What’s wrong with the acorns?

oak galls
What’s wrong with these acorns?

For the past month or so I’ve been seeing what appear to be red acorns, many of them, on the trail between the Dinosaur Trail and Catch the Wind. Most are crushed from being stepped on or run over by Museum visitors or vehicles, but many are whole and unmolested, especially the ones that rest along the side of the path.

I thought it safe to assume that the “nuts” on the trail were acorns since I’ve only found them beneath oak trees.  But, aside from being round, the nuts don’t resemble acorns at all. They are reddish in color, wrinkled, and very small. There are a number of species of oak that produce small acorns, but the trees from which these “acorns” had fallen, and are still falling, produce much larger acorns.

What are these strange seed-like objects beneath the oak trees? “Maybe they’re galls,” I thought, but I don’t remember ever having seen galls fall from the trees like seeds. Also, there’s too many of them. It seems logical that if there were that many galls on the trees then the trees would surely be suffering for it. From what I could see, the trees looked fine.

oak galls
None of the “acorns” measured more than 3/8 inches.

I broke a few of the nuts open to see what was inside but didn’t notice anything unusual, most were moist with a fleshy outer area and a small central core. After cutting a handful of the wrinkly red balls open with a razor (the proper way to do it), I discovered a tiny larva within the central core area of two of the balls. They were galls after all!

oak gall
A larva is visible on the right (white object). I carefully scooped the larva out of the core in order to photograph it more clearly.

Most of the oaks along the trail are very tall, but a few have branches low enough for me to reach. I found several clusters of galls on those low branches.

oak gall
There were several clusters of the galls on some of the branches that I could reach from the ground (note the brown leaf).
oak gall
The galls were all attached to the stem of a leaf. You can see scars on the stem where four of the galls had broken free and fallen to the ground (note that the leaf is turning brown on the left side).
oak gall
Close view of the scars left by fallen galls.

After all this discovery, I still didn’t know exactly what kind of galls they were. Oaks have many types of galls, mostly caused by tiny wasps that lay eggs into twigs or leaves, but these didn’t look like any that I’d seen, or had found while researching for this post.

The wasps that had laid the eggs, which later became the galls, most surely belong to the family Cynipidae which consists of around 600-800 species (depending on the source consulted) in North America and which are all less than one centimeter in length.

If the adult is less than 1 centimeter, how small is the larva? Well, the ones that I photographed are about 1-2 mm long, very small.

oak gall larva
The larva of a gall wasp. This larva is on its back with its belly up, head at top of photo, rear end pointing towards camera.
gall wasp larva
Same larva, side view. The head is clearly visible on the left. The dark markings are mouthparts.

So, what is this wasp that has been so busily galling the oak trees of the Museum this year? The closest I can come to answering that question is that they are Clustered Midrib Gall Wasps (Adleria dimorpha) which is actually quite specific. But, if it’s not that particular species then I would think that they are at least within the same genus. I could be wrong, we all know what can happen when we make assumptions.

If you were to click on this link at BugGuide.net you may be persuaded to agree with me that the wasps in question are Clustered Midrib Gall Wasps. On that page, it states that the wasps lay their eggs in clusters on the midrib or petiole on the underside of leaves. Although none of the galls that I saw were on the leaf itself, they were certainly attached to the petiole, or stem, on the underside of a leaf. White Oak is the host group. All of the galls that I saw were on white oaks or appear to have fallen from white oaks.

But, why are they falling to the ground? Do they need to fall to the ground where the larvae will pupate within the gall among the leaf litter? Galls that are on the twigs or stems of the tree pupate within the gall while it’s still attached to the tree, but a gall that is attached to a leaf is going to come tumbling to the ground sooner or later (when the leaves drop in the fall).

bullet gall
These galls look to be Round Bullet Galls (Disholcaspis quercusglobus) and are attached to a twig. The insects within will mature while the galls are still attached to the tree, unless the branch is broken from the tree.

From what I’ve read, there is considerable variation in the life cycles of the various gall wasps, some even having more than one stage where there are female only generations, male and female generations, and so on. It seems, though, that these wasps, the ones that have been dropping galls all over the paths of the Museum, mature on the ground and emerge as adults 1, 2 or even 3 springs after they were deposited on the leaves as eggs.

Galls produced by gall wasps don’t seem to do much overall harm to the trees or shrubs that host them. Although the leaves of the oaks that I saw, which had galls attached to them, were prematurely brown, or turning brown, they were only a week or so premature in their photosynthetic shut-down.

You learn something every day.

There are more galling adventures to come. Stay tuned!

2 responses to What’s wrong with the acorns?

  1. Avatar
    Dana Weber says:

    I’m so glad to have come across this information! Thank you so much for writing such a detailed, descriptive report. I am always curious about things I discover in nature and I was trying to help my sister identify these galls in her backyard in Long Island, NY. Thanks to your help, I was able to.

    • Greg Dodge
      Greg Dodge says:

      Excellent, glad I could help. Hardly a day goes by that I don’t find something that I’ve never seen before and wonder what it is, or what it does, or how it fits into the scheme of things. Curious, and never really bored!
      Thanks.

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