Fir galls

Balsam gall midge, Paradiplosis tumifex

Continuing with the theme of the left-behind… I do a lot of peering at branches in the winter months, after most life has gone into hiding. Sometimes some interesting discoveries can be made, affixed to (or even within) twigs or branches. I took Raven down to the 100-acre Woods yesterday, the first time we’ve gone in a little while because of hunting season (we’re in the mid-November hunting lull at the moment, but we wore our brightly-coloured jackets, just to be safe; I don’t think anyone hunts on that bit of land, but who knows what goes on there when no one’s around to see it). The forest has a fair bit of Balsam Fir scattered through the damper areas, and I paused at one clump to finger the needles, whereupon I noticed these odd little lumps amidst the foliage. There were a lot of them, mostly at the outer ends of the branches, and mostly (it seemed) on the east side of the tree (though that might just be coincidence).

Determining their identity was fairly easy once home. A Google image search for “balsam fir needle galls” turned up several pages with identical galls to mine. Gotta love the forestry industry’s thoroughness; virtually anything you want to know about pests of commercially valuable tree species is available online. The pest in question here is a tiny midge, Paradiplosis tumifex, which goes by the appropriate English name Balsam Gall Midge.

Eggs are laid on current-year needles as they’re growing (which explains why the galls seemed mostly to be near the ends of branches), and when the egg hatches the larvae burrow into the needle to settle down and feed. The needle forms a gall around the larva and its new home. The larva eats and grows and eats and grows, until mid-autumn when they finally break loose of their gall and drop to the ground. There, they burrow into the soil under the tree and wait for spring. Warming ground temperatures in late April and early May encourage the larvae to pupate, and a week or two later the tiny adults begin to emerge. Females mate, then return to the newly-forming fir needles where they lay their eggs, and the cycle begins again.

Balsam gall midge, Paradiplosis tumifex

Once the larvae leave the needles in the fall, the needles brown and drop from the tree. The branches were scattered with the ones that had caught in the twigs. I noticed on the ones that were still attached to the branch that a few of them seem to be broken off; I’m not sure if this was from the midge larva emerging, or environmental damage post-emergence. The midges are no more than a nuisance, apparently; they do no long-term damage to the tree, beyond perhaps mildly stunted growth that year when under severe outbreak conditions. The galls are dropped by the winter, and even aesthetically the tree looks fine again quickly (important for the Christmas Tree market). Populations are cyclical, with two or three years of high numbers, followed by several of low. I can’t remember having noticed anything unusual about this same tree last year (it grows beside the path), and I’m not sure if that’s because numbers were low last year and high this year, or I just wasn’t looking.

Hard wood willow galls

willow gall, prob. by Rhabdophaga sp.

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.

willow galls, prob. by Rhabdophaga sp.

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?

willow gall, prob. by Rhabdophaga sp.

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.

willow galls, prob. by Rhabdophaga sp.

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.

willow galls, opened

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.

gall inhabitant, prob. midge larva of Rhabdophaga sp.

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. :)

Sacrifices made for good blog material

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. :)