Description of conifer spider mites
Mites feed externally on conifer tree needles. They use piercing-sucking mouthparts to puncture the plant tissue and feed on the liquid within the cells. Feeding injury causes the foliage to be discolored with very tiny yellowish-green speckles. Severe damage causes “bronzing” eventual browning and needle drop. 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. 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.
Twospotted spider mite
The twospotted spider mite is a “warm season” mite favored by hot, dry weather and drought conditions. Adults and nymphs are white with two dark greenish spots (summer coloration). This mite has been reported from over 180 different plants including field crops, lawn, garden and landscape plants, houseplants and weeds. Mites become active in April and May and are active the rest of the summer. Severe damage usually appears in late summer after a period of hot, dry weather.
Spruce spider mite
The spruce spider mite is a common “cool season” mite found on all types of conifers (spruces, pines, junipers and arborvitae). Spruce spider mites are active in the spring and fall. They become dormant during the heat of the summer and survive as “resting” eggs. These eggs and adults resume activity in the fall when cooler temperatures return. Conifers often react slowly to the spruce spider mite feeding. Yellowing and bronzing of the needles may not become apparent until mid-summer, even though the damage occurred the previous fall or spring.
Conifer spider mite management
Several miticides (pesticides that control mites) are available for mite control. Spraying to reduce mite numbers is advised when plants show the characteristic speckling 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, permethrin, and others available at your garden supply store. Soap and oil 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. Keep plants watered and mulched to promote health and vigor and to reduce the impact of mite feeding.
Do you live in Iowa and have an insect you would like identified?
The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them. Please see our website for current forms, fees, and instructions on preserving and mailing insects.
Contact information for each states diagnostic laboratory for U.S. residents. If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.