Oleander Aphids on Milkweed

We had some questions about the bright yellow aphids show up on milkweeds each year.

milkweed-aphids
Oleander aphids

Where did the aphids come from?

Oleander aphids, Aphis nerii, are thought to have been introduced throughout the world from the Mediterranean region, where oleander is native (the species name nereii comes from Nerium, the genus name for oleander). The aphids are now found over much of North America, where they also use milkweeds and a few related plants as hosts.

How do the aphids reach my plants?

Each spring, when the temperatures and other weather conditions are just right, winged females are blown long distances on the wind. When the air becomes calm, the aphids can make directed flights to host plants. Experts have failed to find male oleander aphids in North America, so they believe the winged females produce young without mating (called parthenogenesis). The first offspring are female aphids without wings. The aphids continue to give live birth to more aphids until conditions cause winged forms to be produced. The winged forms move on again.

Why are they yellow?

Like the monarch and related butterflies, these aphids pick up the toxic cardiac glycosides from the milkweed in their bodies. Their bright yellow-orange color serves as a warning to anything that tries to eat them. The chemicals also are secreted from their cornicles (the tiny black tubes on their rear ends).

Does that mean nothing will eat them?

Unlike many other introduced insects, oleander aphids have a number of parasites and predators. You may have seen the results of tiny wasps (for example, Lysiphlebus testaceipes). The female wasp lays its egg by inserting its ovipositor within the young aphids. The wasp's larva then eats the aphid's insides. When the parasite goes through metamorphosis into a pupa inside the body of the aphid, it causes the body of the aphid to turn brown, tan or black, and stiffen. This immobile aphid is referred to as a “mummy.”  When the wasp emerges from the pupa, it cuts a hole in the back of the aphid’s abdomen and flies away, leaving the aphid’s empty body.

milkweed-aphid-wasp

Flower fly larvae and small lady beetles will also eat oleander aphids.

Flower fly (Syrphidae) larva eating an aphid.
Flower fly (Syrphidae) larva eating an aphid.

Flower fly adults feed on flowers and are pollinators (see below).

What should I do about oleander aphids on milkweed?

Let's face it, aphids have a negative reputation. Virtually every gardening book you pick up will list aphids under the "pest" section. These labels are made by humans, however, not the plants or insects. The first thing to realize is that there are a lot of different kinds of aphids, and each species has its own special way of doing things.  Often the aphids that are pests are introduced species, with no or few natural enemies, living on introduced plants growing under less than optimal conditions. Other species of aphids are much more benign. If you are willing spend some time observing the insects you find on milkweed, I think you may be surprised. Believe it or not, you might even find that you like them better once you get to know them.

I have had oleander aphids on my milkweed plants (Asclepias subulata) for 20 years. Every year I do absolutely nothing to them. Every year the wasps appear, the mummies build up and the aphids disappear. The main plant is 20 years old, huge and healthy. The other milkweed plants are also growing well. I don't see any evidence that the aphids are doing harm. Why not leave yours alone for a week or two and see what happens?

What about monarch and queen caterpillars? Will aphids hurt them?

Frankly, you are much more likely to do harm by knocking off eggs and small larvae while trying to control the aphids.

You also might consider that although we humans think that monarch caterpillars eat only milkweed, no one told the monarchs that. Several species of caterpillar are carnivorous (eat insects), including a species that feeds on sundew plants and eats the insects the sundew traps. I would not be at all surprised to find out that monarchs eat an aphid or two from time to time.

Bottom Line:

You can read all sorts of things on the Internet these days. My advice is to be interested and patiently observe what is going on in your own landscape. Do the aphids go away? Do the plants look okay? Do you have a lot of monarch larvae survive? Each situation is unique, and with time and willingness to investigate with an open mind, nature can answer your questions.

Flower fly (Syrphidae) adult
Flower fly (Syrphidae) adult

P.S. Of course I am interested in ants and aphids. I have never seen ants tending oleander aphids here. Have you ever seen ants tending this species?

42 thoughts on “Oleander Aphids on Milkweed

  1. alison mackinder

    Thank you, very interesting, and I will leave the little buggers alone this year!

  2. Francine

    Such a great article! (Linked from Butterfly Encounters Facebook page). I'm particularly fascinated that they're all female! I actually did see ants frantically scurrying around the milkweed plants that are currently hosting aphids. Not sure if they were herding. There were already lots of brown/black shells so it seems the "infestation" was well into it's decline. Will pay closer attention to the ant/aphid dynamic on the milkweeds this summer!

  3. Roberta

    Post author

    I'd love to hear what you find out. It seems ants do occur in the areas where oleanders and oleander aphids are originally from.

  4. Great article and very informative. Thank you for taking the time to write this. I too will wait to see what if any damage these yellow varmints do to my milkweed.

  5. Sharon

    Very informative. But I must say that out of my four milkweed plants, only the one that is overcome with these aphids looks sickly and is dying.

  6. Roberta

    Post author

    Not to be an aphid advocate, but are you sure the aphids aren't doing better on that one because it is already stressed in some other way?

  7. Ann

    Unfortunately the Aphids are killing the milkweed plants and the ladybugs I tried last spring did nothing to control the aphids. Any additional ideas?

  8. Roberta

    Post author

    Typically the store bought ladybugs don't feed on oleander aphids, I suspect the aphids get some protection from feeding on the milkweeds like the monarchs do.

    What kind of milkweeds?

  9. janice

    I have at least 60 milkweed plants at various stages of growth. Most are in pots for a monarch caterpillar exhibit. My most success comes from smashing the aphids on the plant and leaving the residue on the plant. I contaminate the plant with what I call "aphid cholera".

    When I washed aphids off, they reappeared in a week. Now, I have to search to find aphids on the plants.

    This works well with other infestations too. Gather up a bunch of the pests and put them in the blender with some water. Spray the solution on the plant. It works for me.

  10. Roberta

    Post author

    It is often difficult to raise plants under such artificial conditions. You might want to look into consulting with a biological control expert. There are tiny wasps that attack aphids, and only aphids. They wouldn't hurt the monarchs.

    If you are interested in learning more about aphid wasps, I have a post up about them at my other blog.

  11. Emily

    Great article! I am a graduate student studying oleander aphids at the University of Minnesota. I am very interested in many of these observations. We do, occasionally, see ants tending oleander aphids. There are two other species of native milkweed aphid that we observe, and ants are almost always found tending the milkweed aphid (Aphis asclepiadis).
    We rarely see the high levels of parasitism that you report, Roberta, but the generalist aphid predators, including lady beetles, seem to control them pretty well in our part of the world. Otherwise, if they get very dense (many thousands per plant), a strong stream of water can often displace enough to reduce the problems. I am always interested in observations about oleander aphids; if anyone has observations they would like to share or is interested in monitoring aphids on their milkweed plants, please contact me at mohlx006@umn.edu. Thanks!

  12. Roberta

    Post author

    Hi Emily,

    If you want to see more about oleander aphids in Arizona, check some of the posts at my Growing With Science blog, such as Mix Unscrambled.

    Good luck with your studies.

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  14. Dana

    I'm in central Ohio and we get vining milkweed of some type or another. In our yard it likes to grow all over the forsythia bushes along the side of the house. This year there were little yellow aphids all over one of the vines (I haven't looked at the others) so I went googling. This is a very informative article. Thank you. My little girl (6yo) is fascinated by them, particularly by the notion that they only have mommies and no daddies.

  15. Jean

    I have lots of honeyvine milkweed growing in my yard here in KS. Every year it is fed upon by monarch caterpillars. This year the oleander aphids have really been thick to the point that the milkweed leaves are shiny with honeydew. The caterpillars this year have been parasitized by tachinid flies that I think were drawn to the honeydew on the milkweed. Has anyone else observed this?

  16. Roberta

    Post author

    Seems like the flies might survive better with a lot of food, but they are very good at finding caterpillars, no matter what.

    Would be an interesting question for a research project. Any students out there?

  17. Chloe

    Ermmm.... Now i might not want to plant milkweed in my backyard... It attracts too many bugs! Eh ill take good care of it

  18. Molly

    Fascinating! I had planted some butterfly weed in my backyard this spring, but i had to leave for the seashore shortly thereafter. I was gone for 4 months and when I came back I found these little critters sucking on the seed pods. I had planted the milkweed in hopes that the seeds would spread to an abandoned field adjacent to my house. Do you think this will effect the seeds development at all? The plant seems pretty healthy even with the infestation.
    As a side note... There are ants crawling around the plant but they don't look like they're harvesting the aphids. I'll have to observe more...

  19. Roberta

    Post author

    Hi Molly,

    We have had aphids on our milkweeds for years and the plants have seeded readily. We do get milkweed bugs sometimes, and those will feed on the seeds.

  20. Michele

    Your article explains everything going on in my tiny common milkweed garden. The live aphids and dead aphids are all over the plants. Some of the leaves are turning yellow. The garden is full of green flies. Before I read your article I thought I had a fly hatchery or something. The plants are too large with too many seed pods. I'm cutting back branches with the seed pods to protect mine and my neighbors yards.

  21. Hi, I hope you are still seeing comments on this post. I am trying to find out what is causing the Common milkweed pods in our yard to start to turn black. Someone mentioned it could be black soot from the oleander aphids. This patch is at least 6 years old, and I see most of the critters that are on my "Monarch Village" t-shirt, including plenty of aphids each year. I don't remember seeing any black soot in the past. What I'm trying to find out, now that one of the seedpods has opened, exposing its seeds, is whether it is safe for me to give the seeds away, or if the black soot will remain in seeds, causing future plants to have it. As you can tell, I'm not up on plant diseases.

  22. Roberta

    Post author

    Sue,

    Sorry it took a few days to get back to you. The black sooty mold that grows in the honeydew of aphids and scale insects is largely just a cosmetic issue. It really doesn't involve the plant, unless it is so thick on the leaves that the sun can't get through. Your seeds should be just fine.

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  24. Jan

    I have collected the pods from the A. tuberosa in my yard. Thiese little milkweeds have always had aphids, but have always produced a few nice pods. This year I staged a seed bomb event in my Butterfly Club at school. The silk and seeds looked healthy inside the pod. I didn't take the pod until it had started to open. We will toss those bombs out somewhere on our campus in a few weeks. (We are a monarch Waystation)

  25. Helen

    I wonder if the aphids are toxic to just morphed American Toads? I have pools that produce thousands of these little toads. I buy pinhead crickets online but wondered if I could feed these aphids to the little toads without harmful effects.
    Anyone know?

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  27. Roberta

    Post author

    Helen, You might want to try a different kind of aphid. Very few critters eat these kinds of aphids, so I suspect they do take up some of the toxins from the plant.

  28. cam

    I tried the "leave them alone" approach for the past 2 years, and they ruined my milkweed plants. Within a couple of weeks of the aphids' appearance, every milkweed plant I have stops producing flowers because of the damage done by the aphids, and the damage continues all season long (which is a long time in north Florida). No flowers all season means no seed pods at all, as well as no nectar for butterflies and other pollinators. It also stunts the growth of the milkweed plants, since all the new growth is covered in aphids. The plants don't get as big nor look as healthy as they did the first year when there was no aphid infestation for the majority of the season.
    So I'm searching for a way to avoid the infestation this year, before it gets started.

  29. Rhonda Brady

    Nice article. I am in West Central Illinois. Where are you located? I appreciate your comment in the thread above saying "each location is different". A truer statement cant be said especially when dealing with nature. I hope you will give me permission to share it on the IL Monarch Waystation Capital Facebook page. Here in IL we have many varieties of Milkweed- Swamp, Whorled, Butterfly Weed and Common all infested with Oleander Aphids. We do have some ladybug larva eating the aphids, and yet some plants get so infested the honeydew gets moldy and the plant dies.

  30. Kim Engels White

    I have a couple of videos that I took this summer of ants tending to oleander aphids. I have posted them on my FB page 'The Natural Naturalist'.
    I have over 125 Common Milkweed plants in my yard. I have been doing my own study on oleander aphids this year. I found that leaving the aphids on the plants coats the leaves with a film of honeydew. Dust, dirt, and spent exoskeletons can stick to leaves, causing them to appear dark. It can be washed off. I also found that because of the increase in the sweet honeydew, I have a major increase in the quantity of flies of various species. The plant itself doesn't appear to suffer greatly from the infestations, but in some cases, it can get really out of control. One of the things that really intrigues me, is 'why can you have 2 plants next to each other, and one will have a major infestation, and the other will be clean?' It must have something to do with phytochemicals.

  31. Marcie Graham

    Along with aphids, I see small flies and ants AND wasps all scurrying around together on the milkweed leaves. It does appear that the wasps catch and eat the odd ant - but what are the flies (they look like smaller houseflies) doing? Actually, each insect I see must have a purpose, but what? Thanks for all the great information meanwhile.

  32. Roberta

    Post author

    Kim, 125 milkweeds must be quite a sight!
    The aphids seem to do better on the new growing tips, so plants with a new flush of growth may have more aphids.

  33. Marcie Graham

    WOW!!! Phenomenal! I can't wait to go out and watch tomorrow! I'm very new at this, so can you tell me whether Monarchs will be just as happy with butterfly weed to sustain themselves? Milkweed just never seems to result in any caterpillars in my garden, and while I let the milkweed grow wherever it likes, it really looks ugly later in the season. Dare I pull the common milkweed and just keep the gorgeous butterfly weed that I've planted all over (orange and red)? It makes a glorious display!

  34. Roberta

    Post author

    Love your enthusiasm!

    As for pulling, if your butterfly weed is Asclepias tuberosa, it isn't the best host for monarch caterpillars. They don't care for the leaves as much as some of the other kinds of milkweed and will mostly use the buds for food. On the other hand, the migrating adult butterflies will use the flowers for nectar.

    Enjoy your milkweed watching!

  35. Roberta

    Post author

    Sorry for the delayed response. My computer wasn't sending me the comment notifications for some reason.

    One easy way to keep ants off plants is to apply a sticky material around the base of the stem. That won't hurt the butterflies. Scientists use a material called "Tanglefoot." http://www.tanglefoot.com/smg/gosite/Tanglefoot/home

    Many stores with garden centers carry it.

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