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Cottony Scale
Insects
- Scale insects are related to aphids and
do similar damage
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Summary: Scale insects tap into
plants and feed on plant sap, weakening and even
killing plants over time. Cottony scale
insects produce a cottony egg mass from which the
mobile crawler stage hatches. The young crawler
stage is also the easiest stage to control.
Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)
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Scale insects
are closely related to aphids but most don't look
like insects at all, appearing legless and attached
to the plant's leaves or stems (see photo right).
Scale insects feed by tapping into the plant stem or
leaf and withdrawing plant sap. Like aphids, they
are often associated with sticky honeydew which
supports the growth of black, sooty mold.
Some scale insects
produce a cottony sac (see photo right) that
contains hundreds or thousands of eggs. Scale
insects that make these cottony egg sacs are called cottony
scales. Eggs generally hatch in early summer
and release the crawler stage, the only
highly mobile stage in the insect's life cycle. The
active crawler stage is also the one that must be
targeted for effective control.
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Cottony
scale insects with egg sacs, the
white cottony masses.
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There are many scale
insect pests of ornamental plants and they have very
complex life cycles and host plant interactions. One
of the best comprehensive sources of information about
these pests on ornamental plants is Insects That
Feed on Trees and Shrubs by Johnson and Lyon
(see Amazon below right).
Plant damage caused by
scale insects
Plant damage is
related to the sap that scale insects take from the
plant. Over time as more sap is extracted the plant
weakens, leaves may drop and eventually whole branches
may die.
Plants that are stressed
by drought, root damage or disease are better
hosts for many pests including scale insects. These same stressed plants may
be killed by high scale insect populations. In
addition, sooty mold growing on the leaf surface can
interfere with normal processes and can further weaken
the plant.
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Scale insect control
The most important
part of scale insect control is timing.
Control measures must be timed to coincide with
hatching of the crawler stage which usually occurs
in early summer for cottony scales. If
properly timed, and good spray coverage is
achieved, soft insecticides like insecticidal
soaps and oils are just as effective as
conventional insecticides (see Using
Insecticidal Soap For Garden Pest Control).
Over the long run soaps and oils may be more
effective since they preserve the natural enemy
complex which may account for most of the long
term control of these plant pests.
Scout plants starting
in late spring. Use a hand lens to look for scale
crawlers on the underside of leaves or near
cottony egg sacs. Scale crawlers will be about the
size of spider mites but amber in color. Once
crawlers are found control treatments can begin.
It may take several seasons to completely control
a severe scale infestation. Scale infestations
often take years to develop and it is unlikely
that you'll eliminate them overnight.
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Mission: To
provide accurate, up-to-date and unbiased information
for solving common insect and mite problems around
your home, business and landscape using least-toxic
methods.
Jack DeAngelis,
Ph.D.
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