WSU Extension

Hortsense

Douglas Fir
 
Disease
Armillaria root rot 
Laminated root rot 
Rhabdocline needle cast 
Rust 
Stem cankers 
Swiss needle cast 
Upper stem canker 
Yellow-green mottle syndrome 
Insect
Aphids 
Coneworms 
Cooley spruce gall adelgid 
Douglas fir needle midge 
Douglas fir tussock moth 
Douglas fir twig weevil 
Sequoia pitch moth 
Silverspotted tiger moth 
Spruce spider mite 



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Caption: Spruce spider mite adult
Photo by: A.L. Antonelli
  
Douglas Fir : Spruce spider mite
(revision date: 2/14/2019)


Biology
The presence of spruce spider mites is indicated by yellow or bronze stippling beginning near the needle bases. Infestations usually begin on older needles of the lower branches and spread upwards as the mite population increases. Damaged needles may turn brown or reddish-brown. Fine webbing may cover the needles and twigs. The actual spider mites are very small and vary in color from greenish to orange, dark green, or black, with orange legs. Spruce spider mites attack many species of conifers. They are easily spread by wind. These mites are often worst on dusty roadside trees.
Management Options


Non-Chemical Management
  • Spider mites are probably controlled naturally by predatory mites and rain.
  • Try to avoid broad-spectrum insecticides that will kill predatory mites and insects and allow a spider mite population explosion.
  • Hose trees with strong stream of water to wash off spider mites (where practical).
Select non-chemical management options as your first choice!

Chemical Management

Apply oils in early spring to smother over wintering eggs. Begin monitoring for spider mite in the late spring or especially during hot weather. Soaps should be applied in May or early June if mites are present. Repeat applications may be necessary. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • All Seasons Horticultural & Dormant Spray Oil Conc [Organic]
    Active ingredient: oil/pet distillate  |  EPA reg no: 4-80
  • Hi-Yield Dormant Spray
    Active ingredient: oil/pet distillate  |  EPA reg no: 7401-428
  • R-T-U Year-Round Spray Oil
    Active ingredient: oil/pet distillate  |  EPA reg no: 6218-78
  • Safer Brand Insect Killing Soap Conc II [Organic]
    Active ingredient: potassium laurate  |  EPA reg no: 42697-60
  • This list may not include all products registered for this use.
Images

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Caption: Spruce spider mite adult
Photo by: A.L. Antonelli
Caption: Spruce spider mite damage
Photo by: R. Maleike