I find galls tend to draw my interest a lot more in the winter than they do in the summer. Perhaps part of this is that in the summer there’s just so much to look at that I tend to pass over galls that aren’t in-your-face obvious, but in winter, once the leaves are stripped and critters have all disappeared for the season, they stand out in the landscape more. I noticed these galls on a few small willow shrubs in a corner of one of our fields. They’re not large, perhaps 3/4″ (2 cm) long and elliptical. There seemed to be several on each shrub. Mostly they were near the ends of the stems, though there were a few further in. They looked like the stems beyond the gall had died and broken off, though I’m not sure if that’s actually what happened.
I examined a bunch in the field, and then snapped off and pocketed a couple to take back to the house with me. I found a few that had circular entrance plugs such as the one above. These are also visible on goldenrod galls where the larva hasn’t emerged yet, so I began to wonder about the gall’s inhabitant. Was it still in there?
Many galls had exits that looked like this, however. Two different species forming the galls? Or maybe this one was parasitized, and the wasp (which it often is) had already eaten the gall-making host, pupated, and departed. Quite often the holes of parasites look different and are smaller than those of their hosts.
I took a whole bunch of photos and brought a couple back so that I could take a closer look and see if I could figure out the story. Turns out, there either isn’t much on the web about these galls, or I’m searching for the wrong thing. I’m fairly certain that the plants were willows, so I did an image search for willow gall to see what would turn up. The most common are willow pinecone galls (my mom did a post about them back in March) or willow rosette galls, both of which form at the tips of stems, but modify the leaf development to create the gall. I only found two or three pages that had images that looked like mine. And only one of those actually offered info on it.
From that one page, I took the scientific name of the parasite and searched it. Which turned up this useful site. And that’s where I’m getting all my info from. Apparently, not much is known about these galls, or at least, not easily found on the web. That site may or may not be for this species, since no photo is given. The exact species isn’t even known, perhaps not yet described. It provides this description of the galls: “The prolate gall is found on the proximal ends of young willow shoots”. Prolate is a new word for me, and seems to basically mean elliptical or ovoid. Proximal means close to the point of origin, such as the main stem (the opposite is distal, meaning far from; the proximal end of your arm is your biceps, the distal end is your hand). The shape in the description is right, but I don’t know about the location on the stem. Still, I think the genus of the critter is the same, at least.
I believe the parasites are midges in the genus Rhabdophaga. ‘Phaga’ means to eat, and ‘rhabdo’ means rodlike, so I presume that this whole genus targets the stems of their host plants. The other two common willow galls, the pine cones and the rosettes, are also formed by Rhabdophaga midges that bore into their stems at the tip. The website linked to above indicates that the adults of that species emerge in mid-April and deposit their eggs on the leaves of the host plant, a willow such as Salix discolor (pussy willow). The eggs hatch by early May and make their way to the stem, where they bore into the pith (the soft centre). They stay there all summer, with the host plant starting to form a gall by mid-June, about the same time the larva sheds its first skin and enters the second instar (larval stage). It’ll shed again in the fall, and it spends the winter inside the gall in its third-instar form, pupating and emerging as an adult in the spring.
I broke the two galls I brought home open using a knife. One of them appeared to be empty, but the other one I spotted what looked like a cocoon or something.
I plucked the centre bit with the cocoon out, and noticed something stuck to the other end. Upon closer examination, it turned out to be a larva. It lacks any noticeable prolegs, those fleshy protrusions that a caterpillar uses as back legs, which would rule out Lepidoptera to me and suggest probably Diptera or maybe Hymenoptera. I’m sure there’s a more scientific way of figuring out the difference between the larvae of these groups, but I don’t know it.
It was probably all of 5mm (<1/4″) long stretched out. As mentioned above, this is probably the third instar of the midge larva, assuming it is a midge larva, overwintering in the gall. But since my midge-larvae identification skills could still use some work, maybe it’s not a midge larva. Don’t hold me to it. :)
It turns out, those galls are tough. It’s possible to pry into a goldenrod gall with your fingernails, and I didn’t really give it much thought when I tried to do the same with these ones. It’s like trying to carve into a piece of bark, the wood is so toughened (great protection from predators, I guess). I ended up having to saw using the knife to cut through the gall’s wall. So as I tried to open it with my hands as I stood beside the willow shrub, my thumbnail slipped from the gall and gouged my other hand deep enough to draw a fair amount of blood. (The wound isn’t actually that long, fortunately; I shook my hand in pain and then realized I was bleeding, but the shaking caused it to run down my finger.)
The sacrifices I make for a bit of good blog fodder. :)