Spider mites have been found on Colorado
blue spruce at The Morton Arboretum in northeastern
Illinois, which means they are active throughout the state.
Spruce spider mites and other mites that feed on needled
evergreens are most active in the early spring, usually
ceasing their feeding by early May (in southern Illinois) to
early June (in northern Illinois). They spend the summer as
eggs that hatch in the fall for a short feeding time before
Scout for spider mites by vigorously shaking or striking the
foliage over a white piece of paper. Some of the mites will
fall onto the paper where they can be seen as tiny, moving
dots. Smashing them will produce greenish or yellow-orange
streaks. Those making greenish streaks are usually spider
mites that are feeding on the foliage; those making
yellow-orange streaks are usually predaceous mites that are
feeding on the spider mites. Numerous predaceous mites will
reduce the need for chemical control.
Damage appears as light to brownish dots or stippling on the
needles. The mites feed on cells of the leaf, removing and
destroying the chlorophyll. This creates a light spot that
dries and turns brown. From a distance, the foliage appears
bronzish, dirty, or bleached. Heavily attacked foliage will
turn brown, and severe attack can kill branches and trees.
These mites attack spruce, juniper, pine, arborvitae,
hemlock, and Douglas-fir.
Control spider mites with contact miticides such as
insecticidal soap, summer spray oil, dicofol (Kelthane),
and several pyrethroids. Do not use oil on bald cypress. Oil
may cause damage on Savin junipers, spruces, and Douglas-fir
and will take the blue color off of Colorado blue spruce. Be
sure to achieve good coverage with any sprays, particularly
insecticidal soap and summer spray oil. Two treatments a
week apart will be necessary to obtain a high level of
control. Thanks to Dave Shetlar, The Ohio State University,
for information about oil-sensitive plants.