The hot, dry summer we have experienced so far has been conducive to spider mite infestations. These pests are found on a wide variety of plants including arborvitae, spruce, ash, rose, and beans. They are very tiny, about 1/50 inch long, and yellowish. They use piercing-sucking mouthparts to feed on leaves (on the underside) and needles which results in a whitish or yellowish stippled, speckled look to individual leaves or needles and an overall bronzed or yellowed discoloration to the plant. Webbing indicates a spider mite infestation. Severe feeding can stunt the plant's growth and can even kill the plant.
If you find discolored leaves and suspect spider mites, hold a white sheet of paper or paper plate under the leaves and shake the branch or leaves. If mites are present, you will find tiny spider-like creatures drop down and move around on the paper
Keep plants well watered to reduce their susceptibility to spider mites. You can take a hose and spray infested leaves and needles to dislodge some of the spider mites. If you want to treat your plants, consider applying insecticidal soap or horticultural oil. Although they are only moderately effective against mites, they are 'soft' on natural enemies and helps preserve any predators or parasites that are present. The product must directly contact the mites to kill them and repeated treatments may be necessary.
Residual miticides are not widely available to the general public. Effective active ingredients include bifenthrin, deltamethrin, and lambda cyhalothrin. Read the label carefully to be sure the plant you want to treat is listed for the product you intend to use and follow all label instructions.
Published in Yard & Garden News, July 15, 2008