The Bug Geek

Insects. Doing Science. Other awesome, geeky stuff.

Goldenrod gall fly (Eurosta solidaginis)

I remember finding out about these when I was a first year Journalism undergrad.  I was watching the latest lecture-on-tape of my elective class: “Natural History of Ontario”. *

The instructor** pulled a stem of goldenrod out of a bag and laid it on the desk.  We were told the fleshy, spherical structure on the stem was caused by a fly, and that the structure was called a “gall”.  The camera operator zoomed in on the pocket knife blade now held firmly against the gall; I watched as the instructor carefully cut through its thick wall, just deep enough to allow him to pry it cleanly apart into two halves.  The camera zoomed in closer, and lo: a stout white grub, curled in a protective ball, nestled in a tiny chamber at the centre of the bulbous growth.

This seemed amazing to me at the time…like magic.  I had to see if such a thing could be replicated.  Sure enough, a trip to a nearby park quickly led to the discovery of galls on dozens of goldenrod stems.   My very own pocket knife carefully cut in (not too deeply), and then…TA DAAAA!!!!  Magic!  Fly larvae out of a round chunk of plant!  I had to repeat the procedure a half-dozen times or so to convince myself it wasn’t a fluke.

The galls are formed when larvae of the goldenrod gall fly (Eurosta solidaginis) – deposited in goldenrod (Solidago sp.) stems during spring by the female – chew on the stem tissues, inducing a hormone-mediated response in the plant.

Five Goldenrod galls

The thickened tissue provides both food and protection for the developing larva, which overwinters in its gall.

A single, intact gall.

Not all gall flies survive the winter to adulthood, however. The robust white grubs are a highly sought food source for birds such as Chickadees and Downy Woodpeckers, who bore through the galls with their beaks and pluck the tasty treat from within.

Goldenrod gall raided by a Chickadee. The hole is messy and irregular. A neat, round hole suggests woodpecker feeding.

If you find a gall with a small, single round hole (a woodpecker’s feeding hole would be about the same size as the Chickadees, above, only tidier), the larva managed to avoid predation and chewed its way out of the gall in the spring, emerging as  a new adult.

Even after all this time, I still get a little thrill when I find one intact, and carefully split the structure into two neat hemispheres…TA DAAAA!!!! Fly larva, like magic!  It’s one of my favourite nature “party tricks” when I’m outdoors with the uninitiated. 


* Yes, Journalism.  That was my first major.  The Natural History course in question was the catalyst for my switch to a Biology major the next year.

** Natural history genius guru of epic proportions.  My hero. 

15 responses to “Goldenrod gall fly (Eurosta solidaginis)

  1. Morgan Jackson February 11, 2010 at 8:58 AM

    Nice fly post! 🙂 I was on a guided nature hike once where I heard these larvae taste an awful lot like peanuts! I haven’t taken the chance to find out personally, but food for thought!

    Also, Eurosta is fairly heavily parasitized by Eurytomid wasps who consume the fly larvae and take over the gall, so there’s a pretty fair chance that some of the “fly” larvae you’ve seen are actually parasitic wasp larvae!

    • TGIQ February 11, 2010 at 9:15 AM

      Peanuts, eh? Hmm. I’ve known a few entomologists who had the strange urge to taste a lot of their subjects, and I’ve eaten some insects myself, but I don’t think I could bring myself to sample a raw fly larva. Blech. Do you have any photos of the wasp?

  2. MObugs41 February 11, 2010 at 2:40 PM

    I love to find galls. They are such fascinating things. These goldenrod galls are EVERYWHERE! Occasionally I’ve even had the pleasure to see the fly in action (and naturally my camera was elsewhere…grrr). We have a lot of Oak galls around the farm, and some of them get quite large. I’ve never seen anything feeding on them as of yet. Although I did get the privilege of witnessing a woodpecker feeding on a Chinese Mantid Ootheca, and I had my camera!

  3. Pete Yeeles February 11, 2010 at 10:09 PM

    You have birds called Chickadees!! I always thought it was just a slang term for cool kids. You just made my day!

    I remember when I was a kid, out for a walk with some family friends. One of them tried to convince me that the “nuts” all over a particular tree were in fact homes for baby wasps. I refused to believe her, so she grabbed one and brought it home with us. In a similar vein to your story, she then proceeded to cut it open, and I was amazed by the little grub sitting inside. Cool stuff.

    • TGIQ February 11, 2010 at 10:44 PM


      Oh Peter, I TRULY thought you were being sarcastic for a moment…and then I remembered that you live on the other side of the planet. You may have puffer fish and sunny beaches, but YES, I have Chickadees! LOL

      They are super-common and just wonderful little characters. Curious, cheeky, bold, quite fearless actually. I see dozens daily at my bird feeders; this morning I even coaxed one to land on my outstretched hand to take some seed…and these are NOT tame birds! It’s quite extraordinary, the level of trust these little guys seem to innately have with humans.

      So, just for you, a quick pic of one of my beloved “deedees” on my suet feeder:

      • Pete Yeeles February 12, 2010 at 8:14 AM

        No sarcasm TGIQ!! I really was a happy chappy upon finding out there was a bird with such a cool name.

        Nice pic. Sounds similar to a little bird we have in our southern states called a superb fairy wren. Although not naturally fearless, I tend to see them at campsites and these being more used to human contact can be quite cheeky.

        These guys exhibit some great sexual dimorphism. As is usually the case… the bloke is the looker 😉

        (Pic is trawed from the net; not my photograph)

        • TGIQ February 12, 2010 at 8:39 AM

          He’s LOVELY!!!

          We’ve got “wrens” here too (“ours” are Troglodytidae, “yours” are Maluridae), but they’re all LBJ’s: Little Brown Jobs…nothing nearly as flashy,and little to no dimorphism. They’re also much more elusive/shy.

  4. MObugs41 February 11, 2010 at 10:45 PM

    I have the Mantid ootheca and woodpecker picture posted on MObugs under a post about Chinese Mantids. The photo is a little blurry, but I still feel fortunate to have seen such a scene play out. The whole scenario lasted about 60 seconds.

    • TGIQ February 11, 2010 at 10:52 PM

      Found them! What a super-cool thing to witness!!! I’ve come across maybe 2 or 3 ootheca in my life. They’re incredible-looking structures (and tasty, too, apparently!)

  5. christie February 12, 2010 at 2:19 AM

    I really feel like I need to just get into the woods after I read your blog. Hopefully I will be able to get a family expedition together soon.

  6. Pingback: See ya, 2010… « Fall To Climb

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