Sooty mold caused by balsam twig aphid or Cinara aphid activity, as well as spruce spider mite damage, can also be mistaken for rust mite damage. Be sure to check damaged shoots with a hand lens to distinguish between these problems and to find the rust mites themselves. Even with a hand lens, pollen on needles can be mistaken for rust mites. Be sure to look carefully. When to scout .
Rust mites are almost always a springtime pest, but they can also reappear in the fall.
They are not a problem in Fraser fir Christmas trees in western North Carolina every year. Below normal rainfall during extended springtime temperatures encourages rapid increases in rust mite populations. Rust mite activity in the mountains can often be predicted based on early activity in hemlocks in the foothills. It is valuable to determine rust mite presence in a field before the balsam twig aphid treatment window. If rust mites are present and need to be controlled at this time, it can affect the choice of pesticide for twig aphid control.