Master Gardener: Treat plants for scale
insects before they can cause damage
Brian Jervis: Ask a
Master Gardener
Saturday, May 30, 2015
Q: What can I do to
control those white scale insects on my euonymus shrubs? Eva, Tulsa
A: Scales are unique insects, and there are
many varieties. Euonymus scales, like most scale varieties, are pests to a
particular plant species. These insects tend to occur repeatedly and can be
devastating to certain euonymus plants.
Scales are difficult to
treat. Control is best achieved using a combination of scale-resistant plants,
preserving natural predators, and insecticides. There is no one best
insecticide for control. The OSU fact sheet
“Ornamental and Lawn Pest Control,” available online, lists
OSU’s recommendations.
Euonymus plants most
susceptible to scale are Japanese and Wintercreeper euonymus and their
cultivars; euonymus alata (burning bush) is totally resistant to infestation.
The insect can occasionally crossover and involve plant species such as
camellias, hollies and ivies.
Scale insects are
broadly divided into soft and hard varieties. The soft ones, which includes
mealybugs often seen on houseplants, are more easily treated with pesticides
than those with hard shells, such as the euonymus scale.
Control strategies
depend on understanding the life-cycle of scales. Female scales and eggs
overwinter under a brown shell that is fixed in place. In spring, eggs hatch
into small nymphs, called crawlers, which move about the plant and eventually
evolve into males or females. The white males are gnat-like and mobile; the
females, on the other hand, turn brown and fix themselves permanently to the
plant where they restart the cycle. Euonymus scales will have three to four
batches of young in a growing season, a batch every six weeks or so.
Scales have several
predators that are helpful in their control, and efforts should be made to
preserve them. However, the predators are often overwhelmed and an insecticide
may be indicated. Insecticides used are either organics or man-made chemicals.
They work either by contact or systemically (absorbed into the plants’ sap).
There are any number of approaches to take with these products.
OSU’s recommended
insecticides are listed in the fact sheet mentioned above.
An environmentally
friendly treatment is a “superior” grade of horticultural oil, such as the
Sunspray Ultrafine brand, applied before bud break in spring. Because of
multiple generations of scale, it should be used repeatedly three or four times
at four- to six-week intervals during summer. Read the label carefully as some
plants are intolerant of various oil preparations.
During the summer
treatments, a standard insecticide labeled for scale may be mixed with the oil.
Alternatively, a systemic insecticide containing dinotefuran, as found in some
of the Green Light and Ortho products, can be applied as either a drench or
granules to the root zone once a year, in addition to the oil.
Contact the OSU Master
Gardeners at 918-746-3701 for further assistance in controlling euonymus scale.
Garden tips
Insect
alert: Now is the time to be on the lookout for bagworms on juniper and
arborvitae and lace bugs on sycamore, pyracantha and azaleas. Contact Tulsa
Master
Gardeners
for control suggestions.
Do
not work soil if it is too wet. Tilling it while wet will cause damage to the
structure that will take a long time to recover.
Thatch
is a layer of dead and living stems, shoots and roots that pile up on top of
the soil at the base of lawn grasses. If it is more than a half inch thick, it
should be removed with either a core-aerator or power-rake. Now is the time to
de-thatch Bermuda and zoysia grass. De-thatch fescue, if needed, in the fall.
