Blooming Forbs Utilized by Breeding-Season Danaus plexippus in the USA North-Central Region

ABSTRACT. Danaus plexippus (monarch butterfly) populations in North America east of the Rocky Mountains are in decline due to loss of habitat containing Asclepias spp. (milkweed) and blooming forbs. To support D. plexippus recovery, habitat restoration efforts use seed mixes designed to maintain a diversity of species that bloom over the breeding and migration periods. The extent to which these species are uniquely attractive to foraging D. plexippus is unclear. We conducted observational studies at areas in Iowa USA with varying densities and diversities of Asclepias spp. and floral resources to help inform conservation management practices. Forty-two blooming forb species were present across four sites (2-4-ha) in Boone and Story counties from June through September in 2020. Many of the species present are commonly found in pollinator seed mixes. The number of blooming forbs species per site varied from 12 to 24 across the four months; the minimum and maximum density of species-specific inflorescences ranged from 8.45 to 36.3 inflorescence/m2. We observed D. plexippus nectaring on 27 of the 42 species across the sites. Based on total feeding events observed and the species-specific density of inflorescences, Asclepias syriaca, Asclepias verticillata, Helianthus grosseserratus, and Cirsium vulgare were favored for nectaring at each site where they were present. Additional species were favored in some, but not all, sites including Symphyotrichum pilosum, Eupatorium altissimum, Trifolium pratense, Echinacea pallida, and Verbena urticifolia. Findings suggest variation in preference may be due, in part, to site-specific and surrounding landscape features. Future studies on D. plexippus foraging behavior could address the interacting roles of habitat and landscape features, as well as species-specific flower size and shape, scent, nutrition, and nectar abundance. While forb utilization is likely influenced by site- and plant-specific features, preferred species we observed are typically included in recommended native seed mix designed to ensure blooms from June through September to support migratory and non-migratory, breeding D. plexippus.