| Spider mites are very common pests of outdoor
plants. The mites suck juices from the plants,
causing the plants to look dull and unhealthy. Mites
also cause plants to lose vigor so that they may be
unable to overcome a severe infestation, resulting in
the plant's death. In Missouri, the most common
outdoor spider mite is the two-spotted spider mite.
The spruce spider mite infects conifers such as
spruce, juniper, and arborvitae. They are less than
1/60th inch in size and difficult to see without a
magnifying lens. They are not insects but rather a
member of the spider family in which all members
have eight legs.
Symptoms and Diagnosis
Spider mites are among the most ubiquitous of
pests, attacking a wide variety of field, garden,
greenhouse, nursery, and ornamental plants, as well
as several weed species. Infestations of two-spotted
spider mites result in the bleaching and stippling of
leaves. Severe infestations may cause entire leaves to
become bronzed, curled, and completely enveloped
in sheets of webbing. Damage to the foliage may
result in leaf drop and reduction in the overall
vitality of the plant. When a leaf or branch is tapped
over a white sheet of paper, the mites appear as
small specks that resemble dust or pepper and may
be seen to move.
Life Cycle
Adult female spider mites overwinter under loose bark,
in cracks in the soil, in leaf litter, and in other protected
places. In the spring, adults emerge and begin laying
eggs. Each female typically deposits 70 eggs and lives
for only 30 days. Eggs hatch in 5–7 days. As many as
10 generations may occur each year. The young look
similar to the adults, but newly hatched young have
only six legs and do not possess the characteristic dark
spots on the back. Reproduction of the two-spotted
spider mite is favored by hot, dry conditions, so serious
damage is likely to occur in mid-July to September. The
spruce spider mite is most active durng cooler weather
in Spring and Fall.
Integrated Pest Management Strategies
1. Knock mites off plants with water. Spraying with
a strong stream of water (particularly the
undersides of leaves) will provide some control.
Spray plants frequently to control future buildups.
For severe infestations, affected plants or plant
parts can be removed and destroyed. There are
several natural predators that feed on spider
mites. The use of chemical insecticides to control
other garden pests can result in the death of these
beneficial insects and a subsequent increase in
the population of spider mites.
2. Use insecticidal soap. Insecticidal soaps specially
formulated to kill insects and not damage plants
are effective if used frequently until the problem
is under control.
3. Use superior horticultural oil sprays. Highly
refined oils sold as superior or horticultural oils
are also very effective in controlling spider mites.
The oil suffocates the mites. Unlike dormant oils,
these oils are highly refined and under proper
conditions, can be applied to plant foliage
without damage. Follow label directions to avoid
damage to some plants that may be sensitive.
Superior oils are considered nontoxic and are less
likely to kill beneficial insects.
4. Use chemical insecticides or miticides. A very
safe product made from the seeds of a tropical
tree is called Neem. It is commercially available
under the name Margosan-O. Other chemical
controls include malathion, bifenthrin, cyfluthrin, and kelthane. Be sure
to follow all label directions when using
pesticides. Many pesticides are very harmful to
bees and fish when used improperly. |
