Spruce spider mites (Oligonychus ununguis) damage conifer needles by piercing them with their slender mouthparts and extracting sap. This feeding activity creates tiny punctures that result in a stippled or bleached appearance. In heavier infestations, needles turn a dull yellow or brown, often drying out and dropping prematurely. Damage typically begins on the lower inner branches and gradually spreads upward and outward as the infestation worsens.

Fine silken webbing, produced by the mites, is commonly found on the twigs and needles—most noticeably on the undersides of branches—where it collects dust, dead needles, and mite debris. In spruce plantings, infestations can persist for several years, leading to reduced tree vigor and, in severe cases, tree death. Young or recently planted trees, nursery seedlings, and those under environmental stress are especially vulnerable to serious damage.

The spruce spider mite (Oligonychus ununguis) can complete up to eight generations per year, depending on environmental conditions. Adult mites are extremely small—about 0.5 millimetres long—and range in colour from dark green to dark brown. Feeding typically begins in mid-May and continues throughout the growing season.

The mite overwinters in the egg stage, with most eggs deposited on twigs. In regions like the Prairie Provinces, these overwintering eggs hatch in May. The young mites, which closely resemble the adults to the untrained eye, mature into the first generation of adults by early June. By mid-June, females—each about 0.5 mm in length—begin laying 40 to 50 eggs on the foliage. These hatch into the second generation of mites by late June. An additional three to six generations may develop through the summer and into early fall.

From September until the arrival of a hard frost, females lay overwintering eggs that will remain dormant on the plant until the following spring.

Preventing spruce spider mite (Oligonychus ununguis) infestations involves a combination of good cultural practices and mechanical control methods. The first step is to maintain overall tree health, as vigorous trees are less susceptible to serious mite damage. This includes providing adequate water during dry periods, applying mulch to conserve soil moisture, and avoiding excessive nitrogen fertilization, which can promote lush, mite-attracting growth.

Regularly spraying trees with water can also help suppress mite populations. A strong stream of water directed at the foliage, particularly the undersides of branches, helps wash off mites and their protective silk webbing. This also increases humidity around the needles—conditions that discourage mite activity and reproduction.

Pruning and removing infested branches early in the season is another effective way to reduce mite numbers and slow the spread of an infestation. Dispose of pruned material away from the site to prevent reintroduction. Additionally, reducing dust in the growing environment—by watering nearby paths or driveways and occasionally rinsing foliage—can make conditions less hospitable for mites, which prefer dry, dusty surfaces.

Regular monitoring is essential. One simple method is to shake a branch over a white piece of paper and look for tiny moving specks or reddish smears when rubbed—an indication of mite presence. Early detection allows for timely action before populations increase dramatically.

For biological control of spruce spider mite (Oligonychus ununguis), Spical Ulti-Mite is the only beneficial product currently available through our program. This slow-release sachet contains the predatory mite Neoseiulus californicus, which is effective against spider mites under the right conditions. While N. californicus prefers warm, humid environments and is generally more active in higher temperatures, it can help suppress spruce spider mite populations when applied preventively or at low infestation levels, particularly in sheltered or ornamental settings.

However, biological control may not always be sufficient for large or advanced infestations, especially on mature landscape trees where complete coverage is difficult. In such cases, it is recommended to consult a certified arborist or licensed applicator to assess the situation and determine whether a miticide or chemical spray is necessary. They can advise on proper timing, application methods, and product selection to ensure safe and effective control while minimizing impact on beneficial organisms and the surrounding environment.

Combining cultural practices, regular monitoring, and targeted use of Spical Ulti-Mite or chemical control when needed offers the best strategy for managing spruce spider mites long-term.