To distinguish rust mite damage from a nutrient deficiency, examine needles with a hand lens. If the damage is due to rust mites, damaged needles will have tiny marks or scratches running parallel to the midrib. If damage is severe in the spring, affected needles may drop off the tree when they are damaged, or may also shed out prematurely in the fall. Rust mites have many generations over a short period. The weather affects how quickly they reproduce, and how well they can survive.
Rust mites prefer moderate, spring-like temperatures and dry conditions.
Under these weather patterns, they can reproduce quickly. Mite numbers can explode from just a few to more than 50 on a single needle in as little as two weeks. The worst outbreaks of rust mites in white pine and Fraser fir Christmas trees have been when a warm fall is followed by a long, dry spring. Rust mites can quickly cause serious damage in Christmas trees, making some trees unmarketable. However, scouting for rust mites is difficult because of their small size.