Twospotted spider mite
Tetranychus urticae Koch
(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:
Virginia
home
spray
guidelines (html
version)
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 Neoseiulus
(=Amblyseius) fallacis
are
commercially available for release. 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).