Tiny in size, spider mites are hungrily attacking our conifers. By sucking the juices from needles of species such as hemlock, spruce, and fir, spider mites flourish in dry conditions. They can cause damage, particularly to hemlock, older needles, and those close to the main stem. There are two main species of spider mites, the spruce spider mite and the two-spotted spider mite. Spruce spider mites are a pest of most conifers, while the two-spotted spider mite is a serious pest of fruit trees, berries and deciduous ornamentals.
Spider mites are small enough that often they are undetected until they've already done some damage.
Damaged needles appear speckled and dusty with spider-like webs between them. From far away, the needles look yellow when really they are a green and yellow patchwork pattern. The yellow areas are where most of the damage has occurred and killed the needle. Highly damaged needles are usually dropped by the middle of summer. This shouldn't cause you any concern, assures Philip Auger, Extension Educator in Forest Resources in Rockingham County.