Spruce spider mites are a cool season mite species, they are most active in spring and autumn. This in contrast to two spotted and red spider mites which both thrive in hot, dry, summer conditions. Many common evergreens such as dwarf Alberta spruce, Colorado blue spruce, Douglas fir, pine trees, and junipers can be infested with spruce spider mites. They are small(1/50th of an inch) and start as a pale green color before becoming brown and dark green.
Spruce spider mites tend to attack the oldest needles first and can therefore be found at the inner, bottom portion of the tree. As their populations build over successive years they will move into the outer portions of limbs. The earliest symptoms appear in April or May. Damaged needles will appear mottled or stippled with yellow and brown spots. These mites suck juices out from the plant creating this damage. As they continue to feed eventually the whole needle will be yellow. Damage is not normally noticed until June when injured needles turn bronze. Upon closer inspection you may notice spider like webs in the tree and if you shake branches over white pieces of paper you can see the mites themselves with the help of a magnifying glass.
New nymphs hatch in March and April and begin feeding and mating. With a 3 week development period there can be multiple generations in the spring. As summer approaches and temperatures near 90° adults present will lay eggs in the needles of trees and then perish. When fall arrives the summer eggs hatch and new mites renew the same cycle that occurred in spring. Adults present at the end of autumn will mate and lay the eggs that overwinter to the next spring.
Homeowners can prevent spider mites issues by routinely hosing the tree down in the spring and fall. A strong jet of water will dislodge mites and drive them from the plant. Another option would be monitoring the trees closely each spring and fall for the aforementioned symptoms and applying either a summer oil or an insecticidal soap if damage is noticed. Summer oils can cause blue spruces to become green so keep that in mind when choosing your control method. Winter or dormant oils can be applied in December or January to eliminate overwintering eggs as well. If you did not monitor your tree and have found bronzed portions in the summer, those branches will not recover and will need to be trimmed out.