Montana Field Guide

Monarchs are butterflies (order Lepidoptera) in the family Nymphalidae. Originally, six subspecies were named and Danaus plexippus plexippus is the only subspecies in North America. The other subspecies were described from South America (D. p. nigrippus), Puerto Rico (D. p. portoricensis), Virgin Islands (D. p. leucogyne), Tobago (D. p. tobagi), and the Caribbean and Central America (D. p. megalippe). Some disagreement still exists as to the number of subspecies and geographic distribution of Monarch subspecies, therefore Monarchs are treated at the species level under the ESA (Federal Register 2020, USFWS 2020). Western and eastern Monarchs in North America are highly related based on recent genetic evidence (Lyons et al. 2012).

Migrating Monarchs east of the Rocky Mountains are well-documented and better understood than populations west of the Rocky Mountains (Dingle et al. 2005). In general, eastern inland Monarchs converge in Mexico each fall and overwinter, and Monarchs on the east coast migrate to Florida to overwinter (Knight and Brower 2009). On the other hand, most western populations move to southern California or congregate in Mexico to overwinter. As temperatures begin to rise in the spring, Monarchs return to their summer habitats in the northern US or southern Canada migrating =3600 km taking about 75 days to make the journey (Brower 1996). Three to five generations of butterflies make the journey annually with individuals stopping at Milkweeds to lay eggs and blooming flowers to feed along the way.

Monarchs are large butterflies with a wingspan of 8.5-12.5 mm (3.5-5.0 inches). Their wings are orange with black veins and white spots around the margin. Their large size and distinctive coloration make them one of the most well-known butterflies in the US. Males can be differentiated from females by a black dot in the hind wing vein.

Species overwinters in aggregations in coastal California, Florida and Mexico. In early, spring individuals move northward over several flights to summer range. In late summer or fall a final flight moves south to overwintering sites (Scott 1986, Glassberg 2001, and Pyle 2002). Majority of southward-moving individuals west of the continental divide probably overwinter in coastal California, southward-moving individuals east of the divide probably overwinter in central Mexico, but still little data to support this.

Open places, native prairie, foothills, open valley bottoms, open weedy fields, roadsides, pastures, marshes, suburban areas, rarely above treeline in alpine terrain during migration (Scott 1986, Opler and Wright 1999, Glassberg 2001 and Pyle 2002). Reported in Glacier National Park, Montana in mesic montane meadows (Debinski 1993).

Overwintering sites in southern Mexico are much smaller than summer habitats, but the sites must have sufficient amounts of nectar for initiating and completing migration (Alonso-mejia et al. 1997). During the Monarch’s breeding and migration season, they require trees for roosting, and Milkweeds (Asclepias sp.) to lay eggs and blooming flowers with nectar for adults to feed.

Larval food plants include Apocynum, several species of Asclepias (the primary host plant genus), Calotropis, Matelea, and Sarcostemma (Scott 1986, 1992, 2006, Guppy and Shepard 2001, and Pyle 2002). Adults feed on flower nectar (including Achillea, Apocynum, Asclepias, Aster, Buddleia, Chrysothamnus, Carduus, Cirsium, Cleome, Conyza, Cosmos, Daucus, Dipsacus, Echinacea, Echinocystis, Eupatorium, Helianthus, Hesperis, Liatris, Lonicera, Machaeranthera, Medicago, Pastinaca, Phlox, Polygonum, Ratibida, Senecio, Solidago, Sonchus, Symphoricarpos, Syringa, Tagetes, Taraxacum, Trifolium, Verbena, Vernonia, Vicia, Zinnia) and mud (Scott 1986, 2014).

Populations of Monarchs in the eastern United States decreased by 80-97% (Rendón-Salinas et al. 2015, The Xerces Society 2020) and western populations decreased 98% since 1997 (The Xerces Society 2020). The decline of Monarchs may be largely driven by fewer Milkweed (Ascelpias spp.), their larval host plant (e.g., Pleasants and Oberhauser 2013, Zaya et al. 2017) as well as agricultural intensification and pesticide use (e.g. Crone et al. 2019; Stenoien et al. 2018).

Females lay eggs (200 or more) singly under host plant leaves, stems, and inflorescences. Eggs hatch in about 6 days (depending on temperature). Growth is rapid, to L5 and pupation in 18 days after egg-hatch. Adults emerge from pupae (eclose) in about 9-10 days. Larvae build no nests; L1-L2 instars hide in terminal shoots or under leaves, L3-L5 rest openly. Males patrol throughout the day near host plants in search of females; mating may occur at roost sites (Scott 1975b, 1986).

In a news release on December 15, 2020 the US Fish and Wildlife Service announced that after a review of available data on the species' status, the Monarch Butterfly warranted protection under the Endangered Species Act but listing was precluded by higher priority listing actions. The species is a candidate for listing and its status will be reviewed by the service on an annual basis. For more information on this species please visit the USFWS's Monarch Status Page or Monarch web page.

In a news release on December 15, 2020, the US Fish and Wildlife Service announced that after a review of available data on the species' status, the Monarch Butterfly warranted protection under the Endangered Species Act but listing was precluded by higher priority listing actions. The species is a candidate for listing and its status will be reviewed by the service on an annual basis. For more information on this species please visit the USFWS's Monarch Status Page (USFWS 2020) or Monarch web page.