Salix species and Rhabdophaga rosaria

Today’s images come to us from Suzan D. Suzan posted the photos for identification on the UBC Botanical Garden forums in 2004. She cleverly called these “woses.” Original post.

Although the structure in the photo looks very much like a green rose, roses do not grow on willows. And while Suzan never found the insect, we believe this to be a gall caused by Rhabdophaga rosaria, the European rosette willow gall midge. Galls are abnormal growths found on plants that can be caused by parasites such as insects, mites, fungi or bacteria. Insects, particularly wasps and midges, are the most common cause of galls on plants. Plants form the galls in response to ovipositing or feeding by the insect, or from infection by another agent. The gall is an attempt to surround and isolate the invader. In the case of insects, the gall actually forms a protective chamber, where the larvae can develop safely away from predators.

Many types of galls exist. Many are shaped like balls or blisters. They are still unusual things to find in the garden, like this one from the UBCBG forums. In North America they are most commonly found on Fagaceae (oaks), but are also often found on Salicaceae (willows), Rosaceae (roses) and Asteraceae (asters). In general, they cause little damage to the plants and most are not considered to be significant pests.

Further reading on plant galls:

willow gall
willow gall

21 responses to “Salix species and Rhabdophaga rosaria”

  1. Carolina

    Never heard of (insect caused) galls before, this is so interesting. How beautiful they are, until you know there is a larva (eeww!) inside!

  2. Deborah Lievens

    Oh wow! Would I love to see that one! I love galls. Am waiting for someone to write Galls of the Northeast. There is a book for CA. Meantime I will enjoy the links you provided. Also add poplars: we have a poplar petiole gall in NH and I’ve seen galls on the edges of poplar leaves elsewhere.

  3. Ann Rein

    Amazing to think that’s a gall – it would be nice to find out exactly what insect caused it!

  4. Lanie

    wow! at first look it looked like a miniture cabbage, although I knew it wasn’t. it does look like a rose also. how interesting. thank you for posting. I love learning every day from your site

  5. Meg Bernstein

    How interesting. I am used to galls on goldenrod. This is so enlightening.

  6. Eric in SF

    Absolutely stunning! I’ve seen a lot of galls but this takes the cake as the most incredible.
    Here is a much less impressive gall growing on Baccharis pilularis:
    And the lovely larvae at the heart of the gall:

  7. Connie

    I have heard that someone is studying galls in relation to cancer. Something triggers the plant cells around the egg to grow into a gall instead of a normal leaf or stem or whatever. It seems one or more genes are either supressed or activated. Or both? I would like to know more about this, if anyone can point me there…
    Great pictures, I guess it’s no good looking for a “willow rose” in the New World!

  8. bev

    I have never seen a gall that looked so like a flower. Thanks for this interesting photo and discussion.

  9. Adriana

    I have many willows at my home in the midwest. They usually get galls on the stems resulting in abnormal growth. They certainly are not beautiful! If I had to have a willow gall, I sure wish it would be this one! I could cut and sell those to a florist! 😉

  10. Charles Tubesing

    For those interested in identifying galls and the organisms that induce their development, there is the book Plant Galls and Gall Makers, by Ephraim Porter Felt, published in 1940 by Comstock Pub. Co. and reprinted in facsimile by Hafner Pub. Co., New York/London in 1965. The author was Director and Chief Entomologist for Bartlett Tree Research Laboratories, Stamford, CT. The scope is the U.S. After a general discussion of galls, the bulk of the book is comprised of a key to the galls and their causal organisms, organized by plant families.

  11. Deb

    To Carolina, who had never heard of galls caused by insects: I believe that’s what they are referencing, in 19th century English literature — “oak apples.”

  12. Eric La Fountaine

    Connie, note that the photos were taken in Calgary and if you follow the link to the original post on the forums, one of the Friends of the Garden recalls seeing them frequently in the Yukon, so you will find this in the new world. We could only find mention of the European rosette willow gall midge causing this type of formation. The insect probably was brought to N. America unintentionally, but it could be a case of convergence and the gall may have been caused by a different species.

  13. Tom Wheeler

    Superb photo of an intriguing curiosity. As, a child in England I remember the “robin’s pincushion”
    on wild rose. Galls can really start the wheels turning! Thank you, Suzan.

  14. Eric in SF

    The morphology is so close to a ‘normal’ flower that one wonders if the genetics for this particular flower morphology live inside a LOT of plants and this particular gall insect took advantage of it.

  15. Annie Morgan

    It’s a rose! It’s a cabbage! But no, not even a cabbage rose. Utterly fascinating, and quite beautiful. Great pictures.

  16. elizabeth a airhart

    to be or not to be a gall is a good read
    the pictures are ever so good
    my i surely did have the gall to say that

  17. Dana

    Awesome write on galls! I have wondered about their purpose. Besides being protective wouldn’t the extra growth also feed the ‘baby’? I do have one of the rare fungal caused galls on my Forsythia. Also, I met a guy whose uncle wanted to induce galls to his oak tree because he LIKED them!

  18. elizabeth a airhart

    always interesting around here
    i have been outside on the grounds this hot night now that i know what i have found but
    could not put a name to -thank you
    this post took a lot of gall to post did it not

  19. elizabeth a airhart

    sorry about the double post

  20. Justin

    Galls are just fascinating! I’ve never seen one other than the blisters or balls on Fagaceae and Salicaceae! I’ve never even imagined an Aster or Rose gall…any chance of seeing one posted?

  21. Ron Russo

    Dear Folks:
    I read your note on Rhadophaga rosaria and I can find no such species in Gagnes’ 1989 Plant Feeding Gall Midges of North America and you have spelled the genus wrong….there is no H following the r. Should be Rabdophaga.

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