Archive for October 29th, 2009


I recently noticed this little tree standing by the roadside. It caught my eye because it appeared to be decorated with round balls at the ends of its branches. Very odd.

The little tree had already lost most of its leaves, but the few left clinging to the top branches suggested it was an Eastern Cottonwood (Populus deltoides), a member of the poplar genus.


Upon taking a closer look, I found that the balls were actually crinkled masses of some sort of black material. It felt solid to the touch, firm but not rock hard. I looked at the other trees in the area, but only this one seemed to be infected by…whatever it was! My first thought was that it was some kind of fungal infection.

I took a few photographs and later shared them with Birdgirl, who found the answer for me. (It’s good to have a zoologist daughter!) I wasn’t looking at a fungal infection at all. The odd misshapen clumps are the galls of the Poplar Vagabond Gall Aphid (Mordwilkoja vagabunda).


With a bit of research on the internet, I found some interesting information about the tiny insects that cause the galls. Their life cycle is complex and not fully understood. The tiny aphids, ranging in size from 2/100ths to 18/100ths of an inch, can be winged or wingless and pale green to dark brown in colour. There are multiple generations in a year and they can vary in appearance and habit from one generation to the next.

The aphids overwinter as eggs inside old galls and bark crevices. These hatch in the spring and the wee nymphs gather at the tips of the new shoots, where they pierce the new plant tissue and suck the plant juices. This feeding transforms the emerging leaves into twisted, bladder-like galls. The gall encloses the nymphs, which mature rapidly and the young aphids quickly produce hundreds of offspring. As their gall home matures, it becomes brown and dry, splitting to allow new winged generations of aphids to exit and migrate to their unknown secondary host, possible the roots of grasses, where they continue to feed and reproduce over the summer. In late summer, winged aphids return to the dried, brown galls and lay the eggs that will become next spring’s new generation.

As the galls grow older, they change colour from an initial green, to dark brown, to black, and hang on the tree after the leaves have fallen. Although there may be many galls on a tree, they cause little harm to the tree. Since the aphids return to the same galls they left in the spring, the same tree tends to be infected year after year, while nearby trees remain uninfested.

Sleep tight, little aphids-to-be. Enjoy your long winter nap.


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