Rubus galls

Diastrophus gall

I have been seeing these things around our property since moving here. Initially there were just a handful of them, in the Rubus canes along the edge of the woods. I took a photo last winter, the first time I found one. I came home, tried to ID it, couldn’t find anything that quite matched. I found another this fall. Took a photo, came home, tried to ID it, came up blank. A few weeks ago I came across another, same thing. Each time I was sure that I just hadn’t been using the right search term, or looking in the right place, or something like that.

Then earlier this week I found a whole patch of them, in a thicket of canes over at the 100-acre woods. There had to be more than a dozen, clustered into just a few square meters. “Okay,” I told myself, “I really need to get these figured out.” I ran off a bunch more photos then came home, made myself a tea, and forced myself sit down and look till I had an answer. Worse came to worst, I figured, I’d post it to BugGuide and hope Charley Eiseman would help me out on this one, too. ;) But it’s so much more satisfying (and you remember better) if you can identify it yourself.

Diastrophus gall

I think what had been causing me problems is I’d been looking for a photo match. This has always done me fairly well before. Go to Google image search and type in some sort of descriptive term, and in all likelihood someone else has described and ID’d the same thing you have. This didn’t work in this case. It pulled up quite a number of lumpy galls, but none of them looked to be the same as mine. I also tried my favourite recent reference acquisition, Eiseman & Charney’s Tracks & Sign of Insects (go get a copy, if you haven’t already! You’ll thank me!), but none of the photos there looked like my thing, either (though some looked similar). I repeated this process in each ID attempt, and again this week.

It was only on this most recent try that I finally delved into Eiseman & Charney’s text, paying close attention. And it was in there that I think I found my answer. I found three references to galls in brambles (Rubus sp.).

The first stated, “Metallic wood-boring beetles (Buprestidae) include the red-necked cane borer (Agrilus ruficollis), responsible for slight, ridged swellings that are 1 inch (2.5 cm) long in bramble canes”. Which, as cool as it would be for my galls to be wood-boring beetles, didn’t seem to be it. They’re more lumpy and large than slight and ridged, and most are longer than just an inch.

Number two said, “Woody stems and twigs are host to a number of Neolasioptera midges … N. nodulosa [is responsible for] swellings on brambles, sometimes ridged and usually near the tip of the stem.” This seemed a definite possibility. None of them were ridged, but the text only said “sometimes”. Many (though not all) of mine were near the tip. So I Googled it. They definitely do make swellings on bramble canes, but they seem to be much slimmer than what I have. Hm. Back to the book.

The final reference read, “Several species of Diastrophus (Cynipidae) cause pronounced swellings on blackberry stems (as well as roots), up to 6 inches (15.2 cm) long.” I’m not sure which species of Rubus is referred to by “blackberry” (there are at least two that could be called such), but they’re the right plant genus, anyway. And these galls are definitely pronounced, and a couple of them did reach a few inches long. The BugGuide.net page for D. nebulosus looks pretty similar, and the text did suggest that there are several species, which may or may not necessarily all look the same.

Diastrophus gall

So I think these are Diastrophus galls. The insects that made them are wasps. They look less like what we typically think of as wasps, and more like fruit flies – very squat in stature. The lumpiness of the gall is due to the presence of many chambers within. Presumably the adult female lays many eggs into the plant, all clustered together, and they all make their own swelling, which in turns creates one large, lumpy gall. There doesn’t seem to be very much out there on their life history, but from the circumstantial evidence of my observations it would seem they overwinter in their galls and emerge in the spring to mate.

Diastrophus gall

Though the galls on our 30 acres were untouched, more than half of the ones at the 100-acre woods had been investigated by birds. Downy Woodpeckers and chickadees will both drill into galls to extract the grubs overwintering within. They’re most commonly known to do this with goldenrod galls, but obviously these Rubus galls were large enough and promising enough to draw their attention. Where the birds have chipped away the woody cover, you can see the dark holes belonging to each individual chamber. (And it was the discovery of this, more than the abundance of them, that made me really want to get an ID after finding the bramble patch.)

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3 responses to “Rubus galls

  1. You’ve got it! When the galls have distinct longitudinal furrows, like the ones on BugGuide, I think it’s safe to call them Diastrophus nebulosus, but the irregularly blobby ones might be this or some other Diastrophus species.

    The gall on p. 402 I collected near the Massachusetts-Vermont border on April 10, 2004, and adults emerged on June 29. I didn’t save any to find out what species they were (it didn’t occur to me that I might write a field guide to these things at some point…)

    I was using “blackberry” in this case in a loose way–I actually see these most often on dewberries (the one in my book looks like it was R. flagellaris).

    • Thanks for the confirmation, Charley! I noticed that only the furrowed ones were assigned to species, and expected that was the case.

      I’ve run into the same thing myself, in wishing retrospectively to have paid more attention or spent more time with something that I could’ve used in the book, if only I’d known. We had a few species of moth where I did have photos, but they weren’t publication-quality because at the time I’d only wanted a record photo that I could use to try to ID the species later, and so we ended up having to solicit images from other sources.

      I presume identifying these wasps to species would require examination by microscope. I’ve been thinking this year I should save more galls/cocoons/eggs/etc to find out what they become.

  2. I’m trying to figure out of what I’ve found is a Diastraphus gall, but I can’t even determine the Rubrus species! I wish I could post a picture here.
    I don’t know of you’re responding to 5 year old posts any more.
    The moth book is great, BTW.

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