WSU Whatcom County Extension

Integrated Pest Management for Raspberries

Spider Mites

Two-spotted Spider Mites (Tetranychus urticae)

Yellow Mites (Eotetranychus carpini borealis)

Insects and Invertebrates

 

Spider Mites

 

Symptoms

The two species of spider mites most commonly found in raspberry fields are the two-spotted (TSSM) and yellow mite (YM). Spider mites damage leaf surfaces by inserting their “piercing-sucking” mouthparts into foliage tissues and removing plant juices.

 TSSM feeding produces small, yellowish spots on the upper leaf surface. Fine silk webbing is typically present on the underside of the leaves infested with mites. As populations grow and feeding progresses, leaf margins appear dried and leaves turn silver or bronze then yellowish brown before drying up and falling off.

YM symptoms appear as a more concentrated yellowish brown coloration along the secondary and tertiary veins. This species produces very little webbing. Leaves that have been injured by feeding exhibit lower rates of photosynthesis, increased transpiration, deformed leaves, and lower chlorophyll content. This injury causes a mottling of the leaves that may become brown or bronze.

 

Identification

TSSM adult females are pale yellow-green or red with pale legs. They have two large black spots on the back and sides of their bodies. Fine silk webbing is typically present on the underside of leaves infested with this mite.

YM adult females are about a third smaller in length than the TSSM. They are pale yellow or greenish and have two or three pairs of small dark food spots on the body. This species produces very little webbing.

 

Yellow Mite Spider Mite

Yellow Mite (Eotetranychus carpini borealis)

 

Life History

TSSM overwinter as orange-colored adult females within the soil, in basal fruiting canes, and plant debris. They commonly emerge from April to May and begin to feed on older or mid-shoot leaves on fruiting canes before dispersing to the upper canopy in July to August. As these overwintering females feed they take on the species’ normal yellow-green hue and two characteristic dark-colored food spots. Generally five to six overlapping generations occur per year in western Washington. Summer females each lay about 130 eggs during a 30-day life span. Field populations increase rapidly after harvest through early September.  Spider mite females change to orange overwintering forms and begin to migrate from the leaves to overwintering sites.

YM overwinter as bright yellow females without food spots, in soil crevices and along the basal bark tissue of primocanes. This species emerges and disperses earlier than TSSM. It reaches the distal primocane foliage along the top trellis wire of red raspberry in April to May. Summer females lay about one-third as many eggs, yet they produce one or two more generations than the TSSM females. Populations of this species remain on primocane foliage later in the season than the TSSM that have begun their diapause phase. There are several overlapping generations each year.

 

Spider Mites

 

Monitoring

Scout 3-5 sites per field, (depending on field size) and evaluate 10-20 hills, spaced 3-5 hills apart on both sides of the aisle way. In early May, start sampling fields for spider mite activity appearing as white speckling on leaves. Collect ten leaflets per site and use a 10X hand lens to examine leaves for the presence of mites and mite predators. Count the number of two-spotted mites, yellow mites, and mite predators. Record the information at each site. Make note of the predominant mite stage (recent hatch, mixed, or mostly adult). Mite samples should be taken at least every two weeks during May and June.

Mite populations can often remain very low during the early summer and then increase rapidly during harvest. Continue to examine leaves for mites and mite predators as well as leaf bronzing damage associated with spider mite feeding. After harvest, examine leaves to determine population density of spider mites and mite predators. Predators can become quite numerous late in the season, particularly in fields with low insecticide use.

 

Two-Spotted Spider Mite

Two-spotted Spider Mite (Tetranychus urticae)

 

Thresholds and Management

The need to control spider mites is based on the average number of mites per leaf and predator to pest ratios. Factors which influence treatment decisions include; spider mite density, abundance of predators, population trends, damage to foliage, weather conditions (spider mite increase is favored by hot, dry dusty conditions), and miticide pre-harvest interval (PHI). Raspberries appear to tolerate significantly greater densities of yellow mites compared to two-spotted mites.

A pre-harvest spray may be necessary if:

  • Spider mites are approaching ten per leaflet or more and there are few or no predators
  • Populations are increasing and there are numerous eggs
  • Predator to prey ratio is less than 1:10
  • Miticide PHI is three or more days and population is building. A spray decision can be delayed if the available miticide has a short PHI (1-2 days), allowing for treatment during harvest if it becomes necessary.

 

A miticide application may be necessary during harvest if spider mite population is on the rise and the material has a short PHI. Control is usually better if it is timed to catch a building population rather than one that has gotten out of hand. Resample about five days after application to determine effectiveness of treatment. Spider mite population can increase rapidly after harvest through early September. However, treatment is unnecessary unless the population reaches 25 mites per leaflet by September 1.

Integrated management practices that reduce dust on foliage and fruit minimize harmful effects to arthropod predators and potential for severe spider mite outbreaks. Initiate good farming techniques that improve crop health and vigor to better withstand mite feeding impacts. Use miticides that are more selective for spider mite pests and safer on predatory mites.

 

Resources

Washington State University, Extension Bulletin 1959E, Spider Mites on Red Raspberry
http://cru.cahe.wsu.edu/CEPublications/eb1959e/eb1959e.pdf

Washington State University, Small Fruit Pests: Biology, Diagnosis and Management
http://cru.cahe.wsu.edu/CEPublications/eb1388/eb1388.pdf

2010 Pacific Northwest Insect Management Handbook
http://uspest.org/pnw/insects

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WSU Whatcom County Extension 1000 N. Forest St., Suite 201, Bellingham, WA 98225 (360) 778-5800 whatcom@wsu.edu