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.
Nymphs of Oncopeltus fasciatus.
Two adults of Oncopeltus fasciatus mating.
Nymphs often cluster in large numbers.
The nymphs actually eat the seeds of the milkweed plant, and can be found on its seed pods.
The nymph’s proboscis is visible piercing the seed.
Detail of a milkweed seed.
Oncopeltus fasciatus undergoes incomplete metamorphosis — unlike butterflies, the young bugs look similar to the adults.
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.
Steven E. F. Brown 