Q. I have a Hemlock hedge as a privacy screen between a neighboring house and mine. While I was cleaning up leaves, I noticed that the Hemlocks seemed bleached out and off-color. Can you tell what is wrong with my Hemlocks?
A. The hemlock sample had classic symptoms of spruce spider mite damage. On hemlocks, this damage appears as bleached out areas on the needles. It is typically worst where the needles attach to the stem. Spruce spider mites are widely considered the most destructive conifer-feeding mite in the United States and Canada. Most species of conifers are fair game, including arborvitae (Thuja spp.), hemlock (Tsuga spp.), juniper (Juniperus spp.), larch (Larix spp.), pine (Pinus spp.), and spruce (Picea spp.).
These tiny spiders -- you need a hand lens to see them -- feed with piercing-sucking mouthparts, and destroy the cells that contain the green pigment (chlorophyll) necessary for photosynthesis. Severe infestations cause needles to drop prematurely, which reduces the tree's ability to produce sufficient carbohydrate reserves.
Since they are spiders, you may see fine webbing upon close inspection. If you do not have a hand lens, hold a sheet of white paper under the affected area and tap the branch. Mites and debris will fall onto the paper. The tiny dark green or dark brown specks that resemble ground pepper and begin to move around slowly are spruce spider mites. You might also notice some specks moving around quickly on the paper. They are most like predatory mites feeding on the spruce spider mites. Under magnification, adults look like tiny spiders with four pairs of legs.
Spruce spider mites overwinter as eggs under bud scales or where needles attach to the stem. Eggs may also overwinter under fine webbing on stems and branches. Larvae hatch in spring, generally late April into May. They initially have three pairs of legs; the fourth pair appears after its first molt. It takes roughly three days for larvae to develop into nymphs, and six days for nymphs to develop into adults.
Unlike two-spotted and red spider mites that are most active during hot, dry weather, spruce spider mites are most active during the cooler times of the year. Monitor your hemlocks for their activity from April through June and again in September. They remain dormant during hot, dry weather and winter cold.
These pests do warrant control, because they reproduce quickly -- they can produce a new generation every two weeks or so -- and it does not take long for large populations to build up and cause severe damage.
Control recommendations include a dormant application of horticultural oil. Dormant applications are made just before new growth starts in spring, usually late March. There are different types of oil on the market for different applications. Be sure to choose horticultural oil, often sold as ultra-fine or sun oil. It has been refined further and is lighter than true dormant oil (Volck oil).
If you plan to do your own spraying, repeated applications of horticultural oil or insecticidal soap will be most effective. Neither product has any residual activity once the spray has dried. There are a number of new generation miticides that have much longer residual activity that certified applicators are permitted to apply to residential properties, but they are not available to home gardeners.