More Galls.

After discovering the goldenrod galls last week, the boys and I went on a gall hunt to see if we could find more of these fascinating formations. It was a rainy day, but we were not deterred (i.e. I made the boys go out despite the rain).

And our (my) tenacity paid off!  In addition to finding more goldenrod fly galls, we found two other types. We found the gall of a moth, which coincidentally also makes its home inside the goldenrod stem, but differs in that it is elliptical in shape, rather than spherical (see photo below left). We also found a blackberry knot gall, which appears as a strange lumpy growth on a blackberry cane (see photo below right). We took our bounty back to the house to dig a little deeper (litterally) and find out who was living inside each one.

Left to Right: two elliptical goldenrod moth galls; four spherical goldenrod fly galls.

Blackberry Knot Galls

We cut open the spherical gall first. We could see the small grub of the fly (Eurosta solidaginis) tucked inside the hollow space in the center of its home (see photos below). Anna Botsford Comstock, in her "Handbook of Nature Study", likens a gall-dweller such as this to a "boy living in the middle of a giant sponge cake, who when hungry would naturally eat out a larger cave in the heart of the cake."

Goldenrod Gall Fly Larva in left half of gall.

Goldenrod Gall Larva in my craggy hand.

Next we looked at the elliptical shaped gall which is also found on the goldenrod plant. This gall is formed by a moth (Epiblema scudderiana). The moth lays its eggs on the goldenrod's stem in the fall. In the spring, the eggs hatch and the caterpillar buries itself in the stem where the plant tissue starts to swell around it. Over the summer the caterpillar feeds on the inside of the gall, which continually expands to accomodate its growing size.  Before its transformation takes place, the caterpillar chews a small tunnel (see photo below) to the outside, but closes off the entrance with a little door made of debris. The tunnel is constructed in such a way, that the door will only push open from the inside, so as to discourage any intruders. In late summer, the metamorphosis from caterpillar to pupa to moth takes place and the newly formed moth pushes open its trap door and flies off, leaving behind nothing more than its red-brown pupal skin (see photo below).

Goldenrod gall moth empty gall with exit tunnel and pupal skin.

Next we cut open the blackberry knot gall. Inside, there wasn't a single hollowed out center chamber. As its odd outward appearance suggested, the inside of this gall was a random jumble of plant tissue and crevices (see photo below left). The blackberry knot gall is what is called a "colonial" gall, in that it has not one inhabitant, but many. In the spring and summer, the tiny wasp (Diastrophus nebulosus) lays several eggs on the cane of a blackberry bush. The eggs hatch and the larvae spend the winter inside the gall. The more eggs that are laid the larger the gall will become. Each larva has its own "chamber" in which to grow, surrounded by gall tissue on which to feed (see photo below right). In the spring, the adult wasps will chew their way out  and leave behind the apartment home they shared with their siblings.

Blackberry knot gall cut into quarters.

Blackberry knot gall wasp larva in situ.

We kept a few intact specimens and put them in a jar outside to see if we can catch sight of emerging insects in the spring. Of course, we won't be seeing any moths, as they have already vacated their homes. 

Goldenrod fly galls and blackberry knot galls in a jar.

If any of our gall dwellers do indeed reveal themselves, I'll be sure to share the event with you.


By Anna Botsford Comstock