Spider Mites on Conifers
Oligonychus ununguis (Jacobi)


Hanson, T., and E. B. Walker. [n.d.] Field guide to common insect pests of urban trees in the Northeast. Waterbury, VT: Department of Forests, Parks and Recreation. http://www.state.vt.us/anr/fpr/forestry/pubs/pest.html


Acari: Tetranychidae

Hosts: Spruce, fir, juniper, pine, hemlock and others

Evidence: Foliage may appear mottled, stippled, flecked or off-color (a). Conspicuous basal needle discoloration is often the first sign of a problem. Infested trees may appear brownish-gray, and needle loss may occur. Look for mites, starting in May and continuing on a periodic basis, by sharply beating branches over white paper and examining the paper for reddish-brown mites (b), about the size of pepper grains. A hand lens will help you identify them. You may also be able to see eggs with a hand lens, which appear as tiny, shiny red or brown balls laid singly on the twigs and needles. In heavy infestations, webbing may also be conspicuous.

Life Cycle: Winter is spent in the egg stage in needle axils, under webbing on stems or branches, or under bud scales. Hatching occurs in the spring. Under optimum conditions, mites can complete their life cycle in ten days. Each adult lays 40-50 eggs, and there can be several generations per year.

a. Mottled discoloration caused by heavy Spider mite activity

b. Close-up of a Spider mite on spruce foliage.

Management: Control is generally recommended if you find an average of 5-10 mites per branch. If you find many eggs, a superior oil spray in early spring when the buds are still dormant (hard and resinous) will provide control. Otherwise, spray with a registered insecticide in the spring and/or summer as soon as you find active mites in sufficient numbers to cause concern. Frequently, a second application 7-10 days later will be necessary unless the product is ovocidal. Early treatment, before populations build up, is most effective. Some chemical sprays are injurious to predatory mites.

Photo Credits:

Figure a: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT.

Figure b: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT.

References:

Drooz, A.T. 1985. Insects of Eastern Forests. USDA Forest Service Miscellaneous Publication 1426. p 30;

Johnson, W.T. and Lyon, H.H. 1991. Insects That Feed on Trees and Shrubs. 2nd edition. Cornell University Press. p 118-119;

Martineau, R. 1984. Insects Harmful to Forest Trees. Agriculture Canada Government Publishing Centre, Supply and Services, Ottawa. p 38 and 69;

Rose, A.H. and Lindquist, O.H. Revised by Syme, P. 1994. Insects of Eastern Spruces, Fir and Hemlock. Canadian Forest Service Publication. p 94-95.

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