USDA Forest Service - Region 4 - Intermountain Archive, USDA Forest Service, Bugwood.org
Spruce Spider Mite
[Oligonychus ununguis (Jacobi)] has the broadest host range of any spider mite that feeds on conifers. Hemlocks, arborvitae, Juniperus spp., and others are affected. Symptoms of spruce spider mites feeding vary, but usually cause either bleaching/stippled yellowing/whitening at needle bases (arborvitae or hemlock) and/or bronzing/needle drop/webbing (spruce or fir).
The tiny eggs are orange with ridges and have a “thread” coming off the top called a stipe. This “orange-bomb” shape distinguishes them from other spider mites. Eggs hatch 5-10 days after being laid and it takes ~2 weeks for SSM to go through several green/orange nymphal stages and finally molt into the green/yellow-orange adult stage. This entire life-cycle may take only 12-19 days in optimum conditions.
Misinformation persists around the seasonal biology of SSM. Several (5+) generations occur per year, depending upon temperature. SSM is a cool-season species. Temperatures above 85F halt their development. With the current temperatures and dry weather, SSM is poised to resume activity after its summer “sleeping” phase. It is possible to see the highest yearly populations of SSM from September to November.
Miticides are often needed when predators are not abundant.
