In early November I wrote about gall wasps on the Museum’s oak trees. And, as promised, here’s another gall-filled adventure.
Winter’s less than two weeks away and it’s a good time to talk about galls. Insect activity has slowed down, many plants have shut down for the winter, and although there is always something going on out-of-doors, the pace is certainly slower. And, oh yes, the galls are much easier to find when the leaves are no longer on the trees and shrubs.
Over the past year, I monitored (and photographed) several different galls. The Blackberry Knot Gall is one that many people are familiar with, at least in the respect that those people have seen one of these large galls on a blackberry bush sometime in the past. They may not have known what caused the rather strange looking growth, but they’ve at least seen one before.
Like the galls caused by Clustered Midrib Gall Wasps (Adleria dimorpha), discussed in November, the blackberry galls are the plant’s reaction to a tiny wasp with a big name, in this case it’s the Blackberry Knot Gall Wasp (Diastrophus nebulosus). During the spring and summer months, this little wasp oviposits (lays eggs) into the ridged stems of blackberry which stimulates the plant’s tissue into abnormal growth along the stem.
The galls have large rounded ridges along their length. Inside each ridge is a series of chambers which house the wasp’s larvae once the eggs hatch. Secretions from the larvae also stimulate further growth of the gall. The galls may reach 2″ to 6″ in length.
I cut open one of the galls to get a look at the larvae inside and found that each ridge in my specimen could hold 15 or more larvae, each within its own chamber. It seems that the length of the gall is determined by the amount of eggs that the wasp deposited in the stem, the more eggs the longer the gall.
The adult wasps eventually chew their way out of the gall the following spring, leaving tiny holes along the gall’s lumpy ridges.
In early April of this year I noticed some very small wasps walking about on a gall next to the Wetlands in Explore the Wild. Finally, I’d get a chance to photograph an adult Blackberry Knot Gall Wasp.
After taking photos of the tiny wasps (about 3 mm), I uploaded the photos to BugGuide.Net to see if I could get a positive ID of the wasps. I was informed that they, the wasps that I had photographed, were not Blackberry Knot Gall Wasps at all. In fact, the wasps that I saw were members of an entirely different family of wasps.
The wasps that made the galls were in the family Cynipidae (gall wasps). The wasps that I saw, and photogrpahed, on the galls in April were Chalcidoid wasps, in the family Eurytomidae.
Many members in this family of wasps (Eurytomidae) are parasitic, their larvae feed on the larvae of gall wasps. Perhaps the wasps that I photographed did emerge from the gall after all. Of course, that would mean that some of the original occupants, the gall wasps, didn’t make it to adulthood. There were many tiny wasps walking about on the gall that day, and there’s no way to tell for sure if some of them were Blackberry Knot Gall Wasps or not.
It may be, however, that the parasitic wasps hadn’t actually emerged from the gall but were investigating it in search of larvae or pupae of the gall wasp which had not yet emerged from their chambers. If a larva or pupa were found, the parasitic wasp would then lay its egg within that chamber, its larva later feeding on the gall wasp’s larva or pupa. I certainly don’t know enough about Eurytomidae wasps to know exactly what those wasps were up to, don’t even know what species they are. The family (Eurytomidae) consists of 21 genera which includes 250 species. The tiny, black wasps that were walking about on that knot gall one sunny day in April fit somewhere among those 250 kinds of wasps, where, I don’t know.
The important thing in all of this, is that I now know more about these wasps than I did before I stopped and investigated those strange growths on the blackberry bramble, both the Cynipidae (gall wasps) and the Eurytomidae wasps. That’s a good thing. There are dramas being played out every day in the wild. But, if you don’t look, if you’re not out there, you’re not going to see them.
From my research, I’ve discovered that these wasps are not very well known, there is much to be learned about both families. A student (no matter what age or experience) could uncover many details about the lives of these wasps, and probably find a few new species along the way.
Let me know what you discover.
More galls to come….