Milkweed Bugs: Friend or Foe?

What are those reddish-orange and black bugs crawling all over your milkweed, and are they harmful? Like many elements in an ecosystem, the answer fits into the gray zone.

Large Milkweed bugs (Oncopeltus fasciatus) and Small Milkweed bugs (Lygaeus kalmia) love your milkweeds as much as the Monarch and Queen butterflies, and they exist together as part of the natural ecosystem. Although their common names are similar, large and small milkweed bugs differ in more than size.

Large milkweed bugs (LMBs) are herbivorous – they feed on the leaves, stems, and seeds of milkweed using their long proboscis. In general this is not harmful to the milkweed plant, although if there is a large population boom, LMBs can stress the plant and cause misshapen seeds and lower seed production.

Small Milkweed bugs (SMBs) are mostly herbivorous, but can occasionally be scavengers. These insects have been reported feeding on a wide variety of other insects, such as honey bees, monarch caterpillars and pupae, and dogbane beetles. While it may be disappointing to see monarch eggs, larvae or pupae preyed upon, this is all part of a vibrant milkweed ecosystem, and this predatory behavior is likely most common when their preferred food source is scarce.

Milkweed bugs protect themselves by ingesting, then storing, cardiac glycosides from the milkweed, and as a result don’t have an issue with natural predators. The reddish-orange coloration is a warning to predators to stay away.

So should you be concerned about milkweed bugs in your garden? In general, no, they are part of the balanced ecosystem. However, if there is a large population boom, you may elect to reduce feeding pressure by culling some of the bugs, although this is a contentious subject 😉

Resources:

• Monarch Joint Venture
• Illinois Extension: More than monarchs – What are those bugs on my milkweed?
• UW Milwaukee: Milkweed Bugs, Large and Small (Family Lygaedidae)
• Wikipedia: Large Milkweed Bug
• Wikipedia: Small Milkweed Bug