Spruce spider mite

Systematic position: The spruce spider mite (Oligonychus ununguis) belongs to the order Trombidiformes and the family Tetranychidae. It is a major pest that exclusively targets coniferous trees, often causing significant damage to both ornamental landscaping and forestry species.

Host plants: This pest primarily infests spruce, juniper, arborvitae, fir, pine, and larch trees. It is particularly problematic in nurseries and gardens where the microclimate may lack the humidity required to naturally suppress mite populations during dry seasons.

Biology and life cycle: The mites overwinter as eggs deposited at the base of the needles. Larvae emerge in the spring as temperatures rise. Under hot, dry conditions, the population can undergo multiple generations, typically peaking in late spring and again in late summer when host stress is highest.

Symptoms and damage: Damage occurs when mites pierce the needle cells to feed. This results in stippling—tiny yellow or brown spots on the needles. Eventually, the foliage turns bronze or brown and drops off. Infested branches are often covered in fine webbing, which helps trap debris and protects the mites.

Control measures: Integrated pest management includes maintaining plant vigor through proper watering and avoiding drought stress. If infestation levels exceed thresholds, chemical control using miticides is necessary. It is crucial to rotate chemical classes to prevent resistance and ensure coverage on the undersides of needles.