Submitted Photo Elm gall

Even though we have just experienced record low temperatures, and daytime high temps 20-plus degrees below average, spring will soon be here! And when the temperatures do start to climb, the tiny microscopic mites that cause leaf galls will wake up and move to do their damage.

Many of the mites which cause leaf galls winter in cracks and crevasses in the tree bark. When temperatures get warm enough for the tree buds to start to swell, they wake up and move to the buds.

The mites move into the swelling buds and lay their eggs. The eggs hatch and the new mites begin to feed on the emerging tree leaves. This feeding causes the host tree or shrub to build a wall around the mite which is the gall in which the mites lives. This gall actually protects the mite from predators and any control pesticides we might apply!

But there is good news about your infested trees! Control in most cases is not needed! Even though the galls are very alarming in appearance, they do very little if any harm to tree’s health and well-being. The mites take such a small amount of nutrients from the tree that there is not damage done. This is especially true in older trees that are established. The only time you might consider control is if you have a very young tree that had a very heavy infestation in 2017. By heavy infestation I would suggest that every leaf was almost completely covered with galls. Only then would I consider control.

What do we use? Horticulture oil is what is used to control the mites. This can be found in any garden center or nursery. Before using the oil, ALWAYS read and follow the label directions. This is very important as there are a few trees and shrubs that are harmed by this oil. After reading the label and following mixing directions, you are ready to apply.

Two applications of horticulture oil are advised. The first is applied when the buds of the tree have swelled and are just beginning to open. Be sure to get complete coverage of the plant. The second application is made about 10 days later when the buds are opening and the new leaves can be seen in them. Again, complete coverage is very important.

Since the mites are very tiny and almost impossible to see with the naked eye, perfect timing of the oil is not always possible. So don’t expect 100 percent control of the mites and galls. A reduction is what is hoped to be achieved on these young trees. This will allow them to regain their vigor and overcome the feeding competition of the mites. Ideally you should only have to apply the oil to a tree for one or two years. After that the tree or shrub should be large enough to outgrow any feeding being done by the mites.

Ken Eraas is horticulture assistant with NDSU Extension Service/Ward County. He can be reached at kendell.eraas@ndsu.edu.