NC State Extension Publications

Description and Biology

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The hemlock rust mite, Nalepella tsugifoliae, is a virtually microscopic mite in the family Eriophyidae that includes gall mites and bud mites. Hemlock rust mites are "vagrants" because they crawl about on the surface on hemlock needles and bark. They do not cause galls and do not wedge into buds. Females do overwinter in bark cracks. These yellowish to reddish-orange mites have only four legs. They are shaped more or less like the snack food 'Bugles' with the mouthparts and legs all at the wide end. They feed on the needles of hemlock all year, but build up to great numbers in cool, dry springs. Females lay comparitively large eggs from which hatch first nymphs. Eggs are off-white but overwintering eggs are reddish to tan, laid in clusters. First nymphs molt into second nymphs that in a few days molt into new adults. Because they are so small, their tiny mouthparts can only pierce the epidermis. They inject saliva and suck out the predigested plant juice. A tiny pale spot forms at each feeding puncture. The nymphal cast skins are white and contribute to the grayish appearance of heavily infested hemlocks.

Hemlock rust mites are incredibly tiny.

Hemlock rust mites are incredibly tiny.

Hemlock rust mites

Hemlock rust mites are usually on the lower part of hemlock needles.

During the summer, cast skins

During the summer, cast skins are often abundant (but few mites).

Hemlock rust mites survive the winter as eggs

Hemlock rust mites survive the winter as eggs that turn reddish or tan in color.

Eriophyid mites

Eriophyid mites have two nymphal stages.

Host Plants

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Eriophyid mites are usually host specific. The hemlock rust mite feeds on hemlock so there is no danger that it might spread to other plants (although it has been reported from fir.). Heavily infested hemlocks turn yellowish to grayish (spruce spider mites cause hemlocks to bronze) before the needles drop prematurely. Because hemlock rust mites are most abundant in spring, later new growth may be unaffected.

Hemlock rust mites start feeding in very early spring.

Hemlock rust mites start feeding in very early spring.

Residential Recommendations

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Phytoseiid mites feed on hemlock rust mites as do certain lady beetles and lacewing insects. In addition, a parasitic fungus, Hirsutella thompsonii, devastates rust mite populations when the weather is warm and humid. Hemlock rust mites are sensitive to Sevin insecticide. A horticultural oil should give adequate control as well. The Hemlock Pest Management Calendar gives some idea as to when to scout for various pests of hemlock.

This fungus i

This fungus is favored by warm, humid weather.

Other Resources

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For assistance with a specific problem, contact your local Cooperative Extension Center.

This Factsheet has not been peer reviewed.


Professor Emeritus
Entomology & Plant Pathology

Find more information at the following NC State Extension websites:

Publication date: Feb. 5, 2019
Revised: Sept. 23, 2019

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