Naturalis Historia

Fuzzy Orange Galls from Parasitic Wasps on Oak Leaves – Observations from 2013-2020

My daughter is very attentive to the insect population around our house. She spends many hours collecting all sorts of insects and other animals.   In the fall of 2013 she brought me a leaf that she thought had a caterpillar of some sort on it.   She is well aware that bugs make little houses in leaves and stems and after she realized that these little orange furry things were attached to the leaves she thought there must be a bug living inside of it.  She was hoping I could open one up and show here the critter inside.  I didn’t know what it was but a quick look at the oak trees in my yard revealed that there were a LOT of them as you can see in the picture below.

Fuzzy reddish to orange galls on the surface of oak leaves in my backyard. Image: Joel Duff

We have at least 10 large pin oak trees in our yard in addition to a few other in the neighbors. I did a quick scan of the lower branches and then calculated that there must be at least a half a million of these little fuzzy blobs on the leaves in less than an acre of land.   Surprisingly, I also have a large white oak tree right next to a heavily infested pin oak tree and I have yet to find a single one on the white oak tree.    What I found strange about these galls the first time that I saw them was that at the time we had lived in our current home for a bit over a year and I had no memory of having seen these on our trees that previous year.  I don’t know how I would have missed them and yet in 2013 the trees were covered with these galls.

But what are they?  A bit of searching revealed that they are a type of gall produced by a gall wasp.  Most likely they are produced by the tiny gall wasp Callirhytis furva although there are a couple of other possible wasp species that produce galls similar to those we are seeing on our trees.  Below are a few more pictures and a description what this gall wasp is doing.

The picture above and below show a few of these galls a bit closer. You can see that they are found along the main veins of the leaves to which they are attached.   A very small wasp–barely visible to the eye–implants an egg into the growing leaf which then turns into a larva which then entices the pin oak tree to grow these galls which the larva then inhabit and use for protection while they mature into adults. Although the galls look nothing like the leaves or any other part of the tree they are made of plant tissues from the tree.  Yes, the hairs and colors are plant tissue and not from an animal.

Galls along the main vein of a pin oak leaf. Image: Joel Duff

Fuzzy gall wasp galls on leaves.

Close-up of gall wasp gall on pin oak leaf. Image: Joel Duff

About three weeks later in early September the galls began to turn brown after having been a bright orangish-red color.  By late September (in Ohio, presumably later further south and earlier further north) they begin to detach from the leaves and drop to the ground.   The galls seem to have matured and before the leaves change and fall to the ground to rot these galls, detach themselves and fall to the ground where they will rest until the larva matures into an adult wasp and emerge from its protective home.

Galls picked off of pin oak leaves and sliced open. Image: Joel Duff

I plucked some galls off of a leaf and cut them open. They were quite solid.  The small dot in the center is probably the tiny larva which is surrounded by thick plant tissues and a hairy exterior. Over the next year to two years the larva will eat away at the tissue in the gall using that energy to go through its developmental stages to eventually produce an adult gall wasp.

Galls on the ground among the pea gravel under the swing set. Pin oak acorns are also present which give you an idea of how small the galls are. Image: Joel Duff

Above is a picture of some of the galls on the ground not long after I first noticed the galls. Soon the ground was just littered with these little brown fuzzy balls.  From my reading I have found that it may take two years for the larva to mature to an adult and emerge from these little galls.

Interestingly, in researching these gall wasps I found reports from northeastern Ohio of these galls on oak trees from 2007, 2009 and 2011 with the latter two being reported from locations not more than 20 miles from where I live.  The odd years suggest that it is a two-year cycle and this is why I did not see any of these galls on my trees last year.   Well, I guess we had millions of little wasps flying around that weren’t with us last summer but we didn’t seem to notice.  So next year I am expecting that I won’t see them but I am sure that my daughter will be watching each summer to see what shows up next.

The geographical distribution of these wasps/galls

Many people have asked me if what they are seeing in other states near or far from Ohio are the same as what I have observed in the northeast Ohio region.  There is a great website that collects such information from professional and amateur naturalists:  iNaturalist.org.   Here, individuals can report what they observe and upload pictures.  There are hundreds of reports of these same galls and when you pull all of the data from these reports they paint a nice picture of the geographical range and what types of oak trees they are associated with.  It turns out they have been reported on two species of oak trees: Quercus nigra (black oaks) and Quercus palustris (water or pin oaks).  The later are what I have in my yard.  Clearly they have an affinity for some oaks and not others as my pin oaks have million of galls while my white and red oaks right next them are untouched.

Screenshot of the distribution map of reports of oak galls from the wasp Callirhytis furva on iNaturalist.org. Each read square is a location where the galls of this wasp have been reported. If you go to the map you can zoom into to a detailed street map and see pictures if the person reporting the galls uploaded one. https://www.inaturalist.org/taxa/179372-Callirhytis-furva 

A 2014 update: When I wrote about these galls in 2013 I suggested that this was a two-year cycle and so I would predict that there would be no galls in 2014. Turns out my conjecture was right. I’ve looked and looked and none of my oaks have any galls this year.  Next year will probably be the gall year.

August 2015 update:  Around August 1st I saw the first small fuzzy galls on the same oak trees as they were on in 2013!  Two weeks later we have tens of thousands of them and they are growing fast.  They haven’t reached the size they did in 2013 yet but I am sure they will in the next few weeks.  Below are some pictures I took on August 22, 2015 of the galls on the same two trees as those taken above.

August 2017 update:  Like clockwork we observed thousands of galls appear on the tops of the leaves of the same oaks trees in the yard.  The previous year (2016) we didn’t observe any so they are following the every-other-year pattern.

Fall 2018 update:  No galls on the oak trees this fall.  I expect we they will be back next year.

Fall 2019 update:  Interesting year, very few galls. I had expected large numbers but I would estimate there were less than 1/10 of what there were the previous year.

Fall 2020:  Wow, based on past history I didn’t expect to see galls but we have more than we have ever had before.  Pinoaks are loaded and when they dropped (last two weeks of September) they formed a dense cover on the driveway and grass. Maybe the very mild winter of 2019/2020 resulted in more of them surviving.  I have done some more reading and there are some other reports of them working on 2 or 3 year cycles so it seems that many of them waited an extra year this time around.  Based on the comments below and the iNaturalist reports it appears that this year was a huge year for these plant galling wasps. It certainly seems they are becoming  more common over the years.

This year I have saved more samples and I am hoping to do a bit of microscopy and will share pictures when I can. I also may try to extract some DNA or future DNA studies.

Image: Joel Duff

August 22, 2015. Image: Joel Duff

My trees in late September 2020 just as the galls began to separate from the leaves prior to the leaves falling from the trees.

My driveway September of 2020 as the galls dropped off of the trees.