Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Oystershell Scale
Close-up of Oystershell Scale on maple twig.
Oystershell
scale males and females are oyster-shaped, gray, 1/8" long, and with the exuviae accumulated at the narrow end. They commonly occur
on twigs and limbs of lilac, maple, willow, and sometimes on many other hosts.
Heavily infested parts are killed. Crawlers are active in early May about the
time hawthorn is beginning to bloom.
In
general, controls will be more effective if the scale population on a plant is
first physically reduced by pruning out heavily infested and sickly branches.
In some cases, large sized scales can be scrubbed off with a stiff brush.
Horticultural oil sprays kill primarily by smothering, so they will be less
effective against scales crowded together or occurring in layers the plant.
Insecticidal soaps provide a new alternative. They are very effective against
both active and settled crawlers. Oils and soaps are safe to use and are
especially good choices for sensitive areas, such as where people are present
soon after treatment. Because of their short residual, they help to conserve
beneficial species.
Horticultural
spray oils kill all stages of scales that are present at the time of
application, and often give good control. Most trees and shrubs can tolerate
application of light (summer oil) even during the summer months. Refer to the
product label for guidelines on plant sensitivity and any temperature
restrictions. Oil products labeled as summer, superior or Volck
oil are of the highest grade and may be used on tolerant plants during either
the growing season or the dormant season, but at different spray concentrations.
Oystershell Scale on tree limb.
An
alternative to oil sprays are contact insecticides applied during the growing
season when the crawler stages of the scales are present. The presence of
crawlers can sometimes be determined by sharply tapping an infested twig or
branch over a white paper. Crawlers are often orange, brown or purple and
appear as moving specks of dust. Because of their waxy protective covering,
other stages of scales are not readily controlled by contact insecticides.
Contact insecticide sprays will not reach crawlers that have settled under old
scales.
Although
resistance to insecticides may occur in some cases, failure of contact sprays
is more often the result of not timing the applications to coincide with
crawler activity. Even when sprays are timed for optimum effect, complete
control may not be achieved by single applications if crawler activity is
spread over an extended period of time, or if populations are heavy and
crawlers are under old scale shells. Thorough spray coverage is essential for
good control.
Information obtained through University
of Kentucky College of Agriculture
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES