Have you ever wondered about the lumpy growths you see on leaves or stems?
In this case the lumps are galls formed on the petioles of cottonwood leaves. This tree was covered with them.
A lot of different insects and mites cause the plants to form galls, so I was eager to take an example home and see what was inside.
Inside the gall I found numerous tiny insects covered with a waxy powder. Looking under a microscope, it became clear these were nymphs of the poplar petiole gall aphid, Pemphigus populitranversus.
The poplar petiole gall aphid (also known as the poplar petiolegall aphid in scientific literature) has a complex life cycle. The aphids overwinter as eggs on cottonwood twigs. When the leaves unfurl in the spring, the aphids hatch and begin to feed on the leaf petiole. The plant responds to the feeding by producing a localized growth, which becomes the gall. The aphid nymphs move within the gall and continue feeding.
When the aphids complete their life cycles, the galls split open and winged adults fly to plants of Brassica spp. There the aphids feed on the plants’ roots in the soil. In this stage, they have a second common name, which is “cabbage root aphid.” Eventually, at the end of the season, winged forms fly back to cottonwood trees to lay the overwintering eggs.
There are more to aphids than you might think. Many have similar complex life cycles, where they live on annual plant hosts in the summer and fly to woody, perennial hosts in the winter.
Have you ever spotted a gall on a plant? What kind?
These are in every single steam on every single leaf at my family’s acerage. All large cottonwood / poplar trees along the red deer river in Drumheller AB. We do get a fair amount of aphids. I’ve purchased ladybugs at greenhouses to combat a really serious aphid problem in my home in Crossfield AB, but now that I know it’s aphids here .. maybe I’ll get a few more and see if it helps.
The bad news is that ladybugs won’t be able to do much about aphids hiding inside stem galls. The aphids are protected in there.
The good news is these types of galls cause little actual damage to the plant.They are more of a cosmetic or “looks” problem.
The real problem is when you’re trying to grow Brassica near by. We were trying to grow kale and had a cottonwood tree nearby. The aphid issue was horrible until we removed the tree. Sad but necessary for our garden.
Good to know. Many aphids alternate between annual and perennial hosts.
I have huge cotton wood trees in my yard and have been noticing lots of these leaves on them I’m wondering if they can kill the tree? The trees are prob over 30 years old!
The aphids do mostly cosmetic damage, meaning it looks worse than it is. Leaves with galls might drop a bit sooner than those without, but otherwise a big tree should have no problems.
your replies have helped much. I also notice if the gall falls and it is hatching time, sometimes the end of the leaf will be stuck in the ground. When I pull it out there is a hole under the gall or nod. I’ve picked up every galled leaf I find, at least once a day. This is the first year I’ve seen this. My tree is 13 or 14 years old. I have other cottonwoods and they are not infected. They seem to have a defense system, for which I’m grateful.