis Greene (wheel milkweed): a technical conservation assessment. [Online.] USDA Forest Service, Rocky Mountain Region. Available online: Accessed 2 November 2016.

Dingle, H., M.P. Zalucki, W.A. Rochester, and T. Armijo-Prewitt. 2005. Distribution of the monarch butterfly, Danaus plexippus (L.) (Lepidoptera: Nymphalidae), in western North America. Biological Journal of the Linnaean Society 85:491-500.

Flockhart, D.T.T., L.I. Wassenaar, T.G. Martin, K.A. Hobson, M.B. Wunder, and D.R. Norris. 2013. Tracking multi-generational colonization of the breeding grounds of monarch butterflies in eastern North America. Proceedings of the Royal Society B 280:20131087 (Supplementary Material in addition).

Handley, J. and W. Fertig. 2001. Asclepias hallii, Hall’s milkweed. State Species Abstract. Wyoming Natural Diversity Database. Available online: Accessed 2 November 2016.

Herman, W.S. 1981. Studies on the adult reproductive diapause of the monarch butterfly, Danaus plexippus. Biological Bulletin 160:89-106.

Hill, H.F. Jr., A.M. Wenner, and P.H. Wells. 1976. Reproductive behavior in an overwintering aggregation of monarch butterflies. American Midland Naturalist 95(1): 10-19.

Hitchcock, C.L. and A. Cronquist. 1973. Flora of the Pacific Northwest. Seattle: University of Washington Press. 730 pp.

James, D. 2016. Personal communication with Candace Fallon, the Xerces Society for Invertebrate Conservation.

Jepsen, S. and S.H. Black. 2015. Understanding and conserving the Western North American monarch population. In Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly, edited by K. Oberhauser, S. Altizer, and K. Nail, 147-156. Ithaca: Cornell University Press.

Jepsen, S., D.F. Schweitzer, B. Young, N. Sears, M. Ormes, and S.H. Black. 2015. Conservation Status and Ecology of Monarchs in the United States. 36 pp. NatureServe, Arlington, Virginia, and the Xerces Society for Invertebrate Conservation, Portland, Oregon. Available online: Accessed June 2016.

Kartesz, J.T., The Biota of North America Program (BONAP). 2015. North American Plant Atlas. ( Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2015. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)].

Kearney, T. H., R. H. Peebles, and collaborators. 1960. Arizona Flora. University of California Press: Berkeley and Los Angles, CA. 1085 pp.

Lane, J. 1993. Overwintering monarch butterflies in California: past and present. In Biology and Conservation of the Monarch Butterfly, edited by S.B. Malcolm and M.P. Zalucki, 335-344. Los Angeles: Natural History Museum of Los Angeles County.

Leong, K.L.H. 1990. Microenvironmental factors associated with the winter habitat of the monarch butterfly (Lepidoptera: Danaidae) in central California. Annals of the Entomological Society of America 83:907-910.

Leong, K.L.H. 1999. Restoration of an overwintering grove in Los Osos, San Luis Obispo County, California. In 1997 North American Conference on the Monarch Butterfly, edited by J. Hoth, L. Merino, K. Oberhauser, I. Pisanty, S. Price, and T. Wilkinson, 218-221.

Leong, K.L.H., W.H. Sakai, W. Bremer, D. Feurstein, and G. Yoshimura. 2004. Analysis of the pattern of distribution and abundance of monarch overwintering sites along the California coastline. In The Monarch Butterfly: Biology and Conservation, edited by K.S. Oberhauser and M.J. Solensky, 177-185. Ithaca: Cornell University Press.

Lyons, J.I., A.A. Pierce, S.M. Barribeau, E.D. Sternberg, A.J. Mongue, and J.C. de Roode. 2012. Lack of genetic differentiation between monarch butterflies with divergent migration destinations. Molecular Ecology 21:3433-3444.

Martin, W.C. and C. R. Hutchins. 1980. A flora of New Mexico: Volume 2. J. Kramer, Germany.

Masters, A.R., S.B. Malcolm, and L.P. Brower. 1988. Monarch butterfly (Danaus plexippus) thermoregulatory behavior and adaptations for overwintering in Mexico. Ecology 69(2):458-467.

Morris, G.M., C. Kline, and S.M. Morris. 2015. Status of Danaus plexippus population in Arizona. The Journal of the Lepidopterists’ Society 69(2): 91-107. doi:

Nabhan, G., S. Buckley, and H. Dial. 2015. Pollinator Plants of the Desert Southwest: Native Milkweeds (Asclepias spp.). USDA‐Natural Resources Conservation Service, Tucson Plant Materials Center, Tucson, AZ. AZPMC Technical Note No. 16‐1. Available online: Accessed 22 November 2016.

Pellissier, L., G. Litsios, M. Fishbein, N. Salamin, A.A. Agrawal, and S. Rasmann. 2016. Different rates of defense evolution and niche preferences in clonal and nonclonal milkweeds (Asclepias spp.). New Phytologist 209(3): 1230-1239. doi:10.1111/nph.13649.

Pelton, E., S. Jepsen, C. Schultz, C. Fallon, and S.H. Black. 2016. State of the Monarch Overwintering Sites in California. 40+vi pp. Portland, OR: The Xerces Society for Invertebrate Conservation. Available online: Accessed October 2016.

Pleasants, J.M., and K.S. Oberhauser. 2012. Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity 6:135-144.

Pyle, R.M. 1999. Chasing Monarchs: Migrating with the Butterflies of Passage. 307 pp. Boston: Houghton Mifflin Harcourt.

Pyle, R.M. 2015. Monarchs in the mist: new perspectives on monarch distribution in the Pacific Northwest. In Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly, edited by K. Oberhauser, S. Altizer, and K. Nail, 237-247. Ithaca: Cornell University Press.

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Stevens, W.D. 2005. New and interesting milkweeds (Apocynaceae, Asclepiadoideae). Novon 15(4): 602-619.

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Tilley, D., L. St. John, and D. Ogle. 2010. Plant guide for Welsh’s milkweed (Asclepias welshii). USDA-Natural Resources Conservation Service, Idaho Plant Materials Center. Aberdeen, ID. Available online: Accessed 2 November 2016.

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Vandenbosch, R. 2007. What do monarch population time series data tell us about eastern and western population mixing? Journal of the Lepidopterists’ Society 61(1):28-31.

Wassenaar, L.I., and K.A. Hobson. 1998. Natal origins of migratory monarch butterflies at overwintering colonies in Mexico: New isotopic evidence. Proceedings of the National Academy of Sciences 95:15436-15439.

Waterbury, B. and T.K. Ruth. 2015. Survey for milkweed (Asclepias spp.) and monarch butterflies (Danaus plexippus) in Lemhi County, Idaho. Idaho Department of Fish and Game, Boise, USA. 26 pp.

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Species Occurrences Database

Data for the Western Monarch and Milkweed Occurrences Database upon which this website is built were gathered from a wide variety of individuals, agencies, herbaria, online databases and reports, including the following:

  • Arizona Bureau of Land Management
  • Participants of the Biodiversity Information Serving our Nation (BISON) (Arizona State University: Arizona State University Vascular Plant Herbarium, BLM – NISIMS Plants, BLM –Landscape Monitoring Framework – Plant Occurrence Records, Colorado State University: Colorado State University Herbarium, Denver Botanic Gardens: Kathryn Kalmbach Herbarium, Emporia State University (KSTC): Emporia State Herbarium/H.A. Stephens Herbarium, Harvard University: Harvard University Herbaria, Louisiana State University: Shirley C. Tucker Herbarium, Lund University (Sweden): Lund Botanical Museum, Missouri Botanical Garden (MO), Naturalis Biodiversity Center: The National Herbarium of the Netherlands (NHN),, New Mexico Biodiversity Collections Consortium, NPS – Inventory and Monitoring Program, New York Botanic Garden: William and Linda Steere Herbarium, Oregon State University: Oregon State University Herbarium, Real Jardín Botánico (MA): Real Jardín Botánico, Royal Belgian Institute of Natural Sciences, Smithsonian Institution: National Museum of Natural History, Staatliche Naturwissenschaftliche Sammlungen Bayer: State Natural Science Collections in Bavaria, University of Alberta: Vascular Plant Herbarium, University of Arizona: University of Arizona Herbarium, University of British Columbia: UBC Herbarium, University of California, Berkley: University Herbarium, University of California, Davis: Bohart Museum of Entomology, University of Colorado: University of Colorado Museum of Natural History, University of Connecticut: George Safford Torrey Herbarium, University of Illinois, Urbana-Champaign: Illinois Natural History Survey, University of Kansas: University of Kansas Biodiversity Institute, University of Washington: Burke Museum Herbarium, Utah State University: Intermountain Herbarium, Utah Valley University: Utah Valley University Herbarium, Yale University: Peabody Museum)
  • Cal Flora
  • California Academy of Sciences
  • Davenport, K. 2012. Butterflies ten years after the McNally Fire. Unpublished report.
  • Global Biodiversity Information Facility (GBIF) (Arizona State University, California Academy of Sciences, Intermountain Herbarium, Missouri Botanical Garden, Utah State University)
  • Horning et al unpublished data
  • Intermountain Flora
  • James, D.G. 2016. Population biology of monarch butterflies at a milkweed-rich summer breeding site in central Washington. Journal of the Lepidopterists Society 70 (3): 182-193.
  • Journey North
  • Idaho Department of Fish and Game
  • iNaturalist
  • The Nature Conservancy
  • Members of the D-PLEX listserv
  • Members of the Western Monarchs listserv
  • Missouri Botanic Garden
  • Monarch Larva Monitoring Project (University of Minnesota Monarch Lab)
  • Oregon Flora
  • Oregon State University
  • Participants of the Consortium of California Herbaria (Cal State Northridge, California Academy of Sciences, California Polytechnic State University, Carl W. Sharsmith Herbarium, Catalina Island Conservancy, Chico State Herbarium, Dudley Herbarium, Humboldt State University Herbarium, Jasper Ridge Biological Preserve, Joshua Tree National Park Herbarium, Pasadena City College, Pomona Herbarium, Rancho Santa Ana Botanic Garden, Riverside Metropolitan Museum, San Diego Natural History Museum, Santa Barbara Botanic Garden, Sweeney Granite Mountains Research Center, UC Berkeley Jepson Herbarium, UC Berkeley University Herbarium, UC Davis, UCLA Herbarium, UC Riverside, UC Santa Barbara, Victor Valley College, Yosemite N.P. Herbarium;
  • Participants of the Consortium of Pacific Northwest Herbaria (Eastern Washington University, Montana State University, Oregon State University Herbarium, Reed College, University of British Columbia, University of Idaho Stillinger Herbarium, University of Puget Sound, University of Washington Burke Museum, University of Washington Herbarium, University of Wyoming Rocky Mountain Herbarium, Washington State University Marion Ownbey Herbarium, Western Washington University Pacific Northwest Herbarium;
  • Participants of the Xerces Online Milkweed Survey (
  • Participants of the Southwest Environmental Information Network, SEINet (Appalachian State University I.W. Carpenter, Jr. Herbarium, Arizona State University, Brigham Young University, Chicago Botanic Garden, Clemson University Herbarium, Cochise County Herbarium, Coconino National Forest Herbarium, Colorado State University Herbarium, Desert Botanical Garden, Eastern Nevada Landscape Coalition, Fort Lewis College Herbarium, Gila Center for Natural History Herbarium, Gila National Forest Herbarium, Grand Canyon National Park, Herbario de la Universidad de Sonora, Huntington Botanical Gardens Herbarium, Illinois Natural History Survey, Jemez Mountain Herbarium, Kathryn Kalmbach Herbarium, Madrean Archipelago Biodiversity Assessment, Mississippi State University, Museum of Northern Arizona, National Park Service, Navajo Nation Herbarium, New Mexico State University Herbarium, New Mexico State University Center for Natural History Collections Range Science Herbarium, New York Botanic Garden, Northern Arizona University, Northern Arizona University Deaver Herbarium, R.M. Myers Herbarium, Rocky Mountain Biological Laboratory, Rocky Mountain Herbarium, Sagehen Herbarium, San Diego State University, San Juan College Herbarium, Shirley C. Tucker Herbarium at Louisiana State University, Snow College Herbarium, Sonoran Atlas, Southern Utah University, Southwestern Research Station, University of Arizona, University of California Riverside, University of Nevada Herbarium, University of New Mexico, University of South Carolina A. C. Moore Herbarium, USFS SW Region, Utah State University, Utah State University Intermountain Herbarium, Utah State University Uintah Basin, Utah Valley State College; http//
  • Participants of the 2015 USFWS Excel Survey
  • Pyle, R. M. 1999. Chasing Monarchs: Migrating with the Butterflies of Passage. Houghton Mifflin Company, New York.
  • Southwest Monarch Study
  • University of Alabama Biodiversity and Systematics
  • University of Alberta Museums
  • University of Arizona Herbarium
  • University of British Columbia Herbarium (UBC)
  • University of Connecticut
  • University of Kansas Biodiversity Institute & Natural History Museum
  • University of Nevada Herbarium
  • USDA Plants
  • US Fish and Wildlife Service (Regions 1, 2, and 8)
  • US Forest Service (Regions 3 and 6)
  • Utah State University
  • Washington Department of Fish and Wildlife
  • The Xerces Society for Invertebrate Conservation