My now 7-year-old 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.
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 is that we have lived in our current home for a bit over a year and I have no memory of having seen these on our trees last year. I don’t know how I would have missed them and yet this year they are 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 gall wasp Callirhytis furva although there are a couple of other possible species that also partial matches to what 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 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 not from an animal.
About three weeks ago in early September the galls began to turn brown after having been a bright orangish-red color. Then they started 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 are detaching themselves and falling to the ground where they will rest until the larva matures into an adult wasp and emerges from its protective home.
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.
Above is a picture of some of the galls on the ground from a week ago. Today the ground is 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. I collected some and put them in a container which I will keep in my shed to see if they will emerge next year or the year after.
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 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. I thought it seemed very buggy this year. Well, I guess we had millions of little wasps flying around that weren’t with us last summer. So next year I am expecting that I won’t see them but I am sure that my daughter will be watching very closely next summer to see what shows up next.
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 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 10s 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.