The galls that are found on cottonwoods in Montana are of several kinds. Three of the most important are pictured. Others often occur as blisters on the leaves, swollen stems or branches, or rolled leaves.
Popular Leaf-Stem Gall
This gall may be formed anywhere from where the leaf joins the twig up, to the leaf tissue itself. However, the slit running across this gall is typical of it no matter where it is found. The slit acts as an escape hatch for the small aphids which live inside and are responsible for the gall in the first place. As is the case with most aphids, this one has a complicated life cycle. Supposedly the winter months are passed on the roots of plants of the family Cruciferae-the mustards, fan weed, cabbage, and other plants. During March and April, winged aphids fly from the plants to the trunks of cottonwoods where they mate and each female lays one egg. This egg hatches into a “stem mother” who crawls to the developing leaf and feeds on the stem causing the gall to form around her. This stem mother produces living young without ever mating and her offspring, in turn, produce more. Some of these offspring migrate back to the cruciferous plants during summer and fall to begin the cycle over again.
Fortunately, little permanent damage is done by this aphid since the galls are shed with the leaves. Trees are usually not infested to the point where serious damage to the tree will result, so control is usually not necessary.
These galls are also caused by aphids and some trees have actually been reported as killed by this pest. Unlike the preceding gall, this one remains on the tree year round and several years accumulations of galls on a tree makes it stand out conspicuously. These aphids with the improbable name of Mordwilkoja vagabunda, pass the winter as eggs within the old galls on the tree. The black eggs hatch in the spring and the new aphids make their way to the growing shoots where their feeding causes new galls to be formed. New aphids are produced and leave the trees in mid-summer when they disappear for some time.
It is believed that they spend several months on the roots of certain grasses. At any rate, they return to the trees in the fall and lay their eggs inside the galls. For some reason they prefer not to lay their eggs on a tree which does not have galls, consequently, only one tree in a grove may be infested.
No organized scientific efforts have been put forth to find the most efficient means of controlling this pest. However, its life cycle and general knowledge of related species suggests several things which can be done which might be of value. Since it over winters as eggs within the galls, any method of destroying the galls will be helpful. Picking, pruning, or even burning an entire tree if it is heavily infested might be of value to prevent further spread. Spraying would not be expected to be of great value except during the short period when the aphids are exposed on the new growing tips and before the protective galls are formed. Isotox and Malathion can be used for control.
The third type of gall common on cottonwoods is that shown. This gall is caused by a microscopic cigar-shaped mite. These mites are so small that it would take five of them laid end to end to reach across the period at the end of this sentence. The life cycle of this mite has never been worked out, but if it is similar to closely related species, it is very complicated. The first cousin blister and rust mites, each have about ten different forms in their life cycle. It is easy to see why there are only a few men in the world with enough patience to solve the riddles of animals so small and yet so complicated. For most mites of this group, sulfur, lime-sulfur or dormant oil will control them.
I’m sure there is going to be some misunderstanding unless I say what I’m talking about when I say “cottonwood”. Many people have different names for cottonwood, poplar, and aspen. However, they all belong to the same genus, but are different species and consequently I have called the whole group “cottonwoood”!
By George Roemhild, Yellowstone County Extension Service