Conspicuous population declines of the iconic monarch butterfly are the result of loss of summer breeding habitat, extreme weather events, and deforestation of overwintering habitat (Brower et al., 2002, 2012; Casner et al., 2014; Flockhart et al., 2015; Hartzler, 2010; Pleasants and Oberhauser, 2012). Two migratory populations of monarch butterfly exist in North America. The eastern population is the larger of the two and migrates over successive generations ≈4000 km from overwintering grounds in the high-elevation oyamel fir (Abies religiosa) forests of central Mexico to summer breeding grounds in the United States and southern Canada (Brower, 1995). The relatively smaller western population spans ≈500 km from overwintering sites in coastal California to summer breeding grounds west of the Rocky Mountain Range (Dingle et al., 2005).
Monarch larvae (caterpillars) feed almost exclusively on milkweed, a genus of perennial dicotyledonous plants with more than 130 known species native to North America (Woodson, 1954). Larvae acquire protection from predators as a result of the toxic cardenolide glycosides in the milkweed foliage they consume (Malcolm et al., 1989). Monarch migration is closely tied to the timing of milkweed development across the summer breeding range (Stevens and Frey, 2010). Shrinking populations of milkweed due to a combination of extensive planting of genetically modified herbicide-resistant soybean (Glycine max) and corn (Zea mays), widespread herbicide use, and land-use change have reduced the suitability of summer breeding grounds (Pleasants and Oberhauser, 2012). Climate change is predicted to further reduce summer breeding habitat suitability (Lemoine, 2015).
Given the need to restore monarch butterfly summer breeding habitat, recent efforts have promoted milkweed propagation (Cutting and Tallamy, 2015; Dumroese et al., 2016; Landis, 2014; Landis and Dumroese, 2015; Luna and Dumroese, 2013). Much of what is known about milkweed biology and propagation is based on work done with common milkweed [Asclepias syriaca (reviewed in Bhowmik, 1994)]. Propagation protocols have also been developed to facilitate production of other milkweed species, including the federally threatened mead’s milkweed [Asclepias meadii (Baskin and Baskin, 2002)]. Our research focuses on two milkweed species, showy milkweed and narrowleaf milkweed, common to western North America, essential to the western monarch population, and in high demand for habitat restoration efforts.
Showy milkweed is a species distributed across western North America from British Columbia to Manitoba and south to Texas and is found in well-drained soil in open habitats from 0 to 1900 m elevation [U.S. Department of Agriculture (USDA), 2017a]. Narrowleaf milkweed has a much smaller range, covering California, Idaho, Nevada, Oregon, Washington, Utah, and Baja California and is found in dry, barren areas from 50 to 2200 m elevation (USDA, 2017b). Published protocols for the two species use a variety of seed cleaning techniques, either with or without a stratification treatment, and employ an assortment of containers and media, either with or without fertilizer amendments (Bartow, 2006; Leigh et al., 2006; Skinner, 2008; Tilley, 2016). In one protocol for narrowleaf milkweed, seeds were sown directly into flats (1.5 inches deep) containing soilless media, which were then stored in an outdoor coldframe from autumn though the following spring (Leigh et al., 2006). Transplantable seedlings developed within ≈8 weeks of germination and were transferred to a variety of larger pots containing the same soilless media, where they developed into seedlings with fibrous root systems (10–20 cm in length) by the end of the 8-month growing season. In one protocol for showy milkweed, seeds were sown into 10-inch3 containers filled with soilless media amended with fertilizer (Bartow, 2006). Under controlled greenhouse conditions, seeds germinated within 1 week (80% germination). Developing plants were hand-watered as needed and received 20N–8.8P–16.6K water-soluble fertilizer (JR Peters, Allentown, PA) twice during the 5-month growing period. Skinner (2008) sowed showy milkweed seeds directly into 10-inch3 containers filled with soilless media and noted germination was complete within 2 weeks. Plants were watered deeply every 2 d and fertilized (formulation unspecified) weekly for ≈3 months. Tilley (2016) provided a third protocol for showy milkweed, directly sowing untreated seeds into 10-inch3 containers filled with soilless media without fertilizer in a controlled greenhouse environment, providing 20 min of daily irrigation from overhead sprinklers for the first 30 d. Plants were then fertilized [15N–13.1P–12.5K (Miracle Grow All Purpose Plant Food; Scotts, Marysville, OH)] weekly and watered 40–60 min every 2 d for ≈4 months. Although protocols vary, one commonality is the use of small volume (i.e., 10 inch3) containers. Information on the production of milkweed by directly sowing seeds into larger volume containers is lacking.
In restoration plantings, particularly where competition is a factor and environmental stress limits establishment success, larger seedlings may be superior because of the competitive advantage conferred by greater heights and increased root growth potential associated with larger root systems (Grossnickle, 2012). Evidence suggests that not only do larger volume containers yield larger nursery seedlings, but these larger seedlings also have higher survival rates and continue to realize greater growth rates in the field after planting (Pinto et al., 2015; Sutherland and Day, 1988). Similar to increased container volume, identifying optimal species-specific fertilizer application rates will aid in the propagation of superior seedlings in the nursery (Cardoso et al., 2007; Clark and Zheng, 2015), which may confer these same advantages on outplanting. However, a concern in the production of container milkweed plants from seeds is the formation of a firm root plug; i.e., one in which the root system remains intact when removed from the container (Landis, 2014). Because milkweed plants develop rhizomes without many fibrous roots, transplanting or outplanting seedlings can be challenging if root systems are poorly developed. Thus, the objectives of this research are 1) to develop propagation techniques that yield firm plugs for two species of milkweed common to western North America through the manipulation of container volume and fertilizer application rate, and 2) to assess first year field survival of these seedlings.
Bartow, A.L. 2006 Propagation protocol for production of container (plug) Asclepias speciosa Torrey plant plugs. 6 Dec. 2016. <https://npn.rngr.net/renderNPNProtocolDetails?selectedProtocolIds=asclepiadaceae-asclepias-2301>
Baskin, J.M. & Baskin, C.C. 2002 Propagation protocol for production of container (plug) Asclepias meadii Torr. ex Gray plants. 24 Apr. 2017. <https://npn.rngr.net/renderNPNProtocolDetails?selectedProtocolIds=asclepiadaceae-asclepias-1828>
Brower, L.P. 1995 Understanding and misunderstanding the migration of the monarch butterfly (Nymphalidae) in North America: 1857–1995 J. Lepidopterists’. Soc. 49 265 276
Brower, L.P., Castilleja, G., Peralta, A., Lopez-Garcia, J., Bojorquez-Tapia, L., Diaz, S., Melgarejo, D. & Missrie, M. 2002 Quantitative changes in forest quality in a principal overwintering area of the monarch butterfly in Mexico, 1971–1999 Conserv. Biol. 16 346 359
Brower, L.P., Taylor, O.R., Williams, E.H., Slayback, D.A., Zubieta, R.R. & Ramirez, M.I. 2012 Decline of monarch butterflies overwintering in Mexico: Is the migratory phenomenon at risk? Insect Conserv. Divers. 5 95 100
Cardoso, G., Cerny-Koenig, T., Koenig, R. & Kjelgren, R. 2007 Characterizing fertilizer and media pH requirements for greenhouse production of intermountain west native herbaceous perennials Native Plants J. 8 114 121
Casner, K.L., Forister, M.L., O’Brien, J.M., Thorne, J., Waetjen, D. & Shapiro, A.M. 2014 Contribution of urban expansion and a changing climate to decline of a butterfly fauna Conserv. Biol. 28 773 782
Clark, M.J. & Zheng, Y. 2015 Use of species-specific controlled-release fertilizer rates to manage growth and quality of container nursery crops HortTechnology 25 370 379
Cutting, B.T. & Tallamy, D.W. 2015 An evaluation of butterfly gardens for restoring habitat for the monarch butterfly (Lepidoptera: Danaidae) Environ. Entomol. 44 1328 1335
Dingle, H., Zalucki, M.P., Rochester, W.A. & Armijo-Prewitt, T. 2005 Distribution of the monarch butterfly, Danaus plexippus (L.) (Lepidoptera: Nymphalidae), in western North America Biol. J. Linn. Soc. Lond. 85 491 500
Dumroese, R.K., Montville, M.E. & Pinto, J.R. 2015 Using container weights to determine irrigation needs: A simple method Native Plants J. 16 67 71
Dumroese, R.K., Luna, T., Pinto, J.R. & Landis, T.D. 2016 Forbs: Foundation for restoration of monarch butterflies, other pollinators, and greater sage-grouse in the western United States Nat. Areas J. 36 499 511
Flockhart, D.T.T., Pichancourt, J.-B., Norris, D.R. & Martin, T.G. 2015 Unravelling the annual cycle in a migratory animal: Breeding-season habitat loss drives population declines of monarch butterflies J. Anim. Ecol. 84 155 165
Hartzler, R.G. 2010 Reduction in common milkweed (Asclepias syriaca) occurrence in Iowa cropland from 1999 to 2009 Crop Prot. 29 1542 1544
Krischik, V.A., Landmark, A.L. & Heimpel, G.E. 2007 Soil-applied imidacloprid is translocated to nectar and kills nectar-feeding Anagyrus pseudococci (Girault) (Hymenoptera: Encyrtidae) Environ. Entomol. 36 1238 1245
Landis, T.D. 2014 Monarch waystations: Propagating native plants to create travel corridors for migrating monarch butterflies Native Plants J. 15 5 16
Landis, T.D., Tinus, R.W. & Barnett, J.P. 1998 The container tree nursery manual. Vol. 6, Seedling propagation, p. 125–163. In: R.G. Nisley (ed.). Agriculture Handbook 674. U.S. Dept. Agr., For. Serv., Washington, DC
Leigh, M., Pushnik, J.C., Boul, R.D., Brown, M.R., Hunt, J.W. & Koenig, D.A. 2006 Propagation protocol for production of container (plug) Asclepias fascicularis plants potted nursery stock. 6 Dec. 2016. <https://npn.rngr.net/renderNPNProtocolDetails?selectedProtocolIds=asclepiadaceae-asclepias-3087>
Lemoine, N.P. 2015 Climate change may alter breeding ground distributions of eastern migratory monarchs (Danaus plexippus) via range expansion of Asclepias host plants PLoS One 10 e0118614
Luna, T. & Dumroese, R.K. 2013 Monarchs (Danaus plexippus) and milkweeds (Asclepias species): The current situation and methods for propagating milkweeds Native Plants J. 14 5 16
Malcolm, S.B., Cockrell, B.J. & Brower, L.P. 1989 Cardenolide fingerprint of monarch butterflies reared on common milkweed, Asclepias syriaca L J. Chem. Ecol. 15 819 853
Pinto, J.R., Davis, A.S., Leary, J.J. & Aghai, M.M. 2015 Stocktype and grass suppression accelerate the restoration trajectory of Acacia koa in Hawaiian montane ecosystems New For. 46 855 867
Pleasants, J.M. & Oberhauser, K.S. 2012 Milkweed loss in agricultural fields because of herbicide use: Effect on the monarch butterfly population Insect Conserv. Divers. 6 135 144
Scoggins, H.L., Bailey, D.A. & Nelson, P.V. 2001 Development of the press extraction method for plug substrate analysis: Quantitative relationships between solution extraction techniques HortScience 36 918 921
Skinner, D.M. 2008 Propagation protocol for production of container (plug) Asclepias speciosa Torr. plants. 6 Dec. 2016. <https://npn.rngr.net/renderNPNProtocolDetails?selectedProtocolIds=asclepiadaceae-asclepias-3487>
Stevens, S.R. & Frey, D.F. 2010 Host plant pattern and variation in climate predict the location of natal grounds for migratory monarch butterflies in western North America J. Insect Conserv. 14 731 744
Sutherland, D.C. & Day, R.J. 1988 Container volume affects survival and growth of white spruce, black spruce, and jack pine seedlings: A literature review North. J. Appl. For. 5 185 189
Tilley, D. 2016 Propagation protocol for production of container (plug) Asclepias speciosa Torr. plants. 6 Dec. 2016. <https://npn.rngr.net/renderNPNProtocolDetails?selectedProtocolIds=asclepiadaceae-asclepias>
U.S. Department of Agriculture 2017a The PLANTS database. 3 Jan. 2017. <https://plants.usda.gov/core/profile?symbol=ASSP>
U.S. Department of Agriculture 2017b The PLANTS database. 3 Jan. 2017. <https://plants.usda.gov/core/profile?symbol=ASFA>