Facts About Milkweed Bugs and Native Milkweeds

By Susan Camp

Last Thursday, after she read the “Gardening Corner” column about Monarch caterpillars and butterflies, fellow Master Gardener Peggy Cooney emailed me to ask about some red and black bugs that she had discovered eating the seedpods on a milkweed plant.

I hadn’t noticed bugs on our butterfly weed, but the Missouri Botanical Garden is an excellent resource for many topics other than flowers and trees, so that website was my first stop. I entered “milkweed bugs,” and pulled up an entry on the colorful insects.

Peggy has since verified from the photos in the entry that she did indeed find large milkweed bugs (Oncopeltus fasciatus), which, by the way, are members of the order Hemiptera, and are true bugs, because they have piercing-sucking mouthparts and two pairs of wings. Small milkweed bugs (Lygaeus kalmii) are related red and black insects that also eat milkweed. Large and small milkweed bugs primarily differ in size and color patterns. Milkweed bugs overwinter as adults and emerge in our region during the summer and fall. Adults live for about one month.

Milkweed bugs undergo incomplete metamorphosis as they develop, which means that small changes in coloration, pattern, and wing development occur with each of the five molts, which are called instars. Milkweed bugs gather in small groups to feed, and bugs at various stages of development may gather at one time on the same leaf or seedpod, leading to difficulty with identification. Juveniles feed solely on milkweed seed, although adults will eat other seeds, such as watermelon and sunflower.

Large and small milkweed bugs feed by piercing seeds with their sharp mouthparts. They secrete an enzyme in their saliva that liquefies the seed and allows them to suck the contents with the straw-like prosboscis. The bodies of milkweed bugs contain the toxic cardiac glycosides that they ingest from the milkweed plant, making them unpalatable to birds and other predators. Their bright orange-red and black coloration serves as a warning: “Don’t eat! Bad taste”! Small milkweed bugs occasionally feed on Monarch butterflies or pupae.

Milkweed bugs cause minimal damage to plants. The bugs can be removed by hand. Garden sanitation is important. Remove old plant material in the fall to reduce the number of overwintering sites. Insecticidal soap is effective against the insects. There is little reason to use a chemical insecticide, but if you do, follow label directions. Some pesticides are harmful to beneficial insects.

The University of Wisconsin-Madison Master Gardener Program website contains an excellent article “Common Milkweed Insects,” featuring color photos of large and small milkweed bugs, as well as two species of milkweed beetles, aphids, and the milkweed tussock moth, the hairy caterpillars of which Jim and I had tried to identify for the past two weeks. Plant Native Milkweeds

While I was searching for information about milkweed bugs, I found that every Monarch butterfly website stressed the importance of planting only native milkweed, meaning milkweed native only to the region where you reside. About 200 species of milkweed exist in North America, South America, and Africa. Dozens of species are native to North America. The website of Monarch Joint Venture at www.monarchjointventure.org includes a page that divides the United States into six regions and displays photos of milkweed species that are native to each region.

In Virginia, the native milkweeds we should plant include orange-flowered butterfly weed (Asclepias tuberosa), purple-pink common milkweed (Asclepias syriaca), and pale pink to purple swamp milkweed (Asclepias incarnata).

Asclepias curassavica, commonly called tropical milkweed, is an introduced species from Central and South America that has gained popularity in American gardens over the past decade. It is non-invasive, and in warm regions can bloom all year. The long bloom period may lead to decreased Monarch migration numbers, but the primary concern is a potential for the buildup of a protozoan organism on the plant that can weaken or kill Monarch butterflies. To prevent this possibility, plant only native milkweed plants or seeds.

September 12, 2019