Large Milkweed Bug
Oncopeltus fasciatus
Lygaeidae (Seed Bugs)

Identification, a website.

The large milkweed bug eat the seeds of milkweed plants, and occasionally suck juices from other plants, but appear to do no significant damage. Small elongate eggs are bright red. They hatch in spring. Adults overwinter, appearing sometimes in great numbers on warm winter days. There can be more than one generation per year.

Both small and large milkweed bugs have incomplete metamorphosis. The nymphs (immatures) look like the adults except 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 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.

The milkweed plant produces a milky white sap when a leaf is removed. The plants are large (3-4 feet high) with sprays of small white, red, or yellow flowers in the summer. In the fall, seed pods develop which are 4 inches long and 1 1/2 inches wide. Milkweed bugs are found on seed pods piercing the wall of the pod to feed on the The bugs are one of a small group of insects that have the ability to tolerate the toxic compounds in the milkweed plant. They are therefore important in regulating populations of this plant.

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.

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