I was out exploring a nature trail with a group of young students recently. We gathered acorns and their caps (they make great whistles), milkweed pods, dried rabbit-foot clover flowers (they are so very soft), and collected bunches of flowers to take home. It seems like everything that could be picked or picked up was. Most of their findings I could identify with relative ease, but there was one item that left me wondering.
Freddy (not his real name) brought me a stick with this wrinkly dark brown thing attached to the end. Since there were some gardens nearby, I thought it was an old dried-out cockscomb flower, but the stick was all wrong: it wasn’t a flower stem, it was a twig. I looked closer and said, “Hm. It’s a gall of some kind.” I glanced around, saw a couple cottonwood trees, and, based on the twig, determined they were the source of the sample. Beyond that, I had no idea, for it was a new gall to me. By then several of the kids had found sticks with wrinkly galls on them. I told them I would look it up when I got back to work and let them know what I found out.
It took some searching, but it turns out it is indeed a cottonwood gall of a type made by cottonwood gall mites (Eriophyes parapopuli), aka: poplar bud gall mites. These mites, which are so tiny that it would take five, lined up end to end, to stretch across a 12-pt. period, can be found on other members of the poplar family, too, not just cottonwoods. One of the things that I discovered in my research that I thought was rather interesting is that these mites, unlike the overwhelming majority of mites and spiders, have only four legs. Not four pairs of legs (spiders and mites), but four legs…period. That’s just wrong. Despite their small size, and obvious lack of appendages, these miniscule pests travel very well, thank you. How do they get around? By wind, water, insects, birds and yes, even people. They are extremely fertile, producing up to eight generations in a single year (thank goodness they only live about a month as adults).
There are hundreds of species of these eriophyid mites, each causing its own form of damage on plants, from stem and bud galls, to rusts and blisters. These wee pests are very host specific, not only to the plant they feed upon, but also which part of the plant they choose. Some species prefer leaves, others buds, and others stems or flower petals. But in the end, they all do the same basic kind of damage: they enter the plant’s cells and suck the life out of them. Literally. They suck up the cell’s contents. It’s the plant’s reaction to this attack, however, that creates the gall, or blister, that you and I see.
When galls are formed, the plant is reacting to growth regulators that the mites injected into the plant’s leaf or stem tissues. These growth regulators stimulate the tissue into abnormal growth patterns and rates. The end result is a pocket formed around the mites, in which they happily feed and reproduce.
Cottonwood gall mites take up residence at the base of a bud, preventing the development of normal leaves and stems. Instead, these wrinkly, lumpy, irregular growths appear on one side of the twig, eventually covering the entire base of the bud or shoot. At first the galls are green, for they are fresh and new. As they age, they turn red, then brown, and overtime they end up a grey-black color. If you look closely, you can see the holes through which the adults emerge when they are ready to move on to a new host.
Individually these galls are merely an aesthetic problem, but if enough of them form on your tree, the tree could become rather stressed, making it susceptible to other problems. But in general, they are not considered to be a serious problem. If you keep a close eye on your plants/trees, you can detect deformities before they get out of control. Look for discoloration or swellings at the base of leaves and buds. Just prune off the infected twigs and leaves. These can then be burned or bagged and taken to the dump. Pruning, by the way, is best done in the spring before the tree breaks dormancy. If you have a heavy infestation, you can try applying horticultural oils right after the buds break in the spring. This won’t get rid of existing galls, but it may prevent the spread of the mites and development of future galls. Alternatively, you can consider the galls to be interesting modern art, courtesy of Mother Nature.