Summer weather is expected to arrive this week. With hot, dry days, gardeners should be on the look out for spider mite damage to plants, especially in areas of the landscape where plants are near rock, driveways, streets, or other hot reflective surfaces.
Juniper and spruce, particularly dwarf spruce, are very susceptible. Potentilla also seem delectable to these arachnid pests. Although many people look for webbing as the first indication of infestation, a graying discoloration is actually often the first sign of a problem, followed by some dieback of twigs in the interior of the plant. By the time webbing is evident, the spider mite population is huge and very challenging to control.
These critters are tiny and can be seen with a magnifying glass or a hand lens. Or, take a piece of white paper out to a plant you suspect may have spider mites, and tap a branch over the paper. Hold the paper very still and see if what looks like dust particles on the paper move. If so, your plant has spider mites.
As the name implies, spider mites are relatives of spiders and have eight legs. Insects have six legs. Insecticides may or may not kill spider mites. Miticides kill spider mites. But, before you run out and purchase a pesticide, know that you can often easily control spider mites by regularly hosing off the plant or by adjusting a sprinkler head to spray the plant on irrigation days. Mites thrive in a dry, dusty environment. Spraying the plant with water raises the humidity and washes off the dust. Pesticides can actually increase spider mite infestations after the initial kill, because beneficial mite and insect populations that eat the bad mites are also destroyed and chemical resistance develops. This leads an unsuspecting gardener to spray more, without successfully controlling the pests.
If you notice your dwarf spruce thinning out with some dying areas or not looking bright green, thoroughly wash it off with a hose, inside and out. Do this every couple of days until you see new growth developing or the healthy green color returning. Sometimes when an infestation is too advanced, you may want to resort to applying an insecticidal soap that lists mites on the label. A final resort would be using a miticide, but expect the problem to return if you don't raise the humidity on the plant and keep off the dust.
For more information, e-mail skellyj@unce.unr.edu or call me at 887-2252. You can "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu or call your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu.
• JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.