In the fall of the year I turned eight, I remember my mom running into the house one afternoon and yelling for me to come outside. Her urgency made me think something was wrong, so I ran to meet her in the driveway. But instead of an emergency I found serendipity. Hundreds, maybe thousands, of monarch butterflies fluttered through the air, alighting on weeds, bushes, and the blacktop, looking for food, water, and rest. We laid down in the driveway and watched the butterflies move around us, light as dandelion seeds carried in the wind. As quickly as they came, they left; fluttering south toward their ancestral home in central Mexico.
Monarch butterflies (Danaus plexippus) migrate, over several generations, from mountain forests in central Mexico to the most northern borders of the continental United States (Figure 1). They are one of relatively few insect species to migrate such long distances and consistently demonstrate philopatry (a tendency to return to the same place). Along their migratory route, monarchs reproduce several times, often taking multiple generations to travel between their overwintering and breeding habitats.
To travel such long distances requires plenty of food resources, and a favorite of monarchs is milkweed (Asclepias species; Figure 2). Monarchs are specialists (meaning they have a narrow diet range), so these plants serve as important habitat and food, especially for larvae or young monarchs. In recent years monarch populations have steadily declined, in part due to less milkweed habitat available for monarchs. To help reduce habitat loss, many organizations encourage growing milkweed plants in residential areas to provide habitat for monarchs. However, less native milkweed habitat and widespread planting of exotic milkweed species (like Asclepias curassavica; Figure 3) may be leading to declines in monarch populations and changes in migration patterns. (Learn more about how monarchs use milkweed plants at this link or in this video.)
Exotic milkweeds species like A. curassavica bloom year-round, which makes them attractive as ornamentals for residential gardens. However, year-round food resources are encouraging monarchs to stay overwinter in warm regions like Florida, Louisiana, and Texas instead of migrating to central Mexico. A study by Satterfield et al. (2015) showed that sedentary (or non-migratory) monarch populations that overwinter and forage on exotic milkweeds are much more vulnerable to diseases like parasite infection than monarchs that complete the yearly migration to central Mexico.
As temperatures across the southern United States increase with climate change, simply feeding on these exotic milkweeds may also become harmful to monarch populations. According to researcher Matt Faldyn, “monarchs do really well on the exotic milkweed species that’s being widely sold and planted under current environmental conditions. But under warmer conditions, the exotic plant becomes too toxic and monarchs become [less healthy]”.
His recent study examined how changing temperatures consistent with climate change impact monarchs feeding on exotic and native milkweed plants. Under current temperatures, compounds in milkweed called cardenolides help protect butterflies from predators and parasites. The exotic milkweed A. curassavica has more of these cardenolides than many native milkweed species like Asclepias incarnata, so monarchs that fed on the exotic milkweed in this study had a higher level of cardenolides and therefore greater protection against predators. However, as A. curassavica plants were exposed to higher temperatures, they produced even more cardenolides that became potentially harmful to butterflies and larvae. As cardenolides increased, monarch health and survival decreased. Increasing temperatures also led to smaller forewings (or wings closer to the head) in butterflies. This decreased wing length may make migration more difficult for future populations.
So how can we improve habitat for current and future monarch populations?
While tropical milkweed (A. curassavica) provides habitat and food for monarchs in the southern United States, it may be compromising the long-term health of the species. To improve habitat for monarchs, Faldyn recommends increasing the diversity or variety of milkweed species that are planted. This may require garden suppliers to carry more of the 73 native species that grow in the United States, like Larger scale restoration of native milkweed species near agriculture fields may also significantly improve habitat for migrating monarchs.
If you are interested in learning more about native milkweed species in your region, you can find more information here at the Xerxes Society for Invertebrate Conservation.
Featured Image: Monarch butterflies wintering in a forested area of Michoacan, Mexico. Image courtesy of Ashley Riane Booth.