The spider mites, Tetranychus spp., especially the twospotted spider mite, frequently attack strawberry plants. They are distributed widely and can be found in almost every field. They attack a wide variety of plants, including truck crops, shade and fruit trees, and ornamental plants.
The eight-legged adult is about 1/50 inch in length. It varies in color from pale greenish-yellow to green and usually is marked with two dark spots. Feeding and egg deposition occur on the underside of the leaves, and a tangle of fine, silken webs occurs there during 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, the 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 hatch each year. The species overwinters as mature, fertile females in protected places in the field. 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, newly planted fields can become infested quickly.
Heavily infested fields 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. 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, a miticide should be used. It is important to catch population increases early because control of large, established populations is difficult and expensive. In certain fields or certain areas within fields, "hot spots" of mite activity can develop. Keeping an eye on these known hot spots will help warn of an impending outbreak. Thorough coverage is essential when applying miticides, so use plenty of water.