Several species of spider mites (Family Tetranychidae) are occasional pests of turfgrasses in Nebraska. Among the more important are the twospotted spider mite, Tetranychus urticae, Banks grass mite, Oligonychus pratensis, clover mite, Bryobia praetiosa, and winter grain mite, Penthaleus major. Spider mites are not insects, but are more closely related to spiders and ticks. While some spider mite species such as the Banks grass mite feed almost exclusively on grasses, others including the twospotted spider mite also feed on broad-leaved plants and conifers.
Twospotted Spider Mite and Banks Grass Mite
Twospotted spider and Banks grass mites overwinter as adult females and nymphs. Activity resumes as temperatures begin to warm in the spring. Eggs are deposited on the stems and foliage of the turfgrass. Upon hatching, mites pass through several developmental stages. First-stage larvae have six legs, whereas successive nymphal stages and adults have eight legs. These spider mites feed in colonies within a network of fine webbing on undersides of leaves.
Early season spider mite infestations are typically found on grass blades, but spread to other parts of the plant as mites increase in abundance. All life stages of the mites may be present at any given time, and there can be 7 to 10 generations during the growing season. Twospotted spider and Banks grass mites reproduce very rapidly under hot, dry conditions and can reach extremely high population levels especially late in the summer.
Clover mite adults are similar in appearance to twospotted spider or Banks grass mites, but are larger, more reddish-brown in color and have front legs which are nearly twice as long as the hind legs. These mites spend the summer as reddish-colored eggs on the stems, bark and twigs of herbaceous and woody plants, and in the cracks and crevices of buildings.
With the arrival of cooler late summer and fall temperatures, eggs hatch and immature mites move to nearby host plants where several generations are produced before the onset of winter. As colder temperatures approach, adult females migrate to sheltered areas where they continue to lay eggs until extreme cold forces them into dormancy for the winter. In the early spring, newly-hatched immatures and overwintered adults return to plant hosts to feed and reproduce. Turfgrass areas adjacent to overwintering sties can be severely damaged or destroyed.
Winter Grain Mite
Winter grain mites oversummer as reddish-orange eggs attached to the stems and roots of turfgrass. Eggs hatch in October and young mites begin feeding on the grass foliage. The first generation is completed in mid to late November when adult females begin producing eggs of the next generation. These mites become inactive during episodes of extremely cold winter weather. Mites of the second generation are most active from late February through early April and cause a significant portion of the winter injury observed in the turf. Upon maturity, second generation adult females produce eggs that oversummer. By mid to late April the mite population is in rapid decline.
General Symptoms of Spider Mite Damage to Turfgrass
Spider mites injure turfgrasses by sucking plant juices from the outer cell layers of leaves and stems. Mite damage is characterized by general lack of plant vigor and a silvery or pale yellow discoloration on leaves. As damage progresses, grass blades gradually turn yellow, dry out and die.
Twospotted spider and Banks grass mites feed in colonies within fine webbing on lower surfaces of the grass blades. Infestations are more common in areas associated with drought stress such as near buildings, especially on southern exposures, and along sidewalks, driveways and parking lots.
Clover mites have a very broad host range including turfgrasses, clover, grassy and broadleaf weeds, and some ornamental shrubs. While clover mites occasionally cause significant injury to turfgrasses, they are more often a nuisance when they invade homes and other structures in the fall and spring.
Winter grain mites are most abundant during the winter and early spring months and can be observed on sunny days in the thatch and crowns of the grass plants. In many cases, winter grain mite damage, when observed in the spring, is mistaken for weather effects. Winter grain mites seem to prefer Kentucky bluegrass and bentgrass, but will also readily feed on certain fescue cultivars.
Begin sampling for twospotted spider or Banks grass mites in early summer and continue on a regular basis through early fall. Inspect turf stands, nearby vegetation and the sides of buildings for the presence of clover mites in early fall and again in the early spring. Winter grain mite surveys should be conducted from late November through mid March. Spider mites can be detected by carefully inspecting thatch and grass blades with a 10X hand lens or magnifier for foraging clover and winter grain mites, or for colonies of twospotted and Banks grass mites colonies on the undersides of leaves.
Twospotted spider and Banks grass mites are difficult to control because they reproduce rapidly, especially in warm, dry conditions. In addition, some populations may be resistant to available pesticides. Infestations can often be reduced by irrigating the turf to disrupt mite colonies and to reduce moisture stress on the turf. If insecticides are used, it is important to treat twospotted spider and Banks grass mites while colonies are small. Additional treatments may be necessary at 10 to 14-day intervals to prevent mite resurgence. For all spider mite species, apply a liquid insecticide to the infested area (thorough coverage is important) and withhold irrigation for at least 24 hours after treatment. Insecticides/acaricides are normally applied for winter grain mite in mid-March to early April.
Clover mites inside homes and buildings can be collected with a vacuum sweeper (discard the vacuum bag and/or its contents after collection) or sprayed with a household aerosol insecticide. When crushed, clover mites often leave a reddish brown stain, so care should be taken in their removal. Outdoors, the foundation and the surrounding 15 to 20 feet of turf can be treated with an insecticide to reduce mite numbers. Preparing an 18 to 24 inch wide strip around the foundation, cleared of vegetation and planted to plant varieties that are not attractive to the mites should also help reduce invasions of these pests. Examples of flowers that can be used are zinnia, salvia, rose, chrysanthemum and petunia. Shrubs such as juniper, spruce, arborvitae and yew can also be planted, but are subject to infestation by other mite species.