The Red Invaders: Those Colorful Milkweed Bugs

If you've been finding more milkweed bugs than monarchs on your milkweed, join the crowd.

Monarchs are scarce--at least around Solano and Yolo counties--but milkweed bugs are quite plentiful. Sometimes you see them massing on milkweed pods as if they're having a family reunion and trying to figure out who's who during an all-you-can eat buffet. They're blood red, in sharp contrast to the green plants. 

Have you seen the large milkweed bugs, Oncopeltus fasciatus? They belong to the seed bug family, Lygaedidae. We recently spotted them on a patch of showy milkweed (species Asclepias speciosa) in Sonoma.

Large Milkweed Bugs (Oncopeltus fasciatus)

Milkweed bugs are primarily seed eaters, but they're opportunistic and generalists, says Hugh Dingle, emeritus professor of the UC Davis Department of Entomology and Nematology, an insect migration biologist who also researches migratory monarchs.

"They'll get protein from wherever they can find it," said Dingle, author of the popular textbook, Animal Migration: the Biology of Life on the Move. They eat not only eat seeds, but also monarch eggs and larvae and the immature stages of other butterflies, Dingle told us back in 2016. They eat other small bugs and feed on nectar as well. Some scientists have seen them feeding on insects trapped in the sticky pollen of the showy milkweed. 

The bugs feed on the toxic milkweed, rendering them distasteful to predators.  Their warning colors (red and black) also tell prospective predators "Leave me alone; I don't taste good. If you eat me, you'll be sorry." They sequester and store cardenolides (cardiac glycosides) from the milkweed.

In the fall, as the seed pods burst open, monarch enthusiasts scramble to collect the seeds for next year, but they usually have to compete with the red invaders.

If they're still around...