Twospotted spider mite
Tetranychus urticae Say
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.
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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.
Sampling for mites in Virginia using the
leaf-brushing/mite-counting technique is explained under the following:
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.
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
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,
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
Choose insecticides, miticides, and fungicides carefully to prevent
killing the predators.
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
affected (i.e., delayed flowering, large plant size, increased
vegetative runner production).