Choosing the Right Host Plants for Monarchs

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[Blog post originally published in 2016 and archived here with 2018 updates.]

In my previous post about native and alien plant species, I mentioned Doug Tallamy’s research on how native plants supports insect populations. I wanted to follow up with a couple comments about specific host plants for monarch butterflies (Danaus plexippus). Unfortunately the number of monarchs overwintering in Mexico declined again in 2018, though numbers are still better than they were back in 2012. Supporting these insects with appropriate host plants is crucial to their survival.

A couple of the studies that I previously cited claim that butterflies sometimes lay eggs on non-native plants that are actually toxic to their larvae. When the caterpillars hatch and eat these host plants, they become sick and die. Monarchs do this as well. One of the ecologists I’ve been corresponding with recently described the problem in Maine:

We also have at least one poisonous species problem with butterflies… [M]onarch butterflies generally lay their eggs on some species of Asclepias, including our native milkweed. The larvae feed on the leaves of those plants. However, we have a recent invasive species, black swallow-wort [Cynanchum louiseae] (and are about to be invaded by pale swallow-wort), that is in that same family. Monarchs lay their eggs on swallow-wort, but the larvae do not develop/mature normally when they feed on this plant. It’s just one part of the complicated issue of the decline of this insect.

Black Swallowwort. By Jomegat (own work) [ CC BY-SA 3.0  or  GFDL ], via  Wikimedia Commons

Black Swallowwort. By Jomegat (own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons

According to Bulletin #2523 at the UMaine Cooperative Extension, black swallowwort (Cynanchum louiseae) “is a perennial vine with a single non-branching stem that grows up to six feet in length. The vines typically twine and sprawl over other vegetation and die back to the ground each year.” The plant likely came to America in the early 20th century by way of southwestern Europe.1 The Go Botany website reports that the plant’s been found in four southern Maine counties so far, including my own. Nationally its range spreads as far west as Nebraska and then jumps to California. The UMaine bulletin provides information on controlling the plant should you find it on your property. (And as always I urge you to favor mechanical methods over chemical ones when possible.) Volunteers at Marginal Way in Ongunquit, ME have apparently gathered together for the past seven years to pick and dispose of black swallow-wort seed pods in order to discourage its spread along the trail.

In addition to removing this invasive species, we can also help monarchs by propagating native host plants in the Asclepias genus. You can purchase Common milkweed seed (Asclepias syriaca) at the Wild Seed Project website, for instance. However you source the plant, just be sure it’s a milkweed species native to your particular area. Back in 2015, scientists at the University of Georgia published a paper revealing that conservationists in the United States were distributing a tropical species of milkweed, Asclepias curassavica, which blooms during cold weather. Because of the plant’s winter vigor, monarchs didn’t need to migrate south in search of suitable hosts. Instead, they stayed put in hostile climates to feed on the milkweed that was still growing. To make matters worse, the study found that the monarchs that overwintered in the southern US were much far likely to become infected with a protozoan parasite called Ophryocystis elektroscirrha than were the butterflies that migrated south. “In some winter breeding sites,” the report claims, “100% of monarchs… were infected…”2 Infected butterflies are debilitated and short-lived.

We actually experienced the lingering monarch problem at a botanical garden where I worked several years ago. The staff bred the butterflies on the grounds and planted plenty of milkweed. But come autumn, we discovered that many of the butterflies lingered instead of migrating. Apparently some of the milkweed we planted was of the tropical variety, which didn’t decline in the fall as quickly as our native species. And so the butterflies stuck around too long.

This is just one more example of how our plant choices impact our ecosystems.

  1. “Bulletin #2523, Black Swallowwort.” The University of Maine Cooperative Extension Publications. Maine Natural Areas Program and University of Maine Cooperative Extension, 2004. Web. 27 February 2016

  2. Lizzie Wade, “Plan to save monarch butterflies backfires.” Science. American Association for the Advancement of Science, 13 January, 2015. Web. 27 February, 2016