Wednesday, January 29, 2014

Small Milkweed Bug

One insect that can be found at virtually any time of the year is the colorful Small Milkweed Bug, Lygaeus kalmii. Contrary to its common name, this insect feeds on a variety of plants, not just milkweeds. The adult bugs overwinter and often emerge from cracks and crevices on warm winter days. I found one here in Colorado Springs just last week (January 21), in our backyard.

The Smaller Milkweed Bug does not even confine itself to vegetation. It is an opportunistic scavenger on dead insects, and may even prey on the eggs of the Swamp Milkweed Beetle (Labidomera clivicollis), larvae and pupae of the Monarch butterfly, and on other insects trapped by the sticky pollenia in milkweed flowers. I once spotted a pair of adult Small Milkweed Bugs sharing a dead honeybee on a curb in Tucson, Arizona. Adult bugs also feed on the nectar of flowers

These are not large or otherwise imposing insects. Adults measure about 10-11 millimeters in body length. The bright red, black, gray, and white colors warn of the toxic properties of L. kalmii. The bugs, in the nymphal stage, preferentially feed on milkweeds (Asclepias spp.), and so ingest the plant’s defensive chemicals. The bug is not only impervious to the effects of these noxious compounds, but actually sequesters them in special compartments along the edges of its abdomen and thorax.

The chemicals, known as cardiac glycocides, are known to trigger heart arrhythmias and other distressing if not potentially fatal consequences in vertebrate animals. No surprise then that the Small Milkweed Bug is seldom if ever a victim of vertebrate predators; and even spiders and predatory insects tend to avoid them.

Milkweed bugs are in the family Lygaeidae, collectively known as “seed bugs,” and indeed it is the seeds of the host plant that make up the diet of nymphs. Besides milkweed, L. kalmii will feed on the seeds of other plants, especially composites like asters and such. I have also seen them regularly on oleander, a popular landscape plant in Arizona and elsewhere. The Small Milkweed Bug can therefore be considered a “generalist” rather than a specialist or obligate feeder on milkweed.

The range of this species is over virtually the entire U.S. and adjacent southern Canada, but it can be confused with other species of Lygaeus, particularly L. turcicus, the False Milkweed Bug. Note the difference in markings on the head: L. kalmii has a red basal spot or vertical bar on the head, whereas L. turcicus has a Y-shaped marking. The latter species also has few, if any, white markings on the black membrane of the front wing.

Lygaeus turcicus, False Milkweed Bug

Some years tend to be better than others for the Small Milkweed Bug. Here in Colorado Springs, last summer and fall they were everywhere, even outnumbering the usually abundant Eastern Boxelder Bug. It would be interesting to find out what triggers these population booms and busts. Unusually regular, heavy summer rains might have been the reason here last year, providing an abundance of food.

I do like to think of the Small Milkweed Bug as the very definition of persistent, adaptable, hardy, and colorful. What’s not to like about a non-pest bug that can brighten up even an urban vacant lot?

Sources: Berenbaum, May R. 1993. Ninety-nine More Maggots, Mites, and Munchers. Urbana: University of Illinois Press. 285 pp.
Evans, Arthur V. 2008. National Wildlife Federation Field Guide to Insects and Spiders of North America. New York: Sterling Publishing Co., Inc. 497 pp.
Fox, Charles W. and Roy L. Caldwell. Wheeler, A.G., Jr. 1983. “The Small Milkweed Bug, Lygaeus kalmii (Hemiptera: Lygaeidae): Milkweed Specialist or Opportunist?” J. N.Y. Entomol. Soc. 91(1): 57-62.
Root, Richard B. 1986. “The Life of a Californian Population of the Facultative Milkweed Bug, Lygaeus kalmii (Heteroptera: Lygaeidae),” Proc. Entomol. Soc, Wash. 88(2): 201-214.


  1. Thanks for the post and sharing the blog. Valuable and excellent post, as share good stuff with good ideas and concepts.
    lots of great information and inspiration. I just would like to say thanks for your great efforts.
    I appreciate your excellent post.

  2. Eric: It's been a while since we first met at the Portland Zoo where you were managing a bug zoo. I just posted anote but think I hit the wrong button sending it to the rings of Saturn where all emails go.
    Love the Blog. Was looking for info on smaller milkweed bug. I had only seen it in E. WA. But when I planted showy milkweed (Asclepias speciosa) I suddenly had it in W. WA. Apparently that's not unusual. Love your book. You capture exactly the info I want in a very short space. More space of course would be way better, but the blog supplements nicely. Sharon J. Collman, WSU Snohomish County Extension.

    1. Sharon! So great to hear from you. Thank you so much for the generous praise and kind sentiments. Please feel free to make "requests" here for post topics. You can also e-mail me at BugEric247ATgmailDOTcom. I would very much like to get reconnected. :-)

  3. It's an excellent post you have published . Great thanks guy for you post .

  4. It was an amazing article.Thanks a lot for providing us this useful article with us

  5. So awesome and great article - thanks for sharing

  6. Thank you for another excellent post. Some very valid points! I appreciate you penning this post and also the rest of the site is really good.

  7. Used this article for a virtual lab collection @ Texas A&M! Thank you so much for the information!

  8. Thank you for the information. It took a long time to find what this insect was called. They are not very common on the internet, even though they are seen everywhere where I live.


Blog author currently unable to reply to reader comments, nor comment himself. Working to resolve this.