The monarch butterfly is one of the most familiar butterflies of the garden. This butterfly makes a heroic migration, over the lives of several generations, from forests in central Mexico where it overwinters, to summer breeding grounds as far north as southern Canada. The Commission for Environmental Cooperation’s North American Monarch Conservation Plan. published in 2008, states that the “North American migration is considered an endangered biological phenomenon due to threats to the monarch’s habitats during its annual cycle of breeding.”
Monarch butterflies lay their eggs on common milkweed (Asclepias syriaca). The Monarchs progress from egg to larva to chrysalis to butterfly over about 5 weeks. In the spring, the first generation, derived from eggs laid by females who probably overwintered in Mexico, appear in Northern Mexico and the SE United States. There may be three to four more of the summer generations. The adult monarchs present at the end of the summer in the colder climates of the north must retreat to the more hospitable south. According to the Monarch Butterfly Fund, the autumn Monarchs are born in a reproductive state of suspended development called diapause in which the internal sex organs are not mature. In late August through October, they will fly to mountain ranges in central Mexico, living on nectar and helped by warm air currents to cover a distance of up to 2,000 miles.
Once in Mexico, they will settle on the branches of Oyamel fir trees on south-facing mountain slopes and go into a state of torpor, reserving energy until the warmer weather of spring arrives, generally in March. The Monarchs emerge from torpor, end diapause, and again repeat the breeding cycle.
Unfortunately, loss of milkweed habitat due to development of pesticide-resistant crops and pesticide use on agricultural lands has greatly impacted Monarch breeding potential. Suburbanization of agricultural lands has also contributed to habitat loss. Diversion of water in the overwintering sites has increased the distance the Monarchs must fly to reach it, using up valuable body food stores. Forest fires and degradation of habitat by poorly regulated tourist visits may also be contributing to difficulties for overwintering Monarchs.
What can we as gardeners do to help the Monarchs? We can plant common milkweeds for breeding sites, and abundant nectar-producing plants for food sources. A quick internet search will turn up sources for Milkweed seeds and seedlings in our area. Working together, we can aid in restoration of habitat and provision of nectar flowers for our beautiful butterflies.
By Mary Beth Cooper, Denver County Master Gardener