Spider mites are an occasional pest of spruce and pine trees in Iowa. Mites feed externally on conifer tree needles with piercing-sucking mouthparts that puncture the plant tissue and suck out the liquid within the cells. In light infestations the foliage will appear to be speckled with very tiny yellowish-green spots. If the population of mites increases the damage can become severe enough to turn the foliage entirely greenish-yellow and eventually brown. Heavily infested branches will drop their needles. Close examination of infested foliage may reveal very fine webbing on the stems at the base of the needles.
Spider mites are tiny and difficult to see with the naked eye on the plant. A convenient detection technique is to hold a sheet of white paper under a branch and then shake or tap the branch against the paper. The mites, if present, will show up as tiny, slow-moving specks on the paper.
Several miticides (pesticides that control mites) are available for mite control. Spraying to reduce mite numbers is advised when plants show the characteristic damage described above, and the white sheet of paper technique turns up a large number of mites (a dozen or more on each sheet). Sprays available to homeowners include insecticidal soap, horticulture oil (summer rate), malathion, diazinon, Isotox and Orthene.
Insecticidal soap is available from retail stores under various trade names. The active ingredient is "potassium or sodium salts of fatty acids." Mix and apply commercial products according to label directions. You may substitute ordinary dishwashing detergent for the commercial products with little risk of injury to trees. To make your own insecticidal soap spray solution, mix 5 tablespoons of liquid dishwashing detergent in 1 gallon of water. Soap sprays have no residual activity and only control mites and insects that are contacted directly. Thorough spraying is important for control.
On smaller trees and shrubs it may be possible to reduce light mite populations with periodic "hosings." Use the garden hose to apply a forceful stream of water to dislodge mites from an infested plant. Repeat 3 or 4 times on consecutive days.
Hot dry weather and drought conditions favor mite populations. Keep plants watered and mulched to promote health and vigor and to reduce the impact of mite feeding.
This article originally appeared in the August 11, 1995 issue, p. 125.