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Mark Shour, Extension Entomology, (515) 294-5963,
Elaine Edwards, Continuing Education and Communication Services, (515) 294-5168,

Yard and Garden Column for the Week Beginning June 1

When Your Evergreens Turn Color, You "Mite" be in Trouble

By Mark Shour
Extension Program Specialist
Iowa State University Extension

Just as a painter uses texture and color to create a life-like image on canvas, a gardener uses color and texture to create a living work of art in landscape. One of the most common colors in ornamental gardens is green, ranging from the yellow-green of fern fronds to a blue-green of hosta leaves to the medium and dark greens of needled evergreen shrubs and trees. Orange, yellow, brown and gray in arborvitae, fir, hemlock, juniper, pine, spruce or yew, however, may indicate a potential problem. Let's follow the seasonal color changes in these well-known foundation, screen and specimen plants.

A red-brown or orange cast on many evergreens is commonly seen in Iowa's landscapes in early spring. This condition is called winter injury and can result from desiccation or early fall frost. Winter injury mainly affects the needles, but symptoms may vary from only needle tips affected to one or more branches to the entire shrub or tree. Regardless of the cause of winter injury, trees or shrubs may retain their green color until warmer temperatures arrive in the spring. New, light green growth breaks forth from the dormant buds, eventually masking the red-browns or oranges of winter injury.

Yellow or yellow-orange color in the fall on evergreens should not be confused with winter injury. These fall colors signal the beginning of natural needle loss. Older needles near the interior of the tree are being shed. These needles are loosely attached and fall very easily when touched. In contrast, needles affected by winter injury are still firmly attached to their twigs.

An overall grayish, yellowish green or white appearance of evergreens in the summer may indicate the presence of spider mites. Their feeding damage to individual needles appears initially as white or yellow specks that may fuse to form blotches or cover the entire needle. Heavy mite feeding causes the needles to drop prematurely. In addition, spider mites produce fine, silken webbing between the needles; this webbing collects dust, pollen, dead needles and other debris. Generally, the interior portion of a branch and the lower branches are most prone to spider mite feeding and webbing.

The spruce spider mite and the twospotted spider mite feed on many of the evergreens. The spruce spider mite is active during spring and late fall when temperatures are cool. This pest generally restricts its feeding to evergreens, with the exception of yews. Feeding damage often goes unnoticed until the hot, dry summer when spruce spider mites leave the needles and enter a resting period (called aestivation). The twospotted spider mite is active during hot, dry weather. This pest feeds on the evergreens arborvitae, hemlock and juniper, as well as many garden and landscape plants. Shaking an infested branch over a sheet of white paper is an easy method to determine whether spider mites are present. If you dislodge tiny (1/50th inch; 0.5 mm) eight-legged arthropods that have tan heads and dark purple or black abdomens, you have a spruce spider mite infestation. Twospotted spider mites are approximately the same size but light green to yellow with two dark spots on their abdomens.

Spraying a forceful stream of water (syringing) on plants can be effective in controlling spider mite populations in the home landscape. This method requires persistence and dedication. The use of insecticidal soap or horticultural oil (1 to 2 percent) applications also decreases or eliminates spider mite populations. Pesticides are available that are specific to mites (e.g., Hexygon, Mavrik Aquaflow, Ornamite, Morestan), long-lasting, and kill eggs, but these miticides are available only through a professional applicator. Most products available to the homeowner are broad spectrum (e.g., malathion) and kill mites as well as many types of insects.

Other colors and color patterns can be displayed by evergreen shrubs and trees. Discolored needles (purple, yellow, orange or brown); needles with spots or bands (orange, brown or black); twigs with discolored sapwood or heartwood; and resin exuding from twigs, branches or the main stem may signal the presence of an infectious plant disease. Missing foliage, partially chewed needles, white flecks on needles, holes in needles, twigs or branches, entire branch or tree death are indicators of insect pests. It also is possible that animal feeding (rabbits, squirrels or voles), site changes, weather conditions, road salt accumulation and air pollution can cause an evergreen to change color. Samples should be collected and sent to the Iowa State University Plant Disease Clinic at 323 Bessey Hall, Ames, Iowa, 50011, for proper diagnosis and remedial recommendations.


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