A Galling Discovery

We’ve had some good weather lately and I’ve been walking the country roads around my place of work. Earlier in the year I noticed a large section of Thimbleberry (Rubus parviflorus)  alongside the road with a large number of galls on the stems.

I know that galls are usually a plant’s response to the eggs (and subsequently, larva) that are typically laid by a gall wasp but in the past I haven’t really paid close attention to them. It was only after I picked up a copy of Insects of the Pacific Northwest (published by Timber Press) that I started looking closer at them. This book has an excellent section on gall wasps and focuses on identifying the species by its plant association and the visual appearance of the gall.

Old Wasp Gall
An old wasp gall on Thimbleberry (Rubus parviflorus).

With that in mind I took a closer look at these galls this week. They’re pretty old and woody and the few that I broke apart had nothing that looked living inside them. All of them had holes which seemed to me to be an indication that the insect inside had burrowed its way out of its larval home. From what I’ve read, if these old galls were green and on fresh growth they would be full of larva.

A Pteromalid on the surface of an old wasp gall – pretty hard to spot unless you’re looking for it!
Pteromalids are very small – about the size of a black fly. This is a little closer but still cropped quite a bit!

Haggard’s book had a picture of a gall on Thimbleberry that matched these roadside galls perfectly. The gall wasp responsible for the misshaped growths was a wasp called Diastrophus kincaidii.  These tiny wasps turned out to be something very different.

A closer look at a Pteromalid – this image is really cropped!

I was looking at the galls when I noticed two very small (a couple of mm in length) insects crawling around one of the old galls. They were a metallic brown/bronze in colour and seemed to be checking the old gall over. I initially assumed that they were gall wasps but it turns out they are probably Pteromalids, a Chalcidoid Wasp that is a parasite on other insects!

The two wasps were moving fairly slowly over the surface of the old gall. I checked several other galls but couldn’t find any other wasps.

According to Haggard, the gall wasp lays eggs on the fresh growth in the spring. They hatch within a week and begin eating and growing. By fall they’re ready to pupate and spend the winter as pupae, emerging in the spring as adults to start the cycle all over again. Somewhere along the line the Pteromalid enters the story and lays its eggs in the larva. This might explain why these Chalcidoid Wasps were investigating some of the old holes.

Another look at the Pteromalid.

Roger over at The New Dharma Bums wrote about dissecting a Thimbleberry gall and discovering the larva inside. He quotes Jay Scott from the Washington Native Plant Society who describes the inside of the gall as follows:

Dissecting the gall revealed the larvae and tunnels they had made through the plant tissue. Researchers have found this plant tissue rich in extra starch, glycogen, and protein. Outer layers of the gall contain tannin and phenolic compounds. The inside of a gall is therefore food for the herbivorous larvae. The outside is foul tasting armor against predators.

Roger’s photograph of the inside of the gall shows the larva – they’re quite small and easily missed. I’m looking forward to getting a few fresh galls myself and seeing if I can find some larva as well!


I’ve learned a little more about these wasps and my discovery has turned out to be much more interesting that I originally thought. I initially assumed that these were gall wasps given their association with the galls of the Thimbleberry. With a little help from the folks at BugGuide.net I’ve updated my identification to that of a Chalcidoid Wasp, likely a Pteromalid. If anyone can help me narrow this down further (or correct my identification) that would be great!