Sunday we hiked on dry Mt. Diablo. Milkweed along Little Pine Creek had gone to seed, and we found dozens of bright orange milkweed bugs (Oncopeltus fasciatus) and their nymphs on the plants. We brought home a bunch of them and put them on our own milkweed plants. They’re orange, like the oleander aphids (Aphis nerii) and the Monarch butterflies (Danaus plexippus), because, like them, they taste bad from the noxious compounds in the milkweed. The bright color is a warning to predators not to eat them.

Nymphs of O. fasciatus actually eat the seeds of milkweed, and they tend to appear in large numbers when the plant’s seed pods burst open, releasing the seeds on gossamer parachutes.

Oncopeltus nymphs

Nymphs of Oncopeltus fasciatus.

Oncopeltus adults

Two adults of Oncopeltus fasciatus mating.

Nymphs clustered on a leaf

Nymphs often cluster in large numbers.

Nymph eating a seed

The nymphs actually eat the seeds of the milkweed plant, and can be found on its seed pods.

Nymph eating a seed 4

The nymph’s proboscis is visible piercing the seed.

Milkweed seed

Detail of a milkweed seed.

Nymph wing bud detail

Oncopeltus fasciatus undergoes incomplete metamorphosis — unlike butterflies, the young bugs look similar to the adults.

Nymph in seed cloud

Nymph and aphids

Many insects that eat milkweed, such as these oleander aphids seen near an O. fasciatus nymph, are bright orange to warn predators that they taste bad.

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