We review the postulated threatening processes that may have affected the decline in the easte 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.