Spider Mites on Trees and Shrubs

Life Cycle and description

• Spider mites develop through 5 stages from egg to adult.
• In hot weather, the generation time may be as short as one week.
• There may be up to 20 generations in a year depending on the weather. During hot summer weather, female two-spotted spider mites live about 30 days and produce about 100 eggs. This is why spider mite populations often build up so rapidly.
• Spider mite activity can occur any time after plants leaf out in the spring until early fall.
• The spruce spider mite attacks many conifers, especially dwarf Alberta spruce. Unlike other mites, it prefers cooler temperatures in early spring and late fall, and this is the best time to control them. 
• Most species of spider mites overwinter as white, brown, or red eggs. The two-spotted spider mite, which is the most common spider mite species, overwinters as orange adult females that hide in bark crevices. This is the most common spider mite found in homes and greenhouses. 

Damage

• As spider mites feed they suck out the green chlorophyll in leaves.
• The result of this feeding appears as minute white dots or flecks called “stipples”.
• Heavy mite feeding causes yellowing, browning of leaves, and eventual leaf death. During feeding, some mite species may inject toxins that cause varying degrees of leaf discoloration and distortion.
• Some spider mite species may produce webbing that covers leaves and stems when populations are high.The two-spotted spider mite feeds mainly on the undersides of the leaves.

Spruce spider mite

• Damage appears as tiny yellow stipples on needles. The needles turn yellow, and then brown.
• The damage is usually first observed on the older needles at the base (or one side) of the tree (or shrub), eventually moving up the tree (or shrub).
• When the mite population is very high, webbing may be noticed on the needles.

•  Lower branches may lose all of their needles, except new growth on spruce.
• Small trees and shrubs may be killed and large trees may have some dieback.
• This spider mite prefers cooler temperatures and is active in the spring (March - June) and fall (September - November).

Spruce spider mite description 

• Spider mites are very tiny, about 2 mm. They have 8 legs and are yellowish green when young. When mature and fully fed they are grayish black, with a tan area behind the mouthparts.
• The eggs are circular and reddish brown.
• There are several generations a year and eggs overwinter on the bark and needles.
• This mite prefers spruce, pine, hemlock, and arborvitae, but will attack other conifers. 
• There are other species of mites that attack juniper. The damage is similar to spruce spider mites but occurs throughout the summer.

Checking for spruce spider mites

• To check for mites on an evergreen, tap branches over white paper and look for the dark slow moving mites.
• Also look for predator mites, which are fast moving, and tiny round, black ladybeetles that feed on the mites. If predator mites are found, spraying may not be necessary.

Management of spruce spider mite

• Use a horticultural oil spray (1-2% rate) to kill eggs in early spring when temperatures are in the 50 – 55 °F range for a couple of days. Do not use horticultural oil on blue spruce or it will remove the blue color of the needles.

Rust mites 

• Can cause yellowing and dropping of needles on hemlock, fir, spruce, yew golden larch and pines.
• These mites are eriophyid mites, which are very tiny.  
• At 10X magnification, they are visible as light yellow, spindle-shaped forms with four legs.
• On hemlock, the hemlock rust mite feeds on the upper and lower surfaces of the needles. On pine, rust mites are usually found between the needles within the needle sheath.

Monitoring

During the growing season, landscape plants should be monitored every 1-2 weeks for evidence of spider mite damage. Because indoor temperatures are relatively high and constant, plants should be checked on a weekly basis. Examine both sides of leaves; use a magnifying lens if necessary. Signs of active mite infestations include various instars of mites, eggs, webbing, and stippling. If damage is visible, but no live mites are found check leaves or branches higher up on the plant. Mites generally work their way up a plant and often the best place is directly above where the damage is most noticeable. A simple technique for sampling, (especially conifers) is to tap a few terminals or leaves over a piece of white paper. Wait a few seconds and watch for movement. The mites may take a short time before they begin crawling on the paper.

Management of spider mites

Non-chemical 

Plants and small trees can be hosed down periodically with a strong spray of water. (The force of the water will depend on the plant.) Be sure to get good coverage of the lower surface of the leaves. On conifers, thoroughly wash down the entire plant.

Biological 

There are several species of predatory mites available commercially. The predatory mites are usually shipped in containers with pollen for food. They are sprinkled on the plants that are infested with spider mites. The rates will vary with the species of predator purchased. Many garden supply catalogs and biological control businesses sell predatory mites. They will recommend release rates and the species best suited for your needs.

Chemical 

Chemicals should always be considered as a last resort. Use them only if non-toxic methods have failed. Overspraying with insecticides can increase spider mite problems because beneficial insects are negatively impacted. 

The following pesticides are preferred if they are labeled for the plants you want to spray: Safer Insecticidal Soap, Horticultural Oil (e.g. Sun Spray, Volck Oil, etc.). These materials are the safest to use and have the least impact on beneficial insects and mites. The only drawback is that they are short-lived and retreatment may be necessary. Acceptable control will depend on thorough coverage of infested plants, including upper and lower surfaces of leaves. Horticultural oils may also be used at a rate of 3-4%, during the dormant season to control overwintering forms of spider mites on bark. Do not use horticulture oil on blue spruce.

References:

Johnson, W.T. and H.H. Lyon, 1991. Insects That Feed on Trees and Shrubs, 2nd. Ed. Ithaca, NY: Comstock Publishing Associates, Cornell University Press. 560 pp.

Pirone, P.P. 1978. Diseases & Pests of Ornamental Plants, 5th Ed. New York: John Wiley & Sons. 566 pp.

Olkowski, W., S. Daar, and H. Olkowski. 1991. Common-Sense Pest Control. Newtown, CT: The Taunton Press. 715 pp.

Mention of trade names in this publication does not constitute an endorsement by University of Maryland Extension