Large Milkweed Bug

Large milkweed bug on a milkweed leaf
Scientific Name
Oncopeltus fasciatus
Lygaeidae (seed bugs) in order Hemiptera (true bugs)

The large milkweed bug, with its bold black-and-orange pattern, is one of the most beautiful of the true bugs. It is one of several species in its genus, with all having similar coloration but different patterns. In this species, each forewing has, from front to back, an orange, front-pointing triangle, a wide black band, and an orange backward-pointing triangle; then, the outer, membranous portion of the forewing is black. Also note that the pronotum (the shield-like plate, like shoulders, between the head and wings) has a black, forward-pointing triangle bordered on either side by orange.

Learn more about this and other seed bugs on their group page.

Similar species: The small milkweed bug (Lygaeus kalmii) is a widespread species also usually found in open, grassy areas and also feeding on milkweeds. They look quite similar, but on average they are smaller (with a length only to about ½ inch), and their orange-and-black pattern is different: they have a reddish-orange X on the back and a reddish-orange band running across the pronotum.


Adult length: about ½–1 inch.

Like most other seed bugs, this insect usually is seen in open areas, where its food plants are abundant: pastures, old fields, prairies, open woodlands, roadsides, native wildflower gardens, and so on.

As the name suggests, the large milkweed bug feeds on the seeds, leaves, and stems of milkweeds (and dogbanes, which are in the same family). The favorite milkweed is probably common milkweed, Asclepias syriaca. As with other true bugs, this insect uses its long, strawlike mouthparts to pierce the plant and suck its juices.


Life Cycle

Females deposit eggs in or among the pods of milkweed plants in groups of about 30 eggs. A single female can deposit more than 2,000 eggs during her adult life, which is about one month. Large milkweed bugs overwinter as adults, and they usually don't survive Missouri winters. Northern populations, including those in Missouri, are migratory. Adults fly south in fall, and our state is gradually repopulated each spring and summer from populations that survive winters to our south. They are usually seen in Missouri from July until the first hard freezes of fall.

Although in laboratories, large milkweed bugs have been able to survive on the seeds of some other types of plants, including sunflower and watermelon, these are not the preferred foods in the wild, and these insects are not a problem for human crops.

If you are growing milkweeds on purpose, you will probably find large milkweed bugs more of a nuisance (if even that) than a pest. Unless you have an exceptionally large number of these bugs, they generally do not damage milkweed plants very much. Since many people are growing milkweeds to serve as native insect food plants — for monarchs, milkweed tussock moths, and so on — keep in mind that these colorful native bugs are helped by your efforts, too.

The large milkweed bug is one of our most colorful and eye-catching insects. One popular field guide to insects has only a single, big picture of a large milkweed bug on its front cover — of all the insects in North America, that’s the one they chose for the cover photo. If you were publishing a book about insects, which of North America’s nearly 90,000 species would be on your front cover?

Large milkweed bugs, like other seed bugs, play a role in checking the potential population growth of their food plants, by damaging or destroying some of the seeds of their food plants before they have a chance to germinate.

Like many insects that feed on milkweeds, their contrasting black and bright orange colors are a warning to potential predators. As they feed on milkweeds, whose sap contains toxic chemicals, their bodies are able to sequester the toxins so that a predator that eats them will be sickened. The bright colors are memorable, which helps teach predators not to eat creatures that look like them in the future.

There are a wide variety of insects that feed on milkweeds and become distasteful, including monarch butterflies, milkweed bugs, milkweed longhorn beetles, milkweed leaf beetles, and milkweed tussock moth caterpillars. All of these are either orange and black or red and black, sending a clear message to would-be predators: “Don't eat me; I taste terrible.”

Biologists have described the usual red/black or orange/black colorations of milkweed-feeding insects as a “mimicry complex.” It seems clear that the many different insects that share these colors and toxicity benefit from a cumulative educational effect on predators. For example, a blue jay that vomits after eating an orange-and black large milkweed bug might later avoid an orange-and black monarch based on its bad experience with the milkweed bug. Meanwhile, there are some insects that sport the orange/red and black coloration that are not toxic (perhaps they feed on mints, or mustards) — in this case, they avoid predators by mimicking the warning colors of the truly toxic species.

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Similar Species
About Land Invertebrates in Missouri
Invertebrates are animals without backbones, including earthworms, slugs, snails, and arthropods. Arthropods—invertebrates with “jointed legs” — are a group of invertebrates that includes crayfish, shrimp, millipedes, centipedes, mites, spiders, and insects. There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species.