Spruce Spider Mites
Spruce spider mites have been active in Fargo since April this year. Keep in mind that
spruce spider mites are cool-season mites and are often active very early in the growing
season. Mites will cause tiny yellow or white speckles on needles that can be seen with
the naked eye and are easily identified with a hand lens. We often notice damage during
hot, dry summer days. The feeding injury often occurred in the previous spring or fall and
became evident under the stress of hot, dry weather. Since miticides are usually more
effective against adults than eggs, summer applications of miticides may not be effective.
Mites are only 0.5 mm long and are very difficult to see without magnification. As with
most spider mites, a good test for spruce spider mites is to place a white piece of paper
under needles which are believed to be infested and tap the branch. Mites appear as mobile
specks on the white paper. Generally, if ten or more mites are found per sample, some type
of control may be necessary.
Syringing, cultural control, biological control, and chemical controls are all options
that can be used in controlling spruce spider mites. Spraying mites with a forceful jet of
water (syringing) can be an effective method for controlling mite populations in home
landscapes while maintaining natural predators. Lace wings and lady beetles are naturally
occurring predators of spruce spider mites in North Dakota. Insecticidal soaps can be used
to manage spruce spider mites in warm weather, while horticultural oils (1-2% rate) may be
used during the summer and dormant oils (3-4% rate) can be used to kill mite eggs and
adults during the spring and fall. Horticultural oils can injure conifers if applied when
temperatures are not appropriate. Read the labels carefully. Miticides should be sprayed
when adults are active with a follow-up spray 7-10 days after the first spray to control
later hatching nymphs. Always read and follow pesticide labels.
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