One study concluded that «reducing the negative effects of host plant loss on the breeding grounds is the top conservation priority to slow or halt future population declines of monarch butterflies in North America.» (treehugger.com)
Milkweeds are the essential host plant for monarch caterpillars. (xerces.org)
Dissertation: The influence of intraspecific variation in host plants on arthropod and fungal endophyte communities — Joshua G. Harrison — University of Nevada (microbiomedigest.com)