Treating the rapidly
spreading crapemyrtle bark scale
Bill Sevier Master Gardener
Saturday, February 11, 2017
Q: I was told that a dormant oil should be
used to treat scale insects, which have covered my crape myrtles. When should I
do this and what should I use? Mike, Tulsa.
A:
Dormant oil treatment
for crape myrtle bark scale may be environmentally the safest, effective
treatment available. It should be used in dormant concentrations and should be
used now, before the trees come out of dormancy.
The benefit of oil for plant pest control has
been known going back to the time of the Romans. To be effective, these oils
must be applied directly to the insect or the insects’ eggs. The oil kills
insects by blocking the insects ‘breathing passages.
In the past, oils came in two varieties, a thick
heavy oil which was used as a “dormant oil” (used in late winter and early
spring before bud swell and bud break). These oils, if used during the growing
season, were toxic to plants. The other, thinner oil, called a “summer oil” was
used safely in the growing season.
Things have changed now with what is recommended
and available to treat many different pests. Most of the newer horticultural
oils are petroleum-based mineral oil. They are labeled as superior, supreme or ultra-fine
oils. These oils have been refined to remove undesirable compounds toxic to
plants. Therefore, they may be used in winter as dormant oil and in summer as
summer oils by simply changing their concentrations. The winter dormant
preparations are much more concentrated and more viscous than the summer
preparations but are the same oil. Most of these oils contain an “emulsifier”
to help the oil mix with water.
They should only be used during outdoor
temperatures of 40-90 degrees. If used in colder weather, the preparation may
not mix and cover the insects well. If oils are used during high temperatures
or high humidity or on drought-stressed plants, the chance of plant damage is
increased.
Examples of some of the commercial brands of
petroleum oils available at garden centers are Sunspray Ultra-fine, Volck Oil
Spray and Bonide “All Season” oils; there are many others and are available
locally.
Other than the petroleum-based preparations,
there are those of vegetable origin. They are made from neem seeds, cottonseed,
canola, cloves and rosemary. These are used mainly as summer oils. Neem oil is
unique in that it also has effective anti-fungal activity. It is a good choice
for a summer oil. However, for a dormant usage, the petroleum-based mineral
oils are probably best.
These products are effective for many different
types of insects. During the growing season they are used to treat scales,
aphids, mites, whiteflies and others. They are used extensively by commercial
fruit and nut tree farmers.
For homeowners, crape myrtle bark scale and
euonymus scale are significant problems to deal with. A dormant oil applied now
is more effective and safer than synthetic insecticides. As with any pesticide,
one must read and follow the instructions on the label. There are some plants
which are intolerant.
Garden tips
· Most bare-rooted trees and shrubs should be
planted in February or March. The roots of these plants are easily damaged and
should never be left exposed to air. Plant them at the same depth as in the
nursery and make sure good root and soil contact is made by gentle tamping and
irrigation after planting.
· Finish pruning shade trees, summer flowering
shrubs and hedges. Spring blooming shrubs such as forsythia and azaleas may be
pruned immediately after flowering. Do not top trees or prune just for the sake
of pruning.
· With our warm temperatures, applying
pre-emergent herbicides earlier rather than later may be desirable to prevent
crabgrass and other summer weeds.
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