Last week we met the curious Cooleyi spruce gall adelgid living a vagrant life alternating between Douglas fir and spruce trees. While searching for adelgids, I happened across a Colorado blue spruce that was anything but blue. The signature pale blue of this spruce had been replaced by a dirty brown hue. What mischief was this? Needles of many conifers including blue spruce are lined with glands that produce a fine coat of wax. This waxy bloom protects the tree from loosing precious moisture in cold dry climates like the Rocky Mountains where blue spruces evolved. The culprit in turning a blue spruce brown is a tiny mite known as spruce spider mite.
While many insects seek cover and rest when autumn turns to winter, spruce spider mites are busy at work probing needles of spruces, firs, hemlocks, and other conifers to suck nutritious contents of the cells. Using dagger-like mouthparts called chelicerae; mites puncture cells and imbibe the leaking fluids. When spruce trees are attacked by spider mites, their ability to produce wax is reduced and instead of appearing glaucous blue, needles turn yellow, brown, or bronze. In dense populations, as feeding injury accumulates, needles may drop and death may come to branches or entire plants.
As winter deepens, spruce spider mites lay tiny reddish, orange or brown eggs and attach them to needles or stems. Early in spring while temperatures are still cool, eggs hatch into six-legged larvae that pierce and suck cells. Larvae molt into eight legged nymphs which in turn molt again before a final transition into the adult stage. Female spruce spider mites can lay several dozen eggs during a lifetime that spans many weeks. In a curious twist of biology, female mites do not require a mate to reproduce. If there is a male about to fertilize eggs, offspring will be females, but in the absence of a mate, unfertilized eggs hatch into males.
During the cool temperatures of spring and early summer, spruce spider mites complete a generation about every three weeks. Hot days of summer shorten generation time, but temperatures in excess of 85 degrees Fahrenheit are lethal to eggs. So, during sultry summer and fall when other spider mites are rocking, spruce spider mites are rarely found. Cool autumn temperatures invigorate spruce spider mites. Feeding and reproduction resume and damage accumulates on the tree. Spider mites have many predators such as lady beetles and larval lacewings we met in previous episodes of Bug of the Week. Spider mites may also be killed with applications of insecticidal soap, horticultural oil, or miticides. These should be applied according to instructions found on the label. Please take care when applying horticultural oil to blue varieties of plants such as the Colorado blue spruce. Oil dissolves wax and it will turn your blue spruce into a green spruce until the tree can replace the waxy bloom. If you choose the pesticidal approach in dealing with mites, it is always a good idea to consult your local Cooperative Extension agent or specialist to ensure that you have selected the right product and that you are applying it at the proper time.
The interesting article “Spruce spider mite, Oligonychus ununguis (Jacobi) – an integrated approach to management” by Rayanne Lehman was used in preparation of this episode. To learn more about spruce spider mite, please visit the following web site.