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?