Milkweed attracts many insects and invertebrates, including monarch butterflies. Learn about its fascinating link to two insects that coevolved with it.
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What is A Butterfly Host Plant?
A butterfly host plant provides nectar for species of adult butterflies that we call specialists. Specialists prefer a specific plant for laying eggs.
Did you know that monarchs may travel upwards of 3,000 miles on their migratory route? This makes them especially vulnerable to habitat loss, drought, and logging, which reduces wild stands of their host plant, milkweed.
Not only is it the preferred food source for adult butterflies, the caterpillars dine exclusively on it as well.
There are over 100 species of milkweed native to North America, and I happen to have two of them in my yard: honeyvine milkweed (Cynanchum laeve) and butterfly milkweed (Asclepias tuberosa). Swamp milkweed (Asclepias incarnata) is another popular type for wet areas.
Poison in a Flower: Nature’s Protective Strategy
As this plant’s flowering season winds down, it seems like the action might be done for the year. But for this plant, it’s just getting started!
In late summer, you can spot insects such as monarch caterpillars, milkweed aphids, and large milkweed bugs all feeding on the plant—two of these are good, one not so good. All of them are here to dine on the milky sap that give the plant its name.
Interestingly, these insects have evolved to take in the toxin glycoside that exists within the plant’s juices, which then makes them poisonous to anything that might want to eat them.
2 Surprising Insects that Love Milkweed (as much as Monarch Butterflies and their Caterpillars)
Let’s take a look at two insects that love this plant just as much as monarchs!
1: Milkweed Aphids
Honeyvine milkweed is an aggressive vine, and although it’s native to my bioregion, I would never grow it by choice. Unfortunately, our neighbor allows it to take over our shared fence, and it spreads by seed and roots. I’m constantly pulling the vines that pop up EVERYWHERE and threaten to take over our entire 0.10-acre.
The vine is covered each year with milkweed aphids, which are reportedly non-native and invasive. However, it’s unclear whether they help or hurt the situation for monarchs. I expect that it’s a little bit of both, which is usually the way it goes in nature.
On one hand, the aphids cover the plant and may make it hard for 1st and 2nd instar monarch caterpillars, who are vegetarians and dine strictly on milkweed leaves, to get enough to eat.
Garden experts generally recommend a soap and water spray (when caterpillars aren’t present) to wash the aphids off, since there are no known predators that can ingest the now-toxic insects that have been feeding on the toxic milky sap.
On the other hand, the aphids may be an important protein source for 4th and 5th instar monarch caterpillars, which seek out additional nutrition as they prepare for pupating.
What’s more, some sources report that ladybugs can eat milkweed aphids. This reputes the claim that glycoside toxicity makes the aphids unappealing to species that haven’t coevolved with the plant. However, if ladybugs can ingest the aphids, then it may discourage them from preying on monarch caterpillars, thus making the aphids a positive coevolutionary monarch companion.
2: Large Milkweed Bug
As I was doing my daily walk around the garden yesterday—inspecting everything for unusual signs and making harvest lists—I noticed the immature milkweed seed pods covered in red and black bugs.
I’ve seen this in years past and assumed they were some aspect of the aphid’s life cycle. With a little more digging, however, I discovered that they are appropriately called the large milkweed bug (Oncopeltus fasciatus), and also feed on the milky sap.
As far as I can tell, they’re common and although a nuisance, not a threat to monarchs or the plant.
As you can see, milkweed is an interesting plant that is both important for the monarch butterfly as well as a number of other insects that have coevolved to favor the toxic milky sap.
Sow seeds in the fall for blooms the following year. I like this collection of 6 heirloom varieties, or buy your favorite in individual packets.
Are you growing milkweed? Have you noticed any of these insects on your plants? Do you provide habitat for monarchs or other specialist butterflies?
Deb Neyens says
Interesting. We grow milkweed for the monarchs, and just this year we noticed for the first time the red and black bugs. I took a photo, but haven’t had time to research them yet. Have not had an issue with aphids.
Dave Reyman says
I have found that these are described as beetles. Question: are they edible?
A Yellow plastic Solo cup covered with Vaseline decreases the alphids 😉
Anna D. says
That is so interesting!! Will give it a try.
I was so excited to see the milkweed article. Recently out of the blue, milkweed started growing outside under my bedroom window. The flowers were a purple color, and the lovely scent drifted into my window. I have the small pods forming now. I knew that monarchs lay their eggs in the pods, so I’ve been doting over my milkweeds and they are multiplying in number. I hope to find monarchs eventually. They have steadily decreased in number which is alarming to me, and hope that by having my little haven for them, they will prosper here. Thank you for highlighting the milkweed and the monarchs.
Plant it and they will come! (We hope) 🙂
I recently read an article that gave me hope: Monarch Butterfly Population More Than Triples Over Last Year.
Wonderful! Thank you Amy 🙂
We saw a lot of caterpillars in 2016 – not so many this year in spite of the new plantings we have – not sure why
Nature ebbs and flows. I bet annual rainfall and temperatures will affect the numbers from one year to another. Of course, there are more dire reasons why numbers may have dropped, but let’s hope it was just a seasonal ebb 🙂
Sometimes it’s a nearby wasp nest that is taking the larva
sorry but i need to tell you that LADYBUGS are in fact predators who can and do eat the aphids and you can buy them online as well as local garden centers, usually bout 1500 of them in a bag if i remember correctly….. the larva of the ladybugs look kinda scary little things , we call them alligator ladybugs because theyre little flattish black spiky looking bugs before they become the beautiful red and other colored beetles we all know and love….. i use NO chemicals on my yard or flowers ever, and have found mother nature takes care of a lot of our problems, ive seen my milkweeds covered in yellow aphids and only two ladybugs and the next few days theyve eaten all the aphids, and i have a LOT of big milkweeds growing each year that ive planted for the monarchs
forgot to mention also that ive seen monarchs lay eggs on and they seem to actually prefer honeyvine milkweed to regular milkweed , even though it is very invasive, it is still a milkweed plant , its still a great hostplant imo and nectar source for butterflies in general… my neighbors have planted no milkweeds although ive offered seeds etc but they do have honeyvine milkweed vines that have made themselves at home on their hedges, fences etc and weve found monarch caterpillars of all sizes and instars on their vines munching away happily and i see monarchs laying eggs on their vines from across the street when my flowerbeds are full of regular milkweeds . i think if people would educate themselves and each other , theyd find that most plants and trees are actually host plants for butterflies and would let more grow
Lisa Aucoin says
I was pleasantly surprised to find monarch caterpillars on our new swamp milkweed plant in September. Five caterpillars but could find only one chrysalis later on. My butterfly weed had two caterpillars, found at maybe the first and second instar stage but couldn’t find them the next day.
Lots of milkweed bugs and milkweed aphids on my swamp milkweed plant. However, the milkweed did not bloom. I read that it might have been because the plant was still young? It was tall and scraggly, but no blooms. Maybe next year.
Sometimes it will not bloom in its first year. 🙂
jim stricker says
I was told that common milk weed attracts common flys . do you know if there is anything to that?//////////My common milk weeds are next to our deck that is attached to our house.
I wouldn’t be too concerned. House flies are just one of many pollinators that are attracted to milkweed. They typically aren’t the dominant pollinator, and they don’t stick around for long.
My milkweed is covered with flies and wasps. I have also seen a significant amount of monarchs. I was just wondering if the flies and wasps are usually found in large numbers on milkweed.
Yes, it’s normal for all types of pollinators to be attracted to the milkweed flowers.
I am getting more and more into edible permaculture. Marie Viljoen, in her book Forage, Harvest, Feast says that she has only cooked with Common Milkweed, but that the other milkweeds, including Swamp Milkweed (Asclepias incarnata) are also OK to eat cooked. As Swamp Milkweed is less invasive, I would love to grow it and share it with monarch butterflies. I am having difficulty finding more evidence of its edibility by humans. Do you have any suggestions?
I don’t have any information or resources that point to the milkweeds as being edible.
As far as I understand, the milkweed beetles are pretty harmless and you can leave them alone unless they are seriously damaging the plant. I’ve also recently learned that monarch caterpillars prefer young leaves to eat, so trimming back common milkweed is supposed to be one way to ensure you get eggs and caterpillars. I’ve got a pretty good patch of common milkweed in my yard but I never trimmed it so it gets really tall, falling over my sidewalk, and I never see eggs or caterpillars. I’m going to try trimming it (a few plants at a time, staggered over the growing season) to see if I can get smaller, more manageable plants as well as more caterpillars. I do have lots of flowering plants, so I frequently see monarch butterflies. Here’s hoping I can get the next step in their life cycle going!
Terry Harris says
I have a lot of common milkweed in my garden and I frequently get the milkweed buvs. I can’t stand them! Regardless as to whether they are harmful or not. Most of the time they are seen mating and when their eggs hatch, there are millions of those little bugvers! They can’t fly until they are adults, but grossly, I just pick them off and smash them. Also, I have been saving these guys for about 3 years now, and this is the 1st hear that I have had such a hard time finding older cats. As I mentioned I have a ton of milkweed and I have to go out really early to find babies and have a little success with some eggs. The wasps and the lizards are extremely thorough. I have wasps traps up, but they don’t help much. I cannot locate any wasp nest anywhere. I’m having the same problem with my passion flower plant. Last year I could count 20 or more gulf flitterary cats of various sizes on the plant, but this year’s I can’t hardly find one baby! I’m very discouraged with that effort.
Barbara Stavens says
I had two monarch cattapillars on my swamp milk weed. They were large. The next day one was missing and the other was at the bottom of the twig they both consumed. I placed it at the top of another full twig and got busy around the yard. When I went to check it out and hour later it was gone. I figured a bird must have eaten it. Also I noticed that flies are sticking to the leaves. And they are dried up carcuses. Can you tell me anything about that?
Sounds like a predator such as a spider or preying mantis.
Mabel Pepper says
We recently moved to new home. I was informed one of the so-called flowers planted on side of garage wall was milkweed. I planted a peony bush near the milkweed. Will the milkweed interfere in the growth of the peony? Should I consider moving the peony? I don’t know very much about gardening.
Donna Pellegrin says
Amy, First of all, I love your website and all of your articles. I look forward to reading them and I recommend them to others often. So helpful! I have been a part of monarch conservation for several years now, so I was very pleased to see your article on milkweed. I hope you don’t mind, but I have a couple suggestions for you. Honeyvine, Cynanchum laeve, is native to Ohio, and is therefore, by definition, not an invasive plant. It is an aggressive plant for sure, but it is a very important source of food for the monarch butterfly and other native American invertebrates who co-evolved with the plant. I have learned to appreciate it and all of the life that it brings to my yard. Invertebrate populations are crashing globally and we must do what we can to protect them, and that means embracing the native plants that feed our native bugs. Also, you might be surprised to know that aphids often provide a benefit to monarchs when they are present on the milkweed. Aphids (invasive as they are) provide something for lady bugs and other monarch predators to eat other than monarch eggs and the tiny first and second instar caterpillars which have such a low survival rate due to predation. Once monarch caterpillars grow to 4th and 5th instar size, they devour any aphids that are in their way. I have a photograph and a video of one beautiful monarch caterpillar eating an aphid as a snack as it chowed down on its milkweed leaves.