I have been noticing some galls on some plants as I travel about this fall. Most of what has caught my attention has been on willows and goldenrods.
I would suspect that many of you have seen what looks like small, tightly closed pinecones on some willows. Those are willow pinecone galls. hey are caused by a midge, which is a mosquito-like insect. The females lay a single egg on the stem or leaf of a willow near the tip of a twig. Upon hatching, the larvae crawls to the bud at the tip of the twig and burrows into it. The gall then forms from overlapping leaves, thus protecting the larvae. The larvae then pupates in the gall and emerges as an adult the following spring. If you are careful, you can peel apart the gall and find the larvae inside.
I have also been noticing some galls on Canada goldenrod. Two kinds of galls may be observed on the stems of Canada goldenrod now throughout the winter months. The one I have been seeing most frequently is a round gall, about an inch in diameter, on the upper portion of the stem. It is caused by a fly which has laid its eggs in the stem of the goldenrod plant. Similarly, a spindle-shaped gall (or football shaped gall if you relate to that better) may also be observed on Canada goldenrod. This gall is caused by a moth. And like the willow pinecone gall, if one is careful, the larvae may be observed inside. This gall, however, is much tougher to open.
So just what is a gall? It might surprise you, but gall formation is a defense mechanism in response to isolate the insect or pathogen from the rest of the plant. Gall formation is often initiated when an insect oviposits an egg or eggs inside the tissue of the host plant. The gall itself will be composed of plant tissue, but the insect apparently controls the form. The characteristics of the gall are unique to the insect species. The female injects some chemical or chemicals into the plant which stimulates gall formation and when the larva develops it may also continue to produce the substances. In some cases, sugars even accumulate in the gall in response to enzymes produced by the insect, assumedly to ensure energy.
So be on the lookout for galls as you travel about. There are some interesting ecological, physiological, and evolutionary factors at work here.