Dr. Stephen B. Malcolm, professor at Western Michigan University, has been studying monarch butterflies in the field for 28 years, recently with support from National Geographic’s Committee for Research and Exploration. He can tell you all about the monarchs passing through your garden this spring — and some of their mysterious cousins in South America.
Q: Monarchs are floating through neighborhoods across the United States this spring. Where are they coming from and where are they going?
Almost every monarch butterfly east of the Rocky Mountains spends the winter near the tops of forest-clad mountains in central Mexico, about 100 kilometers west of Mexico City. Now that we’ve just passed the spring equinox, the tight aggregations of butterflies are breaking up and the monarchs are flying north to the abundant milkweeds across the southern United States, from Texas to Florida. These butterflies are looking for patches of milkweeds, where mated females will lay their eggs and most of them will die, leaving their offspring to continue to find and use milkweeds distributed all the way to southern Canada.Photo by Stephen B. Malcolm
Top Photo: Basking male and female Danaus plexippus — the northern monarch species — in Michoachan, Mexico. The northern and southern species appear almost identical, except that the trailing edge of the northern type’s forewing is black, while in the southern species it’s orange. Northern monarchs are migrating from Mexico to the northern United States and Canada this spring.
Bottom photo: A male Danaus erippus (southern monarch) feeds on nectar near Tucuman, Argentina. Do these southern monarchs migrate?
Q: What kinds of hurdles do these migrating insects face throughout their range, and are they struggling as a species?
Monarchs do not appear to be struggling as a species because they have a worldwide distribution, but they are confronted by a series of hurdles throughout their life histories. These include both legal and illegal forestry activities in Mexico at the overwintering locations and intensive agriculture in the U.S., Canada and northern Mexico, where monarchs are impacted by both genetically modified crops and the use of insecticides and herbicides.
We also think that the butterflies are being impacted by global climate change both through shifts in available carbon dioxide that change milkweed chemical defenses and changes in temperature regimes that lead to depletion of adult butterfly fat reserves. Increasing urbanization of North America as human populations increase also leads to disappearance of critically important milkweed habitat and milkweed host plant diversity.
Q: What can the average person do to provide support to the monarchs that cross their path?
The most popular measures include “butterfly gardening” — planting attractive milkweed host plants for larvae and flowering nectar resources that help adults store the fat they need for migration and overwintering. But it is important to plant milkweed and nectar plants that occur in your neighborhood rather than species that you can buy online or might have collected elsewhere.
Different milkweeds and nectar plants influence the success of monarchs, and we think that they are responsible for the extraordinary migratory life history of monarchs. For example, in the northern U.S., it makes most sense to grow the common milkweed, Asclepias syriaca, which has a delightful scent and very showy flowers. They can be quite invasive, but in a dedicated butterfly garden with barriers to stop the underground spread of the unusual stems of this species, the plant can be controlled.
Other attractive species include butterfly weed, Asclepias tuberosa, and swamp milkweed, Asclepias incarnata, but these two species have regional forms, and it may be important to use local seed.
In the southern U.S., perhaps the best species is the green milkweed, Asclepias viridis. The most easily grown milkweed species is Asclepias curassavica, also known as blood flower for its showy red and yellow flowers, but this species is really from South America, and I don’t think it should be used intentionally to feed wild monarch larvae in North America.
Photo: Larva of Danaus erippus — the southern monarch — with Danaus gilippus, a relative of the monarch, feeding on a common milkweed plant near Campinas, Brazil.
Q: Your research especially focuses on the southern monarch found in South America, the little-known sister species of the northern variety. When was this species discovered and how?
The southern monarch butterfly was named Danaus erippus by Cramer in 1775, but until 2007 it was thought to be a subspecies of the northern monarch, Danaus plexippus, because the two butterflies are startlingly similar in appearance, the only difference apparently being that the trailing edge of the forewing of plexippus is black, but in erippus it is orange.
However, in 2007 the two species were shown to be distinct, although they are extremely closely related. So because the two species are very brightly colored and common, they have been known since Linnaeus started to catalog species diversity in Sweden in the 18th century. However, unlike plexippus, which has received a large amount of attention from researchers, almost nothing is known about erippus in South America.
We know a great deal about migration in plexippus and overwintering in both Mexico and along the coast of California and the seasonal movements of this handsome butterfly. However, for erippus in South America, we only know from a series of anecdotal papers by Hayward that the butterfly shows seasonal movements in Argentina.
In North America, monarchs exploit approximately 120 species of Asclepias milkweeds as larval host plants, but in South America monarchs only have access to six or perhaps eight species of Asclepias. The butterflies also fly south in the autumn along the eastern edge of the Andes Mountains in Argentina, a very puzzling phenomenon that takes them to colder latitudes for the winter.
Last April we spent two weeks watching 40,000 to 50,000 monarchs a day flying south along the foothills of the Andes, just west of Tucumán in north western Argentina. So far, we haven’t a clue about where they are going for the winter, although we have seen them clustering and nectaring like northern monarchs in the valleys of the eastern Andes near Tucumán. In Bolivia we think that the monarchs may show an altitudinal migration that may perhaps also result in some butterflies migrating south into Argentina for the winter.
Photos: A male Danaus erippus (southern monarch) and a female (lower photo) warm up before setting out to find nectar sources near Tucuman, Argentina. The female southern monarch’s wing color is darker than that of the male.
Q: Unlike the northern monarch, the southern monarch has a very low diversity of available host plants. What is the comparison of the two species’ life histories showing about the role of plant diversity and dispersion in the evolution of migration?
As indicated above, northern monarchs have a very wide choice of available milkweed species, and we think that the northern, common milkweed, Asclepias syriaca, is very important to monarch breeding and migration success. We also think that southern milkweed species are important for the effective defense of the butterflies and their larvae against predators such as birds, mice, wasps, ants and perhaps parasites because the butterfly larvae eat and store toxic chemicals called cardenolides that they use to poison natural enemies. This is perhaps the main reason why monarch butterflies are so brightly colored — by advertising their toxicity they are largely left alone by enemies.
In our research in South America, we are hypothesizing that these factors are also important. For example, we have found the southern monarch exploiting a very toxic, high-altitude milkweed called Asclepias barjoniifolia, at 4,000 meters altitude. It would be exciting if the distribution of this plant species encourages altitudinal migration in southern monarchs because butterflies that feed on this plant are especially toxic to predators.
Photo: Professor Stephen B. Malcolm of Western Michigan University near Sucre, Bolivia, with a favorite plant of the southern monarch butterfly. Malcolm is on the trail of the southern monarch, only recently identified as a separate species from the northern, closely related sister species known in the United States.