There's a tall Fremont cottonwood tree (Populus fremontii) planted at the Plum Pond on the Dipper Ranch. After hiking all the way down there, it's nice to sit next to the pond and relax while watching the dragonflies flit across the pond surface in sync with the fluttering cottonwood leaves. Trees in the Populus family (including the quaking aspen) have long flattened stems and wide blades which cause them to move in the slightest breeze.
In September, I noticed many of the fallen cottonwood leaves had a bulge at the base of the leaf blade. I've frequently planted and hiked among several species of cottonwood trees and never noticed this before. On closer inspection, I found that all the swollen bumps had a small slit on the side. I realized they were probably galls.
Galls are abnormalities in plant tissues caused by some other organism, often an insect. The insect chemically or mechanically stimulates the plant tissue to produce extra plant hormones which in turn greatly increase plant growth in a localized area (like cancer) and this results in a new structure - the gall. Guess what happens inside the gall. The insect either moves into or lays eggs in this new structure. Thus, the insect has caused the plant to create a bug birthing chamber or bug condominium. Certain aphids do this to cottonwood and poplar leaf stems. Ah-hah.
But there's more. The Poplar Petiole Gall Aphid (Pemphigus populitransversus) bears live young inside the cottonwood gall, that turn into winged females who leave the cottonwood to give birth to more female aphids that eat mustard plants. See the naturebyte video at the end of this posting for a fascinating and gross video of the fat, warty mom aphid swarmed by her winged offspring, all sucking inside the gall. The lady mustard version of the family flies back to the cottonwoods in the fall and has a generation of both female and male aphids who lay eggs in the cottonwood twigs or bark since cottonwoods are deciduous and drop their leaves in the fall. In the spring, the leaf buds break, the aphid eggs hatch, the little aphid nymphs chew on the new cottonwood leaf stems causing the galls to form and the whole thing starts over again. Whew.
But there's still more. Today I saw a small bird busily picking gall-laden leaves in the cottonwood tree at the Plum Pond and I saw and heard it peck at each leaf and then drop them. The bird was greyish and seemed to have a small crest. It was getting dark, and as always, I was nervous about the approach of mountain-lion hours, so I couldn't stay for long, but I think it was an oak titmouse (Parus inornatus - photo by Gary Kramer, US Fish & Wildlife Service). A quick internet search finds many research studies on poplar gall aphids, types of poplars infested by them, birds who eat the aphids, and why they occur, where, at what numbers, or not.
This planted cottonwood tree is the only one at the pond or anywhere nearby. Still, the aphids find it, go through their complicated life cycle and the birds find the galls with the aphids. Next time I'm relaxing at the Plum Pond, I will try not to be unnerved by the busy and complicated natural world above me or maybe I will sit under the sycamore tree instead.
Insect and Mite Galls, Univ of Minnesota Extension Service
Backyard Nature with Jim Conrad
Gross & Fascinating Aphid Video by Henry Shenkman
Field Guide to Plant Galls of California and Other Western States, Ron Russo, University of California Press, 2006.