Milkweed & its Many Insect Visitors

Rebecca Vargas is a home gardener, artist, and native plant enthusiast. She runs a blog called realisticgardening.com where she writes about her gardens, her fondness of native plants, her peaceful gardening methods, and other topics related to gardening in the suburbs.

Milkweeds are cool plants. Native plant enthusiasts think of milkweeds in terms of how beneficial they are to insects and ecosystems but this plant as a long history of being used as Native medicine and is currently used in a variety of industries. Milkweed played such an important role in the Americas that in 1753 Carl Linneaus, the father of taxonomy, named the milkweed family after Asclepias, the Greek God of medicine [1].

The most common varieties of milkweed in Northeastern gardens today are the same ones found at local plant sales: butterfly weed (asclepias tuberosa), swamp milkweed (asclepsias incarnata) and common milkweed (asclepias syriaca) but there are hundreds more to suit each location and climate. Milkweeds have become a staple of the native plant garden and this may be one reason why one of the most common questions I see in online native gardening forums is “what are these bugs on my milkweed?!” It’s helpful to remember that milkweed goes through several stages and different insects appear at different times. This is completely normal and part of the milkweed lifecycle.

Oleander aphids on milkweed. Source Save Our Monarchs

One of the first insects you will notice on a milkweed plant are Oleander Aphids. These small round yellow/orange bugs form clusters along the stems, leaf bases, and under leaves. To the newbie native plant gardener or the mainstream gardener who has been taught that bugs are very bad this looks like a big problem, but the truth is they are usually harmless. They do suck the sap out of plants and can overpower or “suck the life” out of a plant, but the chances are low if you keep an organic, chemical free garden that encourages aphid predators like ladybugs and lacewings. Hopefully, you have also planted several milkweed plants so it’s not that big of an issue to lose one plant; it will grow back next year! You can wipe the aphids off or spray them with water to knock them off the plant but doing this means you run the risk of washing away any tiny monarch eggs or caterpillars that may be present on the plant. I prefer to encourage aphid predators and let them work it out.

If there are aphids on your plants, you will also probably see ants. Ants love the sap or “honeydew” produced by aphids and will in essence “farm” the aphids by protecting and relocating them in order to ensure a steady supply of sweet honeydew. The ants themselves are harmless.

Red Milkweed Beetle. Source: Univ. of Milwaukee

The next bug you will notice is the Red Milkweed Beetle. This red and black beetle is easily identified by its long black “horns.” I always see these beetles mating in the cushy milkweed flowers and they seem 100% unbothered. They have evolved to feed on milkweed and their larvae actually feed on the milkweed roots. They seem to have little to no effect on milkweed and are harmless. One cute anecdote about these beetles is that in 1957 a researcher discovered that they can purr and make a “shrill squeak” sound when they “are held in the fingers, are exploring a cage, are stuck in a milkweed blossom, are placed on or fall on their backs, or are fighting.” How cute is that.

Ladybug Larvae. Source: Univ. of California Bug Squad

Throughout the season you will probably notice spiky little orange and black caterpillar things. Those are Ladybug Larva and they are exactly what you want to see if you have aphids on your plants. Speaking of spiky looking caterpillar things, you may also see a furry black, orange, and white caterpillar walking around your plants. This is the Milkweed Tussock Moth caterpillar and in my opinion he looks better at this stage than in his final form: a plain brown moth.

Milkweed Tussock Moth. Source: Bug of the Week

The star of the milkweed show is the Monarch caterpillar, which can only complete its lifecycle on milkweed. Monarch caterpillars start out really tiny and go through five instars (growth periods) before wandering off and forming a smooth green chrysalis. The butterflies emerge in 1-2 weeks.

Photo caption: Monarch caterpillars at different instars - Source

 The last bug you will notice is the Large Milkweed Bug. This orange and black bug moves in towards the end of the season when the milkweed has already grown seed pods. The milkweed bugs feed on milkweed and congregate on the seed pods waiting for them to open and reveal their cache of seeds. These bugs are only really a problem if you’re interested in saving seeds on a large scale because they do eat the milkweed seeds. By eating the seeds, they reduce the amount of seeds that germinate the following year, which can be helpful if you are trying to keep your milkweed from spreading too aggressively. While they may look a little unsightly on the seed pods, milkweed bugs will eventually leave and go south for the winter. As temperatures warm back up the following year, they will make their way back up north.

Large Milkweed Bugs. Source: Missouri Botanical Garden

You may encounter other bugs on your milkweed plants but these are the most common. Bees and butterflies will be all over the milkweed flowers and at the end of the season, it’s not unusual to see a praying mantis (or two) eating the straggler monarch caterpillars that have hatched too late in the season. Those caterpillars will not survive the winter anyway so it’s just as well that the praying mantis eats them before becoming bird food himself. Isn’t garden ecology amazing?

[1] Smith, “A Gardener’s Handbook of Plant Names Their Meanings and Origins” (pp. 47)