Peeping Up Your Petiole

On a lap around the farm last christmas I came across some odd lumps on the petioles (leaf stems) of a cottonwood tree. I suspected straight away they were galls but had no idea the contents. When I came back with the camera I started to promptly and rudely split some galls to photograph what was inside and found little colonies of what I thought were plant lice but turned out to be aphids. I didn’t think aphids were responsible for galls but these are quite common on poplar trees in the U.S. (turns out poplars and cottonwoods are from the same genus). Another interesting thing to note is there were a lot of dead leaves under the tree that looked like they’d beed severed at the galls. I’m wondering if birds (or some other animal) had opened the galls, eaten the contents and discarded the leaves.

Petiole Gall
A gall made by aphids on the petiole of a cottonwood leaf. There are immature aphids (with white waxy coats) on the outside of the gall. The aphids come and go through the slit on the side of the gall. Kyogle, NSW.
Petiole Gall
Winged adult aphids inside the petiole gall are surrounded by nymphs, waxy residue and honeydew marbles.. Kyogle, NSW.
Petiole Gall
Aphid nymphs covered in their waxy residue – possibly useful for keeping sticky honeydew at bay. Kyogle, NSW.
Petiole Gall
Aphid nymphs covered in their waxy residue – possibly useful for keeping sticky honeydew at bay. Kyogle, NSW.
Petiole Gall
Baby aphids, fat aphids and winged aphids among the balls of dried honeydew inside a petiole gall. Kyogle, NSW.
Petiole Gall
The tiny beads of dried honeydew that collect inside a petiole gall with a couple of discarded exuvia. Kyogle, NSW.
Petiole Gall
Aphids of all developmental stages inhabit this gall they’ve induced on a cottonwood tree. Kyogle, NSW.

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