There are a lot of cottonwood tree leaves lying in an area on the Lake Metigoshe hiking and biking trail. All of them are green with a large round growth, or gall, on the leave stalk or petiole. It made me wonder what was going on. So I had to do a little investigating.
Galls can form in several plant parts, such as stems and leaves. The mass of plant cells forming the gall are in response to a mite, insect, or pathogen in an attempt to isolate it 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 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. The gall itself is composed of plant tissue, but the insect apparently controls the form because the characteristics of the gall are unique to the insect species. The chemistry of all this, however, is not well documented.
What I have been seeing is the work of aphids that form what is known as the cottonwood petiole gall. The aphids release a chemical that causes the formation of a blister-like structure around them that serves a sort of covering. At any rate, it apparently works quite well for the aphids and does little damage to the host cottonwood. The leaf drop might be a nuisance for us, but that is about it.
If you spend some time outdoors no doubt you see galls, perhaps on goldenrods, willows, roses, maples, oaks, as well as a variety of other plants. And if you happen to see lots of cottonwood leaves lying on the ground, take a closer look at them. Odds are that you will see a gall on each one, perhaps a half-inch or so in diameter on the leaf stalk. That is the cottonwood petiole gall. And although it may appear like the cottonwood tree is in big trouble, it is nothing to get worried about.
