Click for larger image The discoloration of the needles on this spruce (Picea) was caused by spider mites (Acari)

The spruce spider mite (Olibonhychus ununguis) is a cool season mite unlike the two-spotted and red spider mite, which are most prevalent during hot, dry weather. It is native to North America and is one of the most destructive spider mites in the US. Its preferred host is spruce (Colorado blue, Norway, Dwarf Alberta, and white) but it will also feed on arborvitae, cedar (Cedrus), dawn redwood, Douglas fir, hemlock, juniper, larch, and pine. It can cause considerable damage early in the season before many gardeners are even thinking about spider mites and again in the fall.

Symptoms and Diagnosis

The first indication of spruce spider mite damage is an off-green color, mottling or stippling of the needles; which may not be very noticeable until early or mid summer. Usually older, inner needles are attacked first. Damaged needles turn yellow then bronze in June and fall prematurely. Fine webbing may also be present. In severe infestations, twigs and branches can drop all their needles and die leaving dead areas on the plant. Stressed trees may be killed.

Life Cycle

Small reddish-orange, oval or circular eggs overwinter in bud scales, on the bark and on needles. The young hatch in mid March to late April. Feeding begins and continues through the cool, moist days of spring. When hot, dry conditions begin and daytime temperatures reach 80-90 degrees F., activity slows and resting eggs are laid. The young hatch when the cooler days of fall resume. Spruce spider mites are small, 0.5 mm (1/50 inch), but can be seen with a hand lens. The young are pale green when they first hatch but become very dark green to dark brown as they age. Adults have salmon-pink body-spines and legs. The young have only 6 legs but adults have 8. The spruce spider mite overwinters as eggs in bud scales or on the needles and bark of the host plant. There are 7-10 generations a year. Each can be completed in only 3-4 weeks.

Integrated Pest Management Strategies

1. Monitor plants in early spring and again in early fall. Remember, this spider mite is a cool-season mite. Monitoring for the mites and eggs can be done by tapping the branch over a sheet of white paper. Also check for beneficial mites and insects that may be feeding on the spruce spider mites. If beneficial insects are present, let them do the control. Applying a pesticide will kill the beneficial insects and the population of the problem spruce mite can actually increase. Beneficial mites are usually faster moving, are reddish in color, and leave a red stain when crushed. Adult spruce spider mites are green to brown and leave a green stain when crushed. Ten or more mites per sample with few if any beneficials present, usually warrants application of a miticide.

2. Wash the mites and eggs off the foliage with a strong stream of water. A weekly or bi-weekly “washing” of the foliage in spring and fall can help prevent a population build-up.

3. Use insecticidal soaps or horticultural oil. These pesticides are less harmful to beneficial insects that help keep mite populations in check. Needles of blue-needled evergreens sprayed with an oil spray will turn green. The new unsprayed needles, however, will be blue. Soaps do not have this effect on needle color.

4. Apply Neem or a pyrethroid-based pesticide late in the season to kill adults before they lay eggs that will overwinter.

5. If a fall infestation was severe, apply a dormant oil spray in mid winter to kill overwintering eggs.

6. Have the tree sprayed by a certified arborist or tree care professional. Most pesticides available to homeowners are very weak miticides and will not control spruce spider mites well. Stronger miticides are restricted use pesticides and require application by a certified applicator.

Organic Strategies

Strategies 1-3 and 5 are strictly organic. For organic approaches to Strategy 4, consult the Organic Materials Review Institute (OMRI™) for an appropriate organic miticide product.

More images:

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Browning older needles on hemlock (Tsuga) caused by spider mites (Acari)
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Spruce spider mite damage (Acari) on spruce (Picea)