Monarch Migration and Milkweed

Photo by Tonya Bramlage

Dedicated pollinator gardeners readily know that cutting back milkweed stalks in the late fall or early winter is best done after they have produced seed pods. This practice insures that seeds have had time to mature and encourages a healthy growing cycle. Many flowers, especially native plants, are terrific sources of nectar for butterflies, but milkweed leaves are the only food monarch caterpillars eat. In fact, the monarch butterfly is often referred to as the “milkweed butterfly”. Gardners recommend leaving at least six inches of milkweed stalks to provide habitat for insects throughout the winter and serves as a natural marker for you to identify the location of your milkweed patch. 

The milkweed plant provides all the nourishment to transform the monarch caterpillar into an adult butterfly. But what does the monarch do for the milkweed? Monarch butterflies (Danaus plexippus) and milkweed are a prime example of commensalism. As the adult butterfly flutters from flower to flower looking for food, it helps to pollinate the milkweed; allowing it to successfully produce seeds that will grow and attract more monarchs.

Common milkweed (Asclepias syriaca) has thick, large, rounded leaves and pods with silky downy seeds that seem to sprout out everywhere. Monarch butterflies lay their eggs on milkweed plants so the tiny caterpillars will have access to food the moment they hatch. Each monarch caterpillar will consume 20 or more large milkweed leaves. A four foot milkweed plant will feed only five monarch caterpillars.

The monarch’s entire life cycle revolves around the milkweed plant. Florida’s native swamp milkweed (Asclepias incarnata) with its thin leaves and showy flowers, is a definite monarch pleaser. Many people have heard about the amazing migration of North American monarch butterflies to Mexico. But Florida has a breeding population that doesn’t migrate.

A warm climate and continuous growing season keep the species hanging around throughout the year in southern Florida. Although the north and central Florida colder winter temperatures can kill them, monarchs from south Florida and those migrating from Mexico repopulate our area. Monarchs migrating from Canada and the Northeast bolster the population. 

Imagine the journey flying more than 3,000 miles to Mexico, not knowing where you can rest or where you will have your next meal. 

Monarch butterflies typically live from two to six weeks except for the last generation of the year, which can live up to 9 months. Butterflies in this last generation are members of the generation that migrates south, often called the monarch super generation. Fourth generation monarchs are born in September and October and are biologically different from previous generations. 

Fourth generation monarchs live longer and do not mate or lay eggs until the following spring, after they fly north and reach areas with milkweed. Being very fond of milkweed blossoms, monarchs will reject other flowers when these are available. The plants provide a good nectar flow. However, it is conceivably possible for the monarch and other small pollinators to become trapped in a blossom.

A cluster of butterflies is aptly called a “roost”, and that is precisely what you will see with our Southern Florida monarchs, thanks to the milkweed. Although well intended, many of the things people do to try to help migrating monarchs, such as planting nonnative milkweeds or raising monarchs for release from captivity, may actually be harming the monarch population by interfering with their ability to migrate long distances. Nonnative milkweeds have longer growing seasons, and as a result these plants may lead to more monarchs becoming infected with the OE parasite because the infectious parasite spores can build up on their leaves.

Research suggests there are two main culprits behind the monarch migration losses. The first is the increasing prevalence of a debilitating monarch parasite, which has increased tenfold since the early 2000s. This increase corresponds with increased plantings of nonnative milkweeds throughout the flyway. Many infected monarch caterpillars don’t make it to adulthood. Those that do cannot fly well and do not live long, possibly contributing to the fallout of butterflies along the migration path. The second possible driver is the release of captive-reared butterflies by well-meaning people who are hoping to aid the monarchs. 

Climate change is enabling monarchs to forgo their migration and spend their winters in warmer parts of the United States due in part to warmer temperatures and increased planting of nonnative tropical milkweed. Help for monarchs is best achieved by planting native flowering plants instead of planting tropical milkweed along the migration path in order to facilitate a safe source of nectar for traveling butterflies. Evidence shows that when monarchs are reared in a captive environment, whether indoors or outdoors, they are not as good at migrating. Why are the wintering populations declining while breeding populations are stable? With vigorous debate surrounding the health of the monarch butterfly, milkweed may offer yet another benefit by answering one of the biggest questions baffling monarch butterfly researchers.