Overwintering Monarchs are protected by Oyamel firs: Brower et al., 2009

Journal: Insect Conservation and Diversity
Location: Sierra Chincua and Cerro Pelon, Mexico

Monarch butterflies overwinter in the Sierra Chincua and Cerro Pelon, Mexico, for a period of 154 days. During this time, they cluster on oyamel fir trees, which are on average 1.4 °C warmer than surrounding temperatures at night (protecting from freezing) and 1.2 °C cooler during the day (slowing the loss of fat stores). Older trees (with wider trunks) had more beneficial microclimates.

Monarch Butterfly – Danaus plexippus
Oyamel Fir – Abies religiosa

Brower LP, Williams EH, Slayback DA, Fink LS, Ramirez MI, Zubieta RR, Garcia MIL, Gier P, Lear JA, van Hook T, 2009. “Oyamel fir forest trunks provide thermal advantages for overwintering monarch butterflies in Mexico.” Insect Conservation and Diversity, DOI: 10.1111/j.1752-4598.2009.00052.x
Affiliations: Sweet Briar College, VA; Hamilton College, NY; NASA Goddard Space Flight Center; Universidad Nacional Autonoma de Mexico (UNAM); Huntingdon College, AL; National Cathedral School, Washington, DC.