Spider Mites on Strawberries

Spider mites are distributed widely and can be found in almost every planting. They attack a wide variety of plants, including vegetables, shade and fruit trees, and ornamentals.

The eight-legged adult is about 1/50 inch in length. It varies in color from pale greenish-yellow to green and is usually marked with two dark spots. Feeding and egg deposition occur on the underside of the leaves, and the mites may produce fine, silken threads for dispersal in heavy infestations.

The length of the life cycle varies with seasonal and weather conditions, but may be completed in about 2 weeks. Early in the spring, feeding begins on the undersides of newly produced leaves in small colonies. Reproduction may be continuous from early spring until late fall. The female lays two to six eggs per day, up to about 70 eggs per mite; the eggs hatch in about 4 days. Ten to fifteen generations may occur each year. The species overwinters as mature, fertile females in protected places in the planting. Hot, dry weather favors rapid population increases. Spider mites' small size and habit of feeding on the underside of leaves means that they might be overlooked until the population is so large that serious damage has occurred. Because these mites can be borne by the wind in their silken webbing, new plantings  can become infested quickly.

Heavily infested plants lose their healthy green color, and the undersurfaces of the leaves become brown until the entire leaf looks bronzed. This may be caused by as few as 20 mites per leaf. The mites suck sap from the leaves and can interfere with normal physiological processes such as the production of sugars. Plants might become stunted, and yield can be reduced greatly.

Spider mites have many natural enemies, including insects and other mites, which often keep them in check. Insecticide treatments often cause spider mite outbreaks by destroying these natural enemies. Examine the undersides of leaves weekly during the dormant and spring periods for mites or webbing. Beginning early in the season, examine the undersides of leaves of 50 randomly selected plants for mites or webbing. A hand lens might be necessary because these mites are barely visible to the unaided eye. It is important to catch population increases early because control of large, established populations is difficult and expensive. 

Plants can tolerate low populations of this pest, but if a sharp population increase is noted from one week to the next, or if plant symptoms begin to appear, control methods may be needed.  If mites are found only in certain areas of a planting known as a "hot spot", releasing predatory mites there may provide sufficient control.  If the infestation is more widespread, a miticide application may be needed.  Thorough coverage is essential when applying miticides, so use plenty of water.