Crapemyrtle Bark Scale Insect
Brian Jarvis: Ask a Master Gardener
Sunday, April 14, 2019
Q: I have white spots
that turn to a blackish mold on my crapemyrtles. What is it and how do I get
rid of it? Tricia, Tulsa
A:
We addressed this issue
in this column last year, but some of the recommendations made by OSU regarding
control have changed. The current recommendations come from Dr. Eric Rebek,
state extension specialist for horticultural insects.
It is an insect pest that is called
Crapemyrtle Bark Scale. This insect is relatively new to our area but has
become prevalent. It came to the U.S. from Asia and first appeared in
north-central Texas in 2004. It has subsequently spread northward into the
Tulsa area from nursery stock and other sources of imported crapemyrtles. This
is the reason one should always inspect nursery crapemyrtles for scale insect
before purchase.
Like other scales, the life cycle begins
with either the female scale or eggs overwintering on the crapemyrtle under
loose bark. When the eggs hatch, small mobile “crawlers” are produced, which
migrate on the plant and may be spread to other crapemyrtles by wind or birds.
There may be two to three generations produced per year depending on
temperatures.
Once the female is fully developed, she
mates and attaches to the stems and trunks of the crapemyrtle, where she
remains fixed and lays eggs for the next generation. She dies shortly
thereafter, but the eggs survive under her covering until they hatch.
As the scales feed, they release a
liquid called “honeydew.” This is like the behavior of aphids. The sugars in
honeydew may support the growth of a fungus called “sooty mold.” This
overgrowth produces large black patches on the bark of the crapemyrtle. The
mold is unsightly, which creates a reduction in aesthetic quality, but it is
not significant in terms of the plant’s health.
This pest is easy to identify because it
is the only scale insect to infest crapemyrtles. The adult female is usually
about 2mm long and has a distinctive gray-white felt-like covering. When one of
the females is crushed, a pink blood-like fluid is released.
The current recommendations for control
are removal by hand and the use of winter dormant oils. Scrub down the trunk of
the crapemyrtle with a mild solution of dish soap and water using a
long-handled brush to remove the scale and sooty mold. This method is very
effective. Another recommended treatment is to spray the trunks of the trees
with dormant oil in late winter. Winter dormant oil spray is a stronger
concentration of the petroleum-based oil that is used in summer. Neem oil,
while useful in other applications, will not be effective for this scale in
winter.
Previously, systemic insecticides in the
neonicotinoid family were recommended. These chemicals enter into the
circulation of plants and kill the pest when they feed on the sap. They have
been shown to be effective against crapemyrtle bark scale but are no longer
recommended. The reason for the change in policy is the concern that these
insecticides enter the blossoms of crapemyrtles and would be harmful to bees
and other pollinating insects.
Garden tips
• Don’t spray
insecticides during fruit tree bloom or pollination may be affected. Disease
sprays can continue according to schedule and label directions.
• Mowing of
warm-season lawns can begin now. Cutting height for bermudagrass and zoysia
grass should be 1 to 1½ inches high, and buffalograss 1½ to 3 inches high.
• Harden off
transplants outside in partial protection from sun and wind prior to planting.
• Let spring flowering
bulb foliage remain as long as possible before removing it. That will allow the
energy from the leaf to flow back into the bulb for flower production next
year.
• Prune roses just
before growth starts and begin a regular disease spray program as the foliage
appears on susceptible varieties.
