Spruce spider mites are an occasional pest on conifers in Iowa. Careful monitoring is required to detect damaging mite populations before sever discoloration of the foliage occurs. Natural enemies such as lady beetles and predatory mites often keep mite populations low, but outbreaks are possible.
When Mites Are Active
Spruce spider mites are a cool season mite and are active early in the year and later in the fall, during the warmer months of summer they survive as eggs. Currently we are probably nearing the end of spring activity as temperatures across Iowa are rapidly increasing, but the damage may just now be becoming apparent on the trees. Often the damage is most apparent on the older needles, with the current year’s needles still looking normal. Conifers respond very slowly to spider mite feeding, which is why it is always important to confirm active mites on the tree before treating. Check for mites by holding a sheet of white paper under a branch and then shake or tap the branch against the paper. The mites, if present, will show up as tiny, slow-moving specks on the paper. Dozens of mites on the paper is an indication that populations are high enough to warrant treatment.
Spruce spider mites feed on all conifers, but we receive samples of them most commonly on Colorado blue spruce, Black Hills spruce, dwarf Alberta spruce and arborvitae.
Spruce spider mite feeding damage causes small specks of yellow on the needles that from a distance will make particularly older needles appear sort of gray, bronze or yellowish in color. They do not kill entire needles so any completely discolored needles are probably due to some other factor. Spruce spider mites also leave sticky webbing which can collect debris and leave the needles looking dirty.
Spruce spider mites are fed upon by a variety of predatory insects and mites that help keep their populations under control. When considering treatment options remember that miticides and insecticides will also kill the beneficial insects and this can sometimes lead to worsening pest problems.
On smaller trees and shrubs it may be possible to reduce light mite populations with periodic “hosings.” Use the garden hose to apply a forceful stream of water to dislodge mites from an infested plant. Repeat 3 or 4 times on consecutive days. Keep plants watered and mulched to promote health and vigor and to reduce the impact of mite feeding.
Soap or oil based insecticides can provide management and help preserve beneficial insects and mites because they have no residual activity. However, thorough coverage of the tree is important as these products only kill mites they come in contact with.
Synthetic insecticides labeled for mite control may also be used and will be most effective in the spring and fall. Make sure they are labeled for mites and the type of tree you are treating. Before applying miticides make sure that mites are active by monitoring as described above.
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