Twospotted spider mite
Tetranychus urticae Say
(Acarina: Tetranychidae)

Description:

These mites are typically found on the bottom surface of leaves. Spider mite eggs are spherical and colorless when first deposited, but later become white as hatch approaches. Nymphs and adults are oval shaped and generally yellow or green in color. There are usually one or more dark spots on each side of their bodies.
For additional information on the twospotted spider mite: click here

Biology and damage:

Feeding by the twospotted spider mite, which consists of piercing and sucking of cell contents, occurs on the lower surface of leaves. Damage is expressed as stippling, and bronzing of the leaves and leaf veins. Feeding is particularly damaging during the first 4 to 5 months following transplanting in fall. Their rapid developmental rate (approx. 1-2 wk) and high reproductive potential (about 50-100 eggs per female) enables them to reach damaging population levels very rapidly under good growing conditions. Mite densities of five per leaflet during this critical period of plant growth substantially reduce berry number and overall plantation yield. Plants that sustain infestations of greater than 75 mites per leaflet may become severely weakened and appear stunted, dry, and red in coloration (UC-Davis). The highest mite populations are often observed following the peak spring fruit harvest, and this peak is typically followed by a rapid, natural decline in mite density.

Field scouting/monitoring:

Sampling for mites in Virginia using the leaf-brushing/mite-counting technique is explained under the following: click here
Field scouting also involves direct counts of mites on leaf undersides. Although there is some disagreement on a reliable threshold for strawberry, an economic threshold of 5 mites per leaf is suggested following transplanting (before July 1), then approx. 20 mites per leaf later in the season.

Management:

CHEMICAL CONTROL
Virginia home spray guidelines
California guidelines

BIOLOGICAL CONTROL:
Predators play an important role in keeping twospotted spider mite populations in check. Some predators, such as the mite, Phytoseiulus persimilis, Metaseiulus (=Typhlodromus) occidentalis, and Amblyseius are commercially available for release (UC-Davis). Inoculative releases (i.e., initial releases of a small number of predators) can be made when twospotted mites are first found in the field. Inoculative releases into hot spots (clumped areas of infestations, i.e., windward edges, borders, stressed plants, etc.) may also aid in suppressing infestations. Subsequent innundative releases of predaceous mites may reduce twospotted mite infestations. Following releases of predator mites, it is important to monitor spider mites to determine if they are being maintained below economically injurious levels. Choose insecticides, miticides, and fungicides carefully to prevent killing the predators.

CULTURAL CONTROL:
Strawberry cultivars vary in susceptibility to twospotted spider mite. Short-day cultivars are generally more tolerant of mite feeding than day-neutral cultivars, particularly later in the fruit-production season. Vernalization directly promotes plant vigor. Supplemental cold storage can affect a plant's vernalization. Plants with low amounts of chilling will have low vigor and will often develop intolerable mite infestations. Excessive chilling will promote increased vigor and reduce mite abundance, but other production factors are adversely affected (i.e., delayed flowering, large plant size, increased vegetative runner production).