Pest profile

Hemlock Rust Mite

Nalepella tsugifoliae · Sarcoptiformes: Phytoptidae[3]

The hemlock rust mite is a microscopic, four-legged eriophyid mite that feeds on hemlock needles in spring, turning heavily infested foliage grayish or bluish, then yellow, before needles drop.[1][2]

No licensed photo of this species is available yet; the identification notes below describe what to look for.

How to identify it

  • Mites are yellow to reddish orange, spindle- or wedge-shaped, with only four legs, visible only with 10x magnification.[1][2]
  • White cast skins on the undersides of needles give heavily infested hemlocks a grayish look.[1][2]
  • Unlike spider mites, which bronze hemlocks, rust mites turn them yellowish to grayish.[1]

Look-alikes

Look-alikeHow to tell them apart
Spruce spider miteA spider mite that bronzes hemlocks rather than turning them gray-yellow.[1]

Where it lives

NC State and UMass Extension both report it on hemlock; UMass notes it may be a nursery pest.[1][2]

Biology and life cycle

Mites crawl openly over needles and bark rather than forming galls; they feed all year but build up in cool, dry springs.[1]

Eggs hatch into first nymphs, which molt into second nymphs and, a few days later, into adults.[1]

Damage is most noticeable by midsummer, by which time populations have usually died off.[2]

Damage and signs

  • Each feeding puncture leaves a tiny pale spot; heavy feeding yellows needles and causes early needle drop.[1][2]
  • Because mites peak in spring, later new growth may be unaffected.[1]

Health risk

None to people or pets.[1][2]

Control

Biological control

Predatory mites, lady beetles and lacewings feed on it, and an insect-killing fungus devastates populations in warm, humid weather.[1]

Pesticides

Horticultural oil should give adequate control; NC State notes the mites are sensitive to carbaryl.[1]

Common questions

Why is my hemlock turning gray-blue and yellow in spring?
Hemlock rust mites feeding on the needles can cause that discoloration.[1][2]
Is it spider mites?
Spider mites usually bronze hemlocks; rust mites turn them grayish to yellow and leave white cast skins.[1]
Will new growth be damaged?
Often not, because the mites peak in spring and decline by summer.[1][2]

Sources

  1. NC State Extension, “Hemlock Rust Mite (PDIC Factsheet).” content.ces.ncsu.edu/hemlock-rust-mite (accessed 2026-10-15)
  2. UMass Extension Landscape, Nursery and Urban Forestry Program (Landscape Insect & Mite Guide), “Nalepella tsugifoliae (Hemlock Rust Mite).” www.umass.edu/agriculture-food-environment/landscape/publications-resources/insect-mite-guide/nalepella-tsugifoliae (accessed 2026-10-15)
  3. iNaturalist, “Nalepella tsugifoliae taxon page (order and family).” www.inaturalist.org/taxa/209220 (accessed 2026-10-06)

Updated 2026-10-15. Built from sourced records. Spot an error? Tell us.