Pest description and crop damage Several species of spider mites can cause damage in deciduous ornamentals. Appearance of these mites varies with the species, although all are 0.02 inch or smaller. Adults and nymphs can be yellowish, greenish, or reddish brown, depending on species. Mites damage plants by feeding on leaves, which causes stippling, bronzing, and possibly leaf drop. The reduction in photosynthesis causes loss of vigor. The most common pest species of spider mite on conifers is spruce spider mite. It can cause damage on a wide range of conifers. Adults and nymphs can be yellowish, greenish, or reddish brown. Mites damage plants by feeding on leaves, which causes stippling, bronzing, and possibly needle drop. Webbing occurs with high populations. The reduction in photosynthesis causes loss of vigor.
Biology and life history Most mite species share a similar life cycle. The majority of twospotted spider mites overwinter as adult females. Spruce spider mites, which are commonly found on conifers, overwinter in the egg stage. Mites become active in the spring. There may be eight to ten overlapping generations per year. Spruce spider mites overwinter as eggs laid at the base of needles or scales. Mites become active in the spring, usually around mid-April. There may be eight to ten overlapping generations per year. Spruce spider mite is a cool season mite with greatest rates of egg laying during the spring and fall.
Scouting and thresholds Observe the leaves for mites and webbing and check for the number of pest and predator mites. Tapping branches over a piece of white paper indicates numbers of mites present. Look for evidence of natural enemy activity.
Management- cultural control
Suppression of broadleaf weeds such as mallow, bindweed, white clover, and knotweed with cultivation or grasses may reduce mite numbers. Wash mites from the tree with a strong stream of water. This also dislodges dust and dirt, which favor an increase in mite numbers. Water trees properly, as drought-stressed trees are more susceptible. Avoid excessive nitrogen applications, as this encourages mites
Using an ovicide-larvacide in early spring can give good suppression of this mite.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p.472-477).
PNW Nursery IPM: Spruce Spider Mite (http://oregonstate.edu/dept/nurspest/spruce_spider_mite.htm)
PNW Nursery IPM: Biological Control of Twospotted Mite (http://oregonstate.edu/dept/nurspest/two-spottedmite.htm)