Spider mites are small creatures which feed on trees and plants. Spider mites look green when they are young, later becoming a grayish color, according to the University of Maryland Extension. If you notice yellowing or loss of foliage on your yew bush, it may be caused by these animals. Untreated spider mite populations can quickly increase and threaten plants. Spruce spider mites and rust mites often attack yew bushes and can cause significant damage in just one season.
Spider mites are divided into two categories--cool season mites and warm season mites--based on their period of greatest activity. Spider mites hatch from eggs and reach adulthood in as little as seven days. Spider mites readily reproduce in dry conditions when natural enemies are not as plentiful, according to the Colorado State University Extension. Yew bushes planted in hot areas are especially susceptible to spider mites, which prefer such conditions.
Spruce Spider Mites
Spruce spider mites attack yew bushes, causing small yellow spots on the needles. This often occurs at the base or one side of the yew bush, gradually moving upward. Needles turn yellow, then brown and may eventually fall off. This cool-season mite is active in the spring and autumn, according to the Purdue University Extension. Eggs are laid and stay on the yew bush through the winter, hatching in the spring.
Rust mites are not actually a spider mites, but rather a type of eriophyid mite. Some people may mistake these mites for spider mites due to the similar damage done to plants. They are common on yew bushes, according to the University of Maryland Extension. The damage caused to yew bushes by these mites includes yellowing and dropping needles. Rust mites are extremely small, so their presence is usually determined by the damage done to the yew bush rather than by visual detection of the mites themselves. Rust mites can be differentiated from spider mites by the fact that they are yellow in color and have only four legs. According to the University of Maryland Extension, horticultural oil spray should be used to manage rust-mite infestations.
If your yew bush is showing spotting or yellowing--especially in hot, dry weather--check it for mite activity. Search for round, reddish-brown eggs on the foliage and bark of the yew bush. Webs cover entire sections of branches in cases of significant infestation. Quickly check for spider mites by shaking a branch over a piece of white paper and then looking for small, slow-moving spots.
Infestations found at the beginning of the hot season will most likely need control procedures implemented. According to the Purdue University Extension, it is best control the mites through natural methods, such as hosing down the yew bush to spare natural spider-mite enemies, including lady bugs and predatory mites. The University of Maryland suggests horticultural oil or insecticidal soap during the growing season, along with a dormant oil in the winter to control the egg population.