Spruce spider mites remove the contents of individual cells.

Spruce Spider Mite

Spruce spidermites can web over foliage if not disturbed for a considerable time.

Species: Oligonychus ununguis (Jacobi)

Distribution: Worldwide

Hosts:

In spite of its name, this pest attacks over 40 species of conifers. This includes all trees used in Christmas tree production as well as juniper, arborvitae, hemlock and yew (Taxus).

Damage:

Spider mites have tiny mouthparts which are often grouped in the sucking category along with sucking insects. This is not exactly correct, though these mites are capable of inserting their mouthparts into plant tissues and withdrawing fluids. This causes yellowish spots to occur on the needles. Extensive feeding results in "bronzing" of the foliage and premature needle drop.

Description and Life Cycle:

This mite is closely related to other true spider mites, such as the European red mite and twospotted spider mite. They have egg-shaped bodies which are only 1/64-inch long or less. In the field, they appear to be an olive-green color with reddish-yellow legs. Like all spider mites, the nymphs and adults have four pairs of legs rather than the three pairs found on all insects. Under a microscope, two red eye spots can be seen and a paler stripe is evident along the back. The larvae which hatch from eggs are almost spherical in form and only have three pairs of legs.

Spruce spider mites are considered to be "cool season" mites. That is, they seem to do best during the spring and fall seasons when temperatures are in the 60's and low 70's F. In hot, dry summers, this mite becomes dormant and predators take their toll. Spruce spider mites overwinter in the egg stage attached near the base of needles. These eggs can hatch as early as mid-March but most hatch by mid-April. The larvae feed on needles and molt within a few days into the first stage nymphs, called protonymphs. The protonymphs gain a pair of legs and continue to feed for a week. The protonymph rests for a day or two before molting into the second stage nymph, the deutonymph. Again the deutonymphs feed for a week to ten days before settling again to molt into the adults. The adult female has a rounded abdomen while males are smaller and have pointed abdomens. The entire development time from egg to adult can take from 11 to 24 days, depending on the temperature. Usually six or more generations are common in a season and each female can lay up to 25 eggs in 8-10 days. By late spring, all stages are common and generations overlap considerably. When temperatures consistently reach the upper 80's or 90's F, this mite tends to lay over-summering eggs. Activity often doesn't return until the fall. When summers are cooler than normal, this mite remains active. Conifers seem to react slowly to spider mite attack. The yellow speckles and bronzing may not be evident until hot weather occurs. Therefore, much of the damage which becomes very visible in June and July actually occurred the previous fall and spring.

Control Hints:

Since the damage caused by this pest usually doesn't appear until the populations are beginning to decline from high summer temperatures, constant monitoring is needed to detect damaging populations. Sample for this pest by taking a white colored piece of cardboard or a white tray and hold it under a branch. Strike the branch sharply three or four times with a stick so as to dislodge the mites off the foliage. Wait for about 10 seconds and pour off any of the dirt, loose needles and other trash. The spider mites will remain and will appear as tiny black or olive-green specks walking slowly on the tray. Other mites may be present which do not damage conifers. In order to distinguish these, crush one of the suspected mites in a streak. If the streak is greenish, the mites are probably spruce spider mites. If the streak is black, brown or reddish, the mites are probably scavengers or beneficial mites.

Option 1: Biological Control - Encourage Natural Predators and Parasites - There are numerous predators of spruce spider mites. These include lady beetles and their larvae, predatory mites and dusty wings (a relative of the lacewing). These predators often do a good job during the warmer months. Therefore, be sure to spray for spruce spider mites early or late in the season when the predators are at lowest numbers. Too many growers spray during the summer when the damage becomes visible and the mite populations are declining.

Option 2: Chemical Control - Dormant Oil Sprays - Since these mites overwinter as eggs, dormant oil sprays (3-5%) applied in late fall and early spring can greatly reduce the mite population. Hydraulic sprayers are suggested in order to obtain thorough coverage. Remember that oil sprays on glaucous trees will remove the silvery blush and turn the foliage dark green.

Option 3: Chemical Control - Horticultural Oil and Soap Sprays - Horticultural oil (1.5-2%) and insecticidal soaps have been very effective in controlling this pest in the spring and fall. Refer to the caution on foliage color above.

Option 4: Chemical Control - Standard Miticide Sprays - Registered miticides are most effective if applied in the early spring, usually before mid-May, or in the fall, after mid-September. If damage is detected in June or July, be sure to sample first in order to determine if the mites are active.