title: Monarchs in decline: a collateral landscape-level effect of modern agriculture.
abstract: We review the postulated threatening processes that may have affected the decline in the eastern population of the monarch butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae), in North America. Although there are likely multiple contributing factors, such as climate and resource-related effects on breeding, migrating, and overwintering populations, the key landscape-level change appears to be associated with the widespread use of genetically modified herbicide resistant crops that have rapidly come to dominate the extensive core summer breeding range. We dismiss misinterpretations of the apparent lack of population change in summer adult count data as logically flawed. Glyphosate-tolerant soybean and maize have enabled the extensive use of this herbicide, generating widespread losses of milkweed (Asclepias spp.), the only host plants for monarch larvae. Modeling studies that simulate lifetime realized fecundity at a landscape scale, direct counts of milkweeds, and extensive citizen science data across the breeding range suggest that a herbicide-induced, landscape-level reduction in milkweed has precipitated the decline in monarchs. A recovery will likely require a monumental effort for the re-establishment of milkweed resources at a commensurate landscape scale.
authors.affiliation: Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota, Australia.
authors.forename: Carl
authors.initials: C
authors.lastname: Stenoien
authors.affiliation: Conservation Biology Graduate Program, University of Minnesota, St. Paul, Minnesota, USA.
authors.forename: Kelly R
authors.initials: KR
authors.lastname: Nail
authors.affiliation: School of Environmental Studies, Griffith University, Brisbane, Queensland, Australia.
authors.forename: Jacinta M
authors.initials: JM
authors.lastname: Zalucki
authors.affiliation: CSIRO, Ecosystems Sciences Precinct, Brisbane, Queensland, Australia.
authors.forename: Hazel
authors.initials: H
authors.lastname: Parry
authors.affiliation: Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, St. Paul, Minnesota, USA.
authors.forename: Karen S
authors.initials: KS
authors.lastname: Oberhauser
authors.affiliation: School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia.
authors.forename: Myron P
authors.initials: MP
authors.lastname: Zalucki
mesh_terms: D000818:Animals; D031108:Asclepias / Q000187:drug effects; D002080:Butterflies; D000069599:Crop Production / Q000379:methods*; D004032:Diet; D005998:Glycine / Q000009:adverse effects / Q000031:analogs & derivatives; D006540:Herbicides / Q000009:adverse effects; D007814:Larva; D011157:Population Dynamics; D000097797:Glyphosate
chemical_list: D005998:Glycine; D006540:Herbicides; D000097797:Glyphosate
keywords: Asclepias; Danaus plexippus; citizen science; conservation; migration; transgenic GM crops