25 September 2009

Such Gracious Hosts

The milkweeds (Asclepias spp.) are an astonishing group of plants. The eighty-some species, varieties, and subspecies known to occur within the United States have flowers of nearly every color of the rainbow, ranging from magenta to red to pink to orange to yellow to green to white to cream. The genus is widespread, as at least one member of the genus is known from the flora of 49 of the 50 United States (there are no species of Asclepias in Alaska). The flower structure is quite unique, with horns and hoods in addition to petals and sepals. A few of our favorite milkweed photos from our collection are shown below.

Prairie Milkweed (Asclepias sullivantii), Chicago, Illinois

Showy Milkweed (Asclepias speciosa), Boulder, Colorado

Purple Milkweed (Asclepias purpurascens), LaPorte, Indiana

Fewflower Milkweed (Asclepias lanceolata), Orlando, Florida

Largeflower Milkweed (Asclepias connivens), Orlando, Florida

In addition to their beauty and botanical intrigue, milkweeds also play a very important ecological role. Most people are familiar with the link between milkweed and the Monarch butterfly (Danaus plexippus). A female Monarch lays eggs only on milkweed plants. After the eggs hatch, the larvae feed exclusively on milkweed. Once they are adults, Monarch butterflies feed on nectar from plants in a variety of genera, one of those being, you guessed it, Asclepias. Because the milky sap of milkweed plants is toxic and bitter, the caterpillars that eat the foliage of milkweed plants are also toxic and bitter. In fact, the cardiac glycosides present in milkweed sap actually bioaccumulate in the caterpillar, causing the toxicity levels in the caterpillar to be greater than those in the milkweed leaves. The toxins and bad taste acquired as a caterpillar carries into the adult stage of the insect. Therefore, milkweed does not only nourish the Monarch caterpillars, but it also helps to protect them as they go through their entire life cycle.

Monarch (Danaus plexippus) on Field Thistle (Cirsium discolor)

Monarch caterpillar feeding on Common Milkweed (Asclepias syriaca)

But that's nothing. The Monarch is the charismatic "poster child" for the ecological value of milkweeds (and host plants in general), but Patrick Dailey of Lewis and Clark Community College did a study in which he found 456 other insect species using milkweed plants! With all of the attention given to Monarchs, the rest of these species can easily be overlooked and underappreciated. Below are five insects that rely on milkweeds to the point that they even have "milkweed" in their common names.

Milkweed Tussock Caterpillar (Euchaetes egle) on Common Milkweed

The Milkweed Tussock Caterpillar (Euchaetes egle) is the larval stage of the Milkweed Tussock Moth. Groups of individuals of this caterpillar, which are found throughout the eastern half of North America, have been known to defoliate milkweed and dogbane (Apocynum spp.) plants late in the year. Bats, which are able to consume Milkweed Tussock Moths without experiencing a bitter taste, are a major predator of this species. To avoid being eaten by these bats, Milkweed Tussock Moths have evolved to be able to mimic the high-pitched clicking sound made by moths that do taste bad to bats.

Large Milkweed Bug (Oncopeltus fasciatus) on Butterfly Milkweed (Asclepias tuberosa)

Another insect that can be found on milkweed is the Large Milkweed Bug (Oncopeltus fasciatus), which ranges in size from 10-18 mm. This species, which is known from the eastern and southwestern United States as well as Ontario, feeds on the seeds of milkweed, and thus can often be found on the pods. As a result, Large Milkweed Beetles acquire the bitter taste and toxic qualities of the milkweed sap. Large Milkweed Bugs are also known to feed on flower nectar.

Small Milkweed Bug (Lygaeus kalmii) on Swamp Milkweed (Asclepias incarnata)

The Small Milkweed Bug (Lygaeus kalmii) ranges in size from 10-12 mm, and is found throughout most of the United States and southern Canada. This insect feeds on flower nectar and milkweed seeds, but also has been known to scavenge and hunt other insects when there isn't a lot of milkweed seed or nectar plants available. Like many other insects that feed on milkweeds, Small Milkweed Bugs are toxic to many potential predators.

Swamp Milkweed Leaf Beetle (Labidomera clivicollis) on Common Milkweed

The Swamp Milkweed Leaf Beetle, or Milkweed Leaf Beetle (Labidomera clivicollis) is found in the eastern two-thirds of the United States, as well as in northern Mexico, where it feeds on leaves and flowers of plants in the milkweed family (Asclepiadaceae). This 8-12 mm long beetle overwinters as an adult on the ground, often hidden in leaves of mullein (Verbascum spp.).

Milkweed Aphids (Aphis sp.) on Swamp Milkweed

Milkweed Aphids (Aphis spp.), also known as Oleander Aphids, feed only on plants in the Asclepiadaceae and Apocynaceae. Specifically, they feed on the milky sap, causing them to be poisonous to would-be predators. Milkweed Aphids are introduced in North America. All of the individuals in the photograph above are female. In fact, males aren't necessary for reproduction, as these aphids reproduce assexually. I believe that these aphids are either Aphis nerii or Aphis lutescens.

You will notice that none of these insects are very well camouflaged with their surroundings; in fact, they are all brightly colored. Bright colors in nature are often indicative of an organism that will taste bad or cause harm if eaten, which is the case for all of these milkweed-feeding insects. This is a form of aposematism, which is a condition in which an organism is colored or constructed in a way that indicates special capabilities for defense. It's no wonder that milkweeds are so popular amongst the arthropod world!


Beth said...

Very interesting! I remember playing with milkweed pods when I was a kid, but never realized how beautiful they are or what an important role they play.

Scott said...

Thanks Beth. Many plants have similar roles as hosts for insects. For example, the Federally Endangered Karner Blue Butterfly completely relies on Wild Lupine (Lupinus perennis), as the larvae only feed on this species. This is one reason why preserving habitat is so important.

By the way, I apologize for the formatting of my post. I fought with it for too long, and still can't get the photo captions to be centered without some of the paragraphs also being centered. Oh well.

Bucko (a.k.a., Ken) said...

How interesting that there is such a variety of milkweed. Thanks for the Monarch discussion, I love getting new information (I think that will be the norm here ;o)

Scott said...

Glad you enjoyed the post, Ken. Were you aware that there are 15 species of milkweed within the Chicago Region? You can easily see half of these within an hour of South Bend. Before this post, I knew that a lot of insects used milkweeds, but I had no idea it was that many until I started doing research on the topic.

Thanks for reading!

MartyL said...

I grow two kinds of milkweeds in my garden, the common milkweed and the butterfly milkweed, both are great because they bloom reliably, and are exceedingly drought tolerant - I didn't plant the common milkweed, it volunteered and I let it grow, mainly because I love the smell of their flowers. They grow back year after year and now get about 7' tall. I hadn't realized the common milkweed could be so long-lived. With a little care a common milkweed is actually a nice garden specimen, and the big bonus is the wildly varied array of insects it attracts when blooming, many you probably wouldn't see otherwise. If you enjoy interesting insects and garden, you should have some milkweeds. I just wish I could grow the swamp milkweed (my garden is too dry) - it's the best local sp. for insect observation, IMO.

Scott said...

Hi Marty. I would be that in that sandy soil you could get a few other milkweeds growing in your garden if you wanted to.

I agree that of the milkweeds in the Chicago Region, swamp milkweed seems to attract the most insects, but butterfly milkweed is right up there.