Hemlock Rust Mite in Pennsylvania

Rust mites live on the surface of needles instead of inside a gall or bud.  This native pest is active in late winter and early spring as well as in the fall.  This species can be found on both the upper and lower needle surfaces.  Damage caused by this pest is often misdiagnosed as a nutrient deficiency, winter injury, or drought stress.  A hemlock sample that was submitted in late February contained active life stages of this key pest. 

With a dissecting stereomicroscope, clusters of tiny round eggs may be seen at the base of host plant needles during the winter.  Nymphs hatch from overwintering eggs usually in mid-March well before those of the spruce spider mite.  The hemlock rust mite has a spindle-shaped body with two pairs of legs.  It is very small, and may require a good hand lens (10X-15X) or a dissecting stereomicroscope in order to be observed.  Adults are usually yellowish orange and 0.25 mm long.  Populations build up quickly in the spring, increasing from late February through May depending on the weather.  They decrease during the summer and increase in the fall when eggs are deposited from November through early December. 

This pest has piercing-sucking mouthparts that remove plant fluids from needles.  When many mites are present their damage gives needles a dusty, olive appearance.  To distinguish rust mite damage from a nutrient deficiency, look closely at damaged needles with a hand lens.  If tiny marks that run parallel to the midrib of the needle are seen, it may be a rust mite-damaged needle.  These marks won't be apparent if the rust-colored appearance is due to a nutrient problem.  Damaged needles often drop prematurely in the fall.  If you’re not sure of the causal organism, take a sample to a Penn State Extension office that has a good stereomicroscope, and they should be able to assist you with the accurate diagnosis. 

Scout hemlocks during mid-March through April and again in September.  To effectively manage this key pest, treat infested plants by applying a registered formulation of horticultural oil according to label directions from mid-March through April and again in September if indicated.