Fall-Migrating Monarchs

If you read gardening articles regularly, you probably already know about the plight of the monarch butterfly (Danaus plexippus). Its population is dwindling to dangerously low levels. There are several reasons that this charismatic butterfly is in danger of extinction, and knowing the reasons can provide us a chance to help reverse the trend.  

The unusual life cycle of the monarch butterfly includes an overwintering segment in which the adults are not reproductively active. Once they complete their fall migration to a small area of mountainous fir forests in south central Mexico, they enter a resting state. They cluster tightly on the trunks and branches of the trees and use as little energy as possible during the winter months. They are at risk because illegal logging is destroying this overwintering habitat which is special to them for its unique microclimate. It is interesting to note that species that have special environmental features in their life cycles are usually the ones endangered. Generalist species do quite well within a human-altered environment.

As day length increases and the sun moves north in the sky, the adults with adequate energy mate. Females lay their eggs after they have traveled north and reached areas with sufficient milkweed. Milkweed is their host plant: the only plant that can nurture monarch larvae.

Once they migrate north into the U.S. and Canada, monarchs face danger from lack of milkweed. There are several species of milkweed which support monarch caterpillars, including the weedy-looking common milkweed (Asclepias syriaca) and the iconic orange butterfly weed (Asclepias tuberosa). These generally grow in open, disturbed habitats, like the edge of farm fields. Unfortunately, the use of herbicides on farm fields often kills this host plant along the field edges.

We can help the monarchs thrive by carving out and creating butterfly gardens containing both host and nectar plants. We will also be rewarded with the opportunity to watch their entire life cycle play out before us. The eggs laid on milkweed hatch to become caterpillars. What a joy it is to watch them feed on the milkweed leaves! The caterpillars grow until they turn themselves into a 'J' and hang from a leaf. They encase themselves in a chrysalis and then finally emerge from the chrysalis as an adult. I know gardeners who sequester their caterpillars just before they are ready to pupate (that is, become a chrysalis) to keep them out of harm's way. This also allows them to enjoy watching the process up close.

While it is important to provide milkweed for the caterpillars, it is equally important to provide the adults with nectar food, especially as they get ready for their migration. The flight to Mexico is grueling, even though they try to make most of the trip soaring on favorable winds, reducing the need to burn their fat reserves. Along the way, they often roost, resting and feeding, when weather for traveling is unfavorable. Once back in their fir forests in Mexico, the butterflies with adequate fat reserves are able to survive the winter better and are more reproductively fit when spring returns.

Among the best nectar-producing plants are those belonging to the aster family (Asteraceae). There are several perennial species of this family that we associate with pollinators, such as black-eyed Susan (Rudbeckia hirta), purple coneflower (Echinacea purpurea), and several varieties of aster (Symphyotrichum). Annuals of this group, such as zinnias (Zinnia)and Mexican sunflower (Tithonia rotundifolia), persist all summer and into the fall, thus providing a steady supply of nectar.

Will the monarch butterfly disappear completely if their forest is lost? Perhaps not! While we want to help ensure the survival of these migrating butterflies, there are also non-migrating populations in Florida and on the Gulf coast, as well as west coast monarchs that overwinter in California. The Gulf monarchs are year-round residents and show some genetic differences from migration populations, but they are still considered members of the same species that grace our gardens every summer and fall.