Saturday, February 11, 2017 By: Ask A Master Gardener

Dormant Horticultural Oil for Crapemyrtle Scale

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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