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.
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!
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.
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.
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).
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.