The Mysteries of Large Milkweed Bugs

immature Large Milkweed Bugs (Oncopeltus fasciatus) on Butterfly Weed (Asclepias tuberosa) in my garden.
immature Large Milkweed Bugs (Oncopeltus fasciatus) on Butterfly Weed (Asclepias tuberosa) in my garden.

You might have wondered what are these colorful orange and black bugs. They are most often seen clustered in large groups on milkweed plant seed pods. 

They are Large Milkweed Bugs (Oncopeltus fasciatus). They are two species of Milkweed Bugs, one smaller than the other. The Small Milkweed Bugs (lygaeus kalmii) are between 1/8 – 1/2″  (11-12 mm) long. The Large Milkweed Bug is 1/2 -3/4″ (13 – 18mm) long. 

I saw these Large Milkweed Bugs on the Butterfly Weed (Asclepias tuberosa) in my garden. I noticed tiny yellowish-orange larvae (or eggs?) on the tops of immature Butterfly Weed seed pods. I can never tell if the orange specks are eggs or tiny, tiny larvae. 


I am always passing the Butterfly Weed as I go to and from my car. I keep forgetting to take a hand lens and take a close look at the little orange dots. I can never figure out what they are doing. 

I couldn’t figure out what the doing except sucking the plant juices of the immature seed pods. I thought they just sucked the juices out of seeds when the pods opened. Why are they on the green immature pods. 

The orange and black coloring of the bugs makes it easy to spot among the green foliage. Some hungry birds or predatory insects could easily make a meal of the large cluster of bugs. 

But then I thought, the breeding season is over. There aren’t any hungry chicks to feed. And most of the songbirds have started there southward migration. The year-round resident birds like woodpeckers hunt among the tree bark, not on flowers. 


Even the Chinese Mantid (Tenodera sinensis) that lurks in my garden have left these large insects alone. I notice the mantids were about twenty feet away from the Milkweed Bugs. I had seen the Mantid in the same flowerbed as the bugs, just at the opposite end. The Chinese Mantid was introduced here in the Philadelphia area in 1896, by the way. 

Feeding on the Milkweed means the bug is probably bitter tasting. The latex in milkweed plants transfers the bitter taste to the animals that feed on them.  The latex is a milky substance containing cardiac glycosides termed cardenolides, seep out where cells are damaged. This latex is the source of the bitter taste of Monarch butterfly larva and adults. And birds don’t bother catching Monarchs. 

Monarch Butterfly (Danaus plexippus). Brushfoot (Nymphalids) Family. Milkweed butterflies. Photo by Donna L. Long.
Monarch Butterfly (Danaus plexippus). Brushfoot (Nymphalids) Family. Milkweed butterflies. Photo by Donna L. Long.

It’s interesting how both Large Milkweed Bugs and Monarch butterflies have the same orange and black coloration. And milkweed is the host plant for both insects. I wonder if this phenomenon is by design or coincidence?

Both the Large and Small Milkweed Bugs are seed bugs. Seed bugs (family Lygaeidae) are a diverse group of mostly plant-eating bugs. Seed bugs don’t harm the plant, they suck the seeds and sap. A few seed bugs are predatory. But I don’t see any chewed seeds in the open Butterfly Weed Pods.

immature Large Milkweed Bug and Ladybug in my garden.

Watching these bugs raises many questions for me. 

I wonder why they show up before the milkweed seed pods are formed?

I want to know what they are feeding on before the seed pods mature and split open providing access to the seeds. Maybe it’s secreted plant latex. 

I notice the bugs are at the bottom of the plant in the early morning. They must climb the plant as the day warms and drop off to spend the night in the ground, under leaves or something else. 

Where and when do the Milkweed Bugs lay their eggs? Are the tiny orange things eggs or larvae or both? 


Species Profile

Large Milkweed Bugs (Oncopeltus fasciatus)

Family: Lygaeidae – seed bugs

Size: 1/2 – 3/4″ (13 – 18 mm)

Range: Southeastern Canada throughout the United States and into South America

Elevation: found from mid to low elevations

Active: from spring through fall

Where to Find: adults and larvae can be found on milkweed plants, feeding in dense groups

A Good Scientific Nature Journal Project

Next year I will just have to study the Milkweed Bugs. I have too many questions to not do anything.

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