Large Milkweed Bug – Oncopeltus fasciatus

Large Milkweed Bug – Oncopeltus fasciatus
Family Rhopalidae – Scentless Plant Bugs
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Live adult and nymph milkweed bugs photographed at northern Illinois.

Large Milkweed Bug - Oncopeltus fasciatus
The large milkweed bug ranges east of the Rocky Mountains. They eat the seeds of milkweed plants, and occasionally suck juices from other plants, but appear to do no significant damage.

Both small and large milkweed bugs have incomplete metamorphosis. The nymphs (immatures) look like the adults (meaning they have all the same body parts), except that they do not have full wings and their color pattern is different. Black wing pads appear early in their development. Nymphs have bright orange abdomens, and usually molt 5 times. (5 nymphal instars (stages) before becoming an adult). Eggs take about 1 week to hatch and a month to become adults. Milkweed bugs are usually found in groups on milkweed plants, often on the underside of the leaves. I have seen them in conglomerations of hundreds.

Milkweed bugs have few predators because they concentrate in their bodies bad tasting compounds found in the sap of milkweed plants. The bugs use their bright colors to advertise their bad taste. Inexperienced birds that taste their first milkweed bug are unlikely to try to eat another orange and black insect such as a Monarch or Viceroy butterfly.

Large Milkweed Bug – Adult Male

Large Milkweed Bug and Nymphs
Large Milkweed Bug and Nymphs

The milkweed plant produces a milky white sap. These plants are large (3-4 feet high) with sprays of small white flowers in the summer. In the fall, seed pods develop which are 4 inches long and 1 1/2 inches wide. When the seeds ripen, the seed pods open up releasing the seeds which float on fluffy white parasols. Milkweed bugs can be found on seed pods piercing the wall of the pod to feed on seeds.  Milkweed bugs are one of a small group of insects that can tolerate the toxic compounds in the plant.

Order Hemiptera: True Bugs number almost 5,000 species in North America, and 40,000 worldwide. They have mouthparts formed into a beak, adapted for sucking plant juices or the liquefied insides of their animal prey.
Suborder Auchenorrhyncha – Cicadas & Planthoppers
Suborder Sternorrhyncha – Aphids, scales, mealybugs, jumping plant lice