Many species of spider mites (order Acariformes), distributed throughout the North Temperate zone, attack a wide variety of plants. The spruce spider mite, Oligonychus ununguis, is the mite pest of greatest concern in British Columbia container and bareroot nurseries. However, recent infestations of Trisetacus species at several bareroot nurseries (Figure 99) indicate that this mite is also a problem. Other mites are often observed on seedlings and are usually harmless. Recently, large numbers of the genus Linopodes have been found at several nurseries. They have long, slender legs, the two front ones being twice as long as the others, enabling them to jump and walk backwards quickly. They are not pests of plants.
The spruce spider mite feeds on the foliage of several conifers, including cedars, Douglas-fir, true firs, hemlocks, larches, pines, and spruces. It inserts its stylet-like mouthparts into needles and tender twigs and sucks out the cell contents, causing a mottled, bleached discoloration and subsequent drying of the needles (Figure 100). Severely affected foliage turns dingy-yellow to dull, rusty-brown and the needles drop off. Immature and adult mites spin a fine, silk webbing around and among the needles of infested twigs. The abundance of this webbing (Figure 101), especially noticeable when branches and twigs are viewed from the underside, increases as the season progresses. The webbing usually contains eggs and cast mite skins, and protects the mites from dislodgement and from some of their natural enemies. The mites can be seen directly on foliage with a hand lens or by the tapping of an infested twig over a white sheet of paper.
To date, the spruce spider mite has been a sporadic pest on bareroot and container stock at coastal and interior nurseries. Certain seedlots are affected more than others. However, the potential for damage is probably greatest in southern interior nurseries because hot, dry conditions favor the pest. Low host vigor, host crowding, and absence of natural enemies also enhance spider mite outbreaks. Consequently, damage is most likely to occur in late summer to early fall when seedlings are often being stressed for water and nutrients to induce dormancy.
Full knowledge of host range, distribution, and damage of the Trisetacus mite has not yet been attained. It has been identified on 2+0 production stock, nursery outplanting stock, and grafting stock, and in an interior reforestation site. The natural range is coastal, but if introduced into the Interior it may become established. Its hosts include lodgepole, shore, and Scots pine. Interior provenances of trees are more seriously affected than are coastal provenances. These mites live in colonies in needle sheaths where they damage epidermal tissue, destroying the entire needle base which becomes brown, necrotic, and sometimes calloused. Needles (Figure 102) then become chlorotic, stunted, and often twisted or crinkled with needle growth reduced up to 70%. Infested needles may drop. Most damage occurs in the spring. Locally the problem is called "kinky disease."
There are five stages in the life cycle of the spruce spider mite: egg, larva, two nymphal stages, and adult. Eggs are pale yellow when laid and gradually become reddish brown. During a warm, dry season, hatching begins in late spring to early summer. Development of larvae and nymphs requires 3-6 days, respectively. Newly hatched larvae are pink, becoming needle-green after feeding. They have three pairs of legs. Nymphs are mottled, needle-green to dark-green. Like the adults, they have four pairs of legs. Nymphs become adults by early summer and may live up to 30 days. Adults are very small (0.4-0.6 mm long), dark-green to dark-brown, and move about fairly rapidly. Each female lays as many as 50 eggs. There may be up to seven succeeding generations of mites per season - that is, about one new generation each 15 days. Larvae, nymphs and adults all feed, the spring generation on old foliage and subsequent generations on current foliage. Overwintering eggs are laid in autumn under loose bud scales and bases of needles. Outbreaks can originate from overwintering eggs or be windblown into nurseries.
Trisetacus mites are very small, less then 0.3 mm long, and very slow moving. They are light yellowish-white, elongate, and wormlike with all their legs at the anterior end. Colonies found in needle sheaths, where they overwinter as well, can have up to 200 mites, but damage can be caused by 10 to 20. The population overwinters as both adults and eggs within the needle sheaths. During the time of candle elongation in the spring, the mites move to the new growth and lay several overlapping generations of eggs. It is at this time that they cause considerable damage to the new needles producing the symptoms of kinky disease. During the summer, as necrotic tissue begins to develop at the needle base, the mites often disappear; presumably they move on to healthy needles.
Mite infestations can build up rapidly and management is often necessary. Spider mite populations on trees around the nursery should be monitored, particularly during long, warm-dry summer periods, because these mites can be windblown into the nursery. Where practical (e.g., in small outbreaks), affected seedlings can be washed daily with a strong stream of water to wash away the mites and break up the webbing protecting them and their eggs. Sometimes, miticide use may be necessary, especially where water washings might be undesirable, such as in large outbreaks or where the water might break dormancy of stock destined for storage or shipping. When using a miticide, current recommendations should be checked; insecticides are usually ineffective against mites. Miticide applications must be made frequently since eggs are resistant, and several different life stages are usually present simultaneously. Outbreaks should be managed late in the season before stock is infested with overwintering eggs and sent to reforestation sites.
Specific controls for Trisetacus mites have yet to be established, but as already mentioned, stock destined for reforestation sites should be free of mites. Symptomatic seedlings can be examined with a powerful hand lens, by gently removing the needle sheath separating the two needles, and examining their inner surface for mites. The best time to control this pest is from mid-spring to early summer during the period after candle elongation, but prior to needle elongation. At this time the mites move out of last year's needle sheaths and migrate up the shoots into the elongating needles of the new growth. Adding oil to the spray mix helps insecticides penetrate through or around the sheath to the infested area.
Doidge, D.F., and V.G. Marshall. 1971. Spruce spider mite in British Columbia. Dep. Fish. and For., For. Res. Lab., For. Pest Leafl. 33, Victoria, B.C.
Hunt, R.S. 1981. Trisetacus (Acarina: Eriophyoidea) on Pinus contorta in British Columbia: distribution, symptoms, and provenance effect. Can. J. For. Res. Vol. 11: 651-653.
Peterson, L.O.T. and V. Hildahl. 1969. The spruce spider mite in the prairie provinces. Dep. Fish. For. For. Res. Lab., Liaison and Services Note MSL07, Winnipeg, Man.
Steward, K.E. and L.O.T. Peterson. 1960. Control of the spruce spider mite. The Queen's Printer and Controller of Stationery, Publ. 1078, Ottawa, Ont.
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Principal, locally grown hosts |
Host age and season when damage appears |
|
Nursery type and location |
|
|
|
|---|---|---|---|---|---|---|
|
Bareroot |
Container |
|||||
|
Age |
Season |
Coastal |
Interior |
Coastal |
Interior |
|
|
Several species |
1+0 |
Late summer to early fall |
Yes |
Yes |
Yes |
Yes |
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Figure 99. Trisetacus mite damage on bareroot pine.
Figure 100. Mountain hemlock damaged by the spruce spider mine.
Figure 101. Webbing of the spruce spider mite on affected foliage (courtesy of G.B. Neill).
Figure 102. Trisetacus mite damage on lodgepole pine. Note stunted, chlorotic needles.
Figure 103. Life history of spruce spider mite (several generation per year).