Twospotted spider mites, Tetranychus urticae, are tiny (0.25 to 0.5 mm long) eight-legged arthropods with two large spots on both sides of the body. Mites are typically found on the undersides of leaves but may colonize entire plants during outbreaks. Silk webbing on the undersides of leaves, and bronzing, stippling and burning of leaves are characteristic signs of spider mites.

The female mites overwinter in the soil and in plant debris in and around mint fields. Females become active in the spring as temperatures warm, and they begin depositing eggs on the undersides of leaves. Eggs hatch in 4 to 5 days and the entire life cycle from egg to adult may require 1 to 3 weeks, depending on the temperature. Infestations of mites may occur as early as March during warm weather. During the summer months, mite populations can increase very quickly, particularly in hot, dry weather, along dusty roads, during periods of water stress and even in response to insecticide applications. Therefore, fields should be inspected at least weekly to detect the buildup of damaging populations.

The practice of spring flaming mint fields west of the Cascade Mountains to control rust contributes to the suppression of spider mite populations (Hollingsworth and Berry, 1982b). Additionally, predator mites occur in mint fields and can be a significant factor in limiting spider mite populations and problems. Learn to distinguish between spider mites and predator mites.


Two spotted spider mite has become progressively more important as a pest of mint throughout the Northwest. Probable reasons may include: 1) development of resistance in populations of mites in mint fields which have been repeatedly treated with miticides such as Kelthane (dicofol), 2) reduced use of flaming for spring rust control, 3) changes in production practices or pesticide use patterns, causing increased mortality of natural enemies, or 4) improved awareness of their pest status and the use of proper sampling methods. Either fall plowing or spring flaming of mint contributes greatly to the cultural control of mites in mint fields by reducing early season populations that are initiated from overwintering female mites (Hollingsworth and Berry, 1982b).

Correct sampling procedures for spider mites involve close observation of leaf samples taken from different areas in a field (Hollingsworth, 1981; Hollingsworth and Berry, 1982b; Hollingsworth et al., 1984). Include leaf samples from plants in areas that are high, excessively dry, and have had past problems with mites, such as dusty areas or field margins. The use of a 10X or 15X hand lens will enable growers to distinguish between spider mites and predator mites (see below). By walking in a "Z" or "M" pattern in the field, randomly collect stems and inspect the leaves from the bottom, middle and top of the stems for mites (count adults, nymphs and eggs). Classify the leaves as "infested" or "not infested" if the mites (adults and nymphs) number five or more. For each 30 acres, 14 individual field sites should be monitored for mites by examining a total of 45 leaves (15 leaves each from the bottom, middle, and top) from 15 randomly selected mint stems per site. It also is important to count the number of predator mites on each leaf. These natural predator mites help reduce spider mite populations below economic levels through harvest. If predator mites are present, the field may not require immediate treatment and should be rechecked at a later date. Treatment of spider mites is justified if there are no predator mites and if 18 or more of the leaves in the 45-leaf sample taken at each site are infested with five or more spider mites. It is also important to estimate the number of spider mite eggs on the leaves, because their number will help predict an emerging infestation. Hollingsworth et al. (1984), A Sampling Plan For Twospotted Spider Mites in Mint, PNW 251, explains an effective system for monitoring infestations of spider mites in mint. To convert the number of leaves with 5 or more mites to an estimate of the mean mites per leaf, use the spider mite sampling calculator.

Injury to mint plants begins to occur when populations of spider mites reach five per leaf. Feeding injury caused by densities greater than five mites per leaf increases water stress (DeAngelis, 1981; DeAngelis et al.1982; 1983a), reduces photosynthesis (DeAngelis et al., 1983b), and alters terpene metabolism, resulting in elevated levels of menthol and neomenthol and decreased levels of pulegone (DeAngelis et al., 1983c). Peppermint leaf phenolics reduce spider mite fecundity and increase developmental time (Larson, 1983; Larson and Berry, 1984).

Spider mite populations can increase rapidly during hot, dry weather and even after an insecticide application. Fields should be inspected twice weekly during these times. If treatment with a miticide is justified, refer to the insecticide table for registered acaricides and rates for use on mint to control spider mites). Be sure to refer to additional information concerning biological control of spider mites., and to recent OSU Entomology Dept. Research Progress Reports (Berry et al. 1992, 1994). Even though Kelthane is registered on mint it is highly toxic to predator mites, and we do not recommend its use. Unlike Comite, Kelthane should not be used when predator mites are contributing to natural control. Also, the use of Kelthane in the spring is not recommended because it kills predators.

Spring flaming for rust control west of the Cascade Mountains in Oregon and Washington reduces the incidence of spider mites early in the growing season. This may result in fewer applications of miticides during the growing season (Hollingsworth and Berry, 1982a, 1982b). Fewer miticide applications will help delay the development of mite resistance, protect beneficial predators, and reduce production costs.