Nalepella tsugifoliae

Hemlock rust mites. Photo: Sandy Gardosik, Pennsylvania Department of Agriculture.
Scientific Name: 
Nalepella tsugifoliae
Common Name: 
Hemlock Rust Mite
Growing Degree Days (GDD's): 
7–450 GDD's, Base 50F, March 1st Start Date (Source: Cornell Cooperative Extension.)
Host Plant(s) Common Name (Scientific Name): 
Fir (Abies spp.)
Golden larch (Pseudolarix spp.)
Hemlock (Tsuga spp.)
Spruce (Picea spp.)
Yew (Taxus spp.)
Insect Description: 

The hemlock rust mite is not a spider mite, but rather an eriophyid mite. These are tiny, wormlike, and slow moving insect relatives that require magnification to be seen on their host plants. The hemlock rust mite feeds on the needles of its hosts openly by removing fluids from the needles. When mite populations are high, foliage may appear blue-ish in color and turn yellow before dropping from the plant. The hemlock rust mite causes most of its damage in the spring and may be a pest in nurseries. Damage by this eriophyid mite is most noticeable by mid-summer, but by that time the populations typically die off. With 10X magnification, adult mites are yellow and spindle-shaped with four legs. Thin, white colored cast skins may be visible with magnification, as well as the tiny, yellowish-orange eggs. Several generations may occur per year with adults overwintering in the cracks and crevices of bark. 

Damage to Host: 

Both surfaces of host plant needles may be fed upon. Causes off-color foliage and needle drop. This pest often goes undetected until damage is very apparent. Activity begins in the spring. Usually active up to early summer before populations are reduced. 

Monitoring: 

In the springtime, scout hemlock needles using a 10X hand lens to search for the spindle-shaped eriophyid mites. Populations may linger further into the summer if weather conditions are cool and dry.

Cultural Management: 

Prune out and destroy individual, infested branches when possible.

Natural Enemies & Biological Control: 

Predatory mites and predatory insects are sometimes reported as natural enemies of hemlock rust mites.

Chemical Management: 

Abamectin (NL)

Carbaryl (L)

Pyrethrins (L)

Pyrethrin + piperonyl butoxide (L)

Horticultural oil (L)

Spiromesifen (L)

Notes: 

Active ingredients that may be applied systemically include: abamectin (injection).

Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.

Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: January 2024. To check current product registrations in Massachusetts, please visit: https://www.mass.gov/pesticide-product-registration .