Spider mites on coneflowers. (C) Jo Ellen Meyers Sharp
Those specks of rusty gold on the leaves of coneflowers, tomatoes, dahlias, arborvitae, spruce and dozens of other plants in the garden are one more thing to blame on the excessive heat and drought.
The stippling is caused by spider mites, creatures that aren’t really spiders, but are closely related. Spider mites flourish in hot, dry weather. The weather also suppresses a natural fungus that helps control the mites, allowing the bugs to remain unchecked.
Spider mites puncture the cells of leaves and suck out the chlorophyll, leaving small rusty, gold or creamy spots. Each mite can puncture 100 cells an hour and there may be thousands of mites on a plant. There are several different mites and they tend to be fairly specific to certain types of plants. For instance, the mites on a dahlia are not the same as those on an arborvitae.
Usually you see the damage before you see the tiny yellow, green or brown mites. They work on the undersides of leaves and branches, forming visible webs on plants to help them move about. To test for mites, shake a leaf or branch above a white piece of paper. If dots that move appear on the paper, the plant likely has spider mites.
Fortunately the mite damage is not usually deadly. The first line of defense is a strong spray of water from the hose to wash off the mites from plants. Remove any visible webs. Insecticidal soap, a natural insecticide available at garden centers, also can be used to control heavy infestations of spider mites. Horticulture oils smother spider mites and frequently are recommended for evergreens. Horticulture oils are temperature sensitive, so make sure the one you use is seasonally appropriate. Make sure to spray the undersides of leaves. Always read and follow the label directions.
For more information about this pest, download Purdue University’s Spider Mites on Ornamentals [1] or Ohio State University’s Spider Mites and Their Control [2].