Twospotted spider mite
Tetranychus urticae Koch
These mites are typically found on the bottom surface of leaves.
Spider mite eggs are spherical and colorless when first deposited,
later become white as hatch approaches. Nymphs and adults are oval
shaped and generally yellow or green in color. There are usually
more dark spots on each side of their bodies.
For additional information on the twospotted spider mite: click
Biology and damage:
Feeding by the twospotted spider mite, which consists of
and sucking of cell contents, occurs on the lower surface of
Damage is expressed as stippling, and bronzing of the leaves and
veins. Feeding is particularly damaging during the first 4 to 5
following transplanting in fall. Their rapid developmental rate
(approx. 1-2 wk) and high reproductive potential (about 50-100
female) enables them to reach damaging population levels very
under good growing conditions. Mite densities of five per leaflet
during this critical period of plant growth substantially reduce
number and overall plantation yield. Plants that sustain
of greater than 75 mites per leaflet may become severely weakened
appear stunted, dry, and
red in coloration (UC-Davis). The highest mite populations are
observed following the peak spring fruit harvest, and this peak is
typically followed by a
rapid, natural decline in mite density.
Sampling for mites in Virginia using the
leaf-brushing/mite-counting technique is explained under the
Field scouting also involves direct counts of mites on leaf
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
later in the season.
Predators play an important role in keeping twospotted spider mite
populations in check. Some predators, such as the mite, Phytoseiulus
persimilis, Metaseiulus (=Typhlodromus) occidentalis,
commercially available for release. Inoculative releases
(i.e., initial releases of a small number of predators) can be
when twospotted mites are
first found in the field. Inoculative releases into hot spots
areas of infestations, i.e., windward edges, borders, stressed
also aid in suppressing infestations. Subsequent innundative
of predaceous mites may reduce twospotted mite infestations.
releases of predator mites, it is important to monitor spider
determine if they are being maintained below economically
Choose insecticides, miticides, and fungicides carefully to
killing the predators.
Strawberry cultivars vary in susceptibility to twospotted spider
Short-day cultivars are generally
more tolerant of mite feeding than day-neutral cultivars,
later in the fruit-production season. Vernalization directly
plant vigor. Supplemental cold storage can affect a plant's
vernalization. Plants with low amounts of chilling will have low
and will often develop
intolerable mite infestations. Excessive chilling will promote
increased vigor and reduce mite abundance, but other production
affected (i.e., delayed flowering, large plant size, increased
vegetative runner production).