Spider mites, also known as two-spotted mites, become
a particular problem for the gardener through the winter. Normally, they
hibernate in ground litter or under the bark of trees or shrubs. However, if
they stowaway onto a plant being brought indoors, the artificial lights, and
warm, dry, conditions of most heated homes will allow them to keep infecting
The spider mite, Tetranychus urticae, is a tiny,
8-legged pest related to the spider & tick. Adults are normally green or
yellow but turn red when the day lengths shorten in the autumn. They attack
plants by stabbing the underside of the leaves and sucking out the sap. This
damage causes a distinctive stippling effect due to the loss of chlorophyll. As
their numbers increase, the number of white speckles on the leaf increases and
the leaf eventually dies. Once the spider mites begin reproduction, a
distinctive “webbing” forms, usually under the leaf and then at the growing tip
of the plant.
What makes this pest truly difficult to control is its
rate of reproduction. Each female will lay up to 12 eggs per day. Mating is not
required for egg production. At 21ºC, these eggs will hatch in as few as 3
days and will develop into adults in only 14 days. If left unchecked, 10 spider
mites in May will become 100,000 by July!
Spider mites have been found in greenhouses across
North America and Europe and are known to attack over 200 species of plants
including azalea, camellia, citrus, evergreens, hollies, ligustrum,
pittosporum, pyracantha, rose, and viburnum; fruit crops such as blackberries,
blueberries and strawberries; vegetables including tomatoes, squash, eggplant,
cucumber; and trees such as maple, elm, ash, black locust, and poplar.
The Predatory Mite
Unlike the spider mite, the predatory mite,
Phytoseiulus persimilis, is a welcome insect in the garden & greenhouse. It
is a fast moving insect with an orange teardrop-shaped body. The species is a
specialized predator the two-spotted spider mite and feeds on all stages of its
prey, from egg to adult.
The adult P. persimilis is a voracious eater, eating
between 5-20 prey per day. It uses its sense of smell to find plants infected
by spider mites. As soon as it comes into contact with spider mite webbing, it
will intensify its search.
P. persimilis can be purchased from many biological
control companies. Often it is shipped in a glass vial or on trays of bean
leaves. The easiest method of application is to sprinkle about 20 adults on
each plant. These predatory mites prefer to work their way up a plant,
searching for food so try to introduce them as low down as you can. In
addition, if many of your plants are infected, keep them close together with
their leaves touching so that these predators will be able to easily move from
one plant to another.
Unlike the spider mite, P. persimilis prefers humid
conditions and Misting will not only help it multiply, but will keep down the
numbers of the spider mite. A relative humidity of 70% is ideal for P.
persimilis. Once its food supply is exhausted, the numbers of P. persimilis
Other cultural controls
As previously mentioned, misting plants at least twice
a day will keep spider mite numbers down. Mite populations can also be reduced
by spraying the underside of the leaves with a jet of water to break up the
webs and wash the mites off.
Soap sprays are also very effective at controlling
spider mites. The Active ingredient, potassium or sodium salts of fatty acids,
is not toxic and can safely be used indoors. A homemade spray can also be
prepared using ordinary dishwashing detergent. Mix 5 tablespoons of detergent
in 1 gallon of water and spray the plants, especially the underside of the
Terra Viva Organics www.tvorganics.com – a source of
University of Florida Dept of Entomology
http://www.ifas.ufl.edu/~insect/orn/twospotted_mite.htm- life cycle of the
Cornell University Biological Control
A guide to P. persimilis, the predatory mite
Arzeena Hamir is an agronomist and President of Terra
Email: Arzeena Hamir