Each fall, millions of monarch butterflies (Danaus plexippus plexippus) migrate to the sacred fir (Abies religiosa) forests of central Mexico. Monarch butterflies have a very narrow temperature range in which they can live and reproduce, causing them to seek the cooler temperatures and increased food available in northern locales (such as Toledo) in the spring, and repeat the migration south in the fall when frosts eliminate their food sources.
This migration takes five or more generations each year. In other words, it may be the great-great-great grandchildren of the spring monarchs that return to Mexico the following fall. This migration of 3,000+ miles is the largest and longest migration by invertebrates on Earth, and it is unique to monarchs east of the Rocky Mountain range. Although monarch populations west of the Rockies do migrate (to California and isolated non-migratory populations in Florida, Cuba, Hawaii, New Zealand and Australia), none occur in such vast quantities as the eastern North American monarchs.
Unfortunately, over the past 15 years, monarch populations have shown huge declines as illegal logging threatens their overwintering range, and milkweed (the monarch host plant, Asclepias spp.) becomes increasingly rare across their northern summer range. Monarchs face numerous challenges, whether it be global climate change that results in earlier warming, more severe temperature extremes or the increased reliance on herbicides across the American Midwest which destroys the milkweed plants the monarchs rely on to survive.
In recognition of the monarchs’ plight, the Toledo Zoo has started breeding the butterflies for release. Early this summer, 25 eggs were collected from milkweed plants at the Zoo and reared on milkweed grown by Wild Toledo staff. These individuals were then bred and their resulting offspring reared.
As the daylight hours decrease and temperatures cool, the most recent generation receives biological cues that it is time to migrate. The Toledo Zoo is releasing more than 100 of these monarchs into the wild to complete their migration to Mexico.
In addition, partnering with Monarch Watch at the University of Kansas, each monarch released carries a specially designed tag. As the released monarchs make their migration, citizen scientists who spot the tagged butterflies can email or call and the university to report the unique ID each butterfly carries. This not only allows us to track our butterflies to Mexico, but it provides researchers with valuable information about the timing and direction of the great monarch migration.