ORNAMENTALS
NORTHWEST
ARCHIVES
Jan.-Feb.-Mar.
1982
Vol.6, Issue 1
Pages 17-19
Joe Capizzi (OSU Extension
Entomologist), P.H. Westigard (OSU
Entomologist, Southern Oregon
Experiment Station), and Jim Witt (Ag
Chemist, OSU Extension Service).
ONE OF OUR UNDER-UTILIZED INSECTICIDES -HORTICULTURAL SPRAY OILS
Horticultural spray oils ("Superior" oils) continue to be one of the best available pesticides
to control scales, mites, plant bugs, psyllids, certain moths, mealybugs, some aphids, mite
eggs and some insect eggs. The "Superior" oils are effective on dormant, semidormant, and
actively growing plants. The "Superior" type horticultural spray oils, initially developed for use
on trees in leaf (summer oil sprays), are now also recommended for use during plant dormancy.
Horticultural spray oils ("Superior" type oils) are "contact" insecticides which interfere
physically rather than chemically with pests. They only control pests and eggs present on the
plant at the time of application. Insects and mites migrating to a treated plant will generally not
be affected by oil residues.
Confusion about proper use of horticultural spray oils is caused by variation among product
labels with respect to use recommendations and ingredient and caution statements and by the fact
that many of the caution statements are a carry-over from the labels of 30 years ago and, in
part, not applicable to today's "Superior" oil. As a result, horticultural spray oils are not
used as extensively as they possibly should be. In the last 25 years horticultural spray oils have
been greatly improved and must now meet established specifications to be described as "Superior
type" oils. Caution statements on an up-to-date label regarding phytotoxicity of such oils
should be few.
Johnson (1980) found no published literature reporting phytotoxicity to any species of deciduous
tree or shrub in the U.S. or Canada from using a "superior" oil in the dormant stage. "The same is
true for conifers, however there will be a cosmetic and perhaps an economic effect if oil is
sprayed on glaucus conifers such as Colorado blue spruce. Oil sprayed on such trees will result
in removal of the bluish frosted material from the needles. Two or three years may pass before
normal color returns. Labels for summer application of oils, likewise, have erroneous caution
statements not applicable to superior oils." There are but few deciduous trees that are sensitive to
light superior oil if the dosages are in line with prescribed rates and the plant is not under
moisture stress.
Some kinds of injury may occur with or without the involvement of oil. It is Capizzi’s Law
that phytotoxicity will occur with any application of anything to plants under unfavorable
conditions.
However, the superior oils are designed to combine full pesticidal actions with minimum plant
injury hazards.
Specifications for superior oils
1. Measurements of refinement:
UNSULFONATED RESIDUE (UR)
-The UR value is an index of the amount of the product free from unsaturated hydrocarbons.
Injury to plant leaves (phytotoxicity) is related to the percentage of unsaturated hydrocarbons in
an oil. An oil with a UR of less than 92% is considered unsafe on green plant leaves.
GRAVITY - The higher the gravity reading, the more paraffinic the oil. A number of 30 degrees
or greater indicates a highly paraffinic oil; this is important both in insecticidal effectiveness and
plant safety. A range between 30 and 40 degrees is suitable for dormant treatments.
2. Indicators of oil persistence on the plant:
Oil sprayed onto the plant should be persistent enough to kill the pests, but not so persistent as to
injure the plant.
VISCOSITY: Viscosity of spray oils ranges from 60 to 200 seconds. Heavier oils (100 seconds
or higher) lay down more persistent deposits. Dormant or semi-dormant trees will tolerate
moderate deposits that would be phytotoxic to plants in leaf. Even so, for dormant oil sprays,
oils having viscosities of 60 -70 seconds are effective and have a greater margin of safety
than do oils having viscosities of 100 or more.
An exception to the general recommendation of lower viscosities for greater safety is Volck
Supreme oil which has a viscosity of 140 and is safe.
DISTILLATION PROFILE AT 10 mm Hg: While viscosity readings may provide a good
general indication of oil heaviness, the distillation profile is a more universally reliable indicator
of oil heaviness. Distillation range is indicative of the speed of evaporation, or persistence,
of the oil spray. Distillation at 10 mm of mercury (a vacuum situation) at 408-425 F degrees will
produce a product with the correct evaporation rate. Over 80% of the product should distill
within a narrow temperature range of 80 Fahrenheit degrees or less; the temperature at which
50% or more of the product distills should be a narrow range between 409-425 degrees
Fahrenheit. (Johnson, 1980).
Distillation range is also a measure of the oil's purity. A narrow distillation range indicates
that more of the oil molecules are of a similar nature than an oil with a wide distillation
range. For some "narrow cut" spray oils, 80% of the oil may be distilled over as little as a 70
degree Fahrenheit temperature range. Providing these molecules are in the insecticidally
active range (see viscosity), then a larger proportion of the oil will give control than in an
oil with a large range in distillation.
3. POUR POINT: Pour point indicates the temperature required to have the oil in a liquid state.
Any superior oil that remains in the liquid state at 20 degrees Fahrenheit should be safe for all
routine field applications. Storage of oil at lower temperatures may break the emulsifier.
Select an oil based on pps label specifications (see Table 1 for specification standards).
Horticultural spray oils ("Superior" type oils) are sold under many different product names. Most
oil commonly bought by arborists and nurserymen comes from agricultural chemical companies
who purchase from the refinery and sell it, unchanged, under their own labels: Dormant Oil,
Dormant Spray, Spray Oil, Miscible Oil, Emulsive Mineral Oil, Volck Supreme, Acme Dormant
Oil, Scalecide, Orthol-D, Orchex 796, Bonide Dormant Spray Oil, E&B Dormant, Unico Spray
Oil, Sun Spray, Superior 70, Volck 70 Supreme, etc.
Traditional Caution Statements:
Traditionally, it has been stated that dormant oils should not be applied if the temperature is
below 40 degrees F or if the temperature will approach freezing soon after application of the
dormant oil spray. Cautionary statements have been made that some evergreens may show injury
symptoms if freezing temperatures occur within three weeks following application, and that
dormant oils applied after October 1 may increase cold damage to some trees.
In addition, it has been suggested that oil sprays should not be used on sugar maples, hemlock,
larch, cryptomeria, Japanese maples, beech, hickory, walnut, butternut, mountain ash, and
maidenhair ferns; it has also been stated that oil may injure Douglas-fir buds and lessen the blue
color of blue spruce.
Even though it is difficult to substantiate these cautionary claims, the labels should be followed.
Caution statements in an up-to-date label regarding phytotoxicity should be few ...none-the-less,
follow label directions or find a product with a modern label.
Applying superior oil sprays ... Tips to reduce the likelihood of injury and insure
effectiveness:
1. Calibrate the sprayer using water before applying oils to insure proper dosage and thorough
coverage.
2. Check agitation to make sure it is operating with every tank of spray. Do not allow spray tank
to stand without agitation.
3. Do not apply oil to stressed tree or shrubs. This is tricky because stress is not always apparent.
4. Avoid application in near freezing temperatures or when such weather is predicted. Superior
type oil spray should be applied when the spray will dry quickly, and when the temperature is
between 40 and 75 degrees F. The best spraying time is a dry, mild, sunny morning.
5. Use oil type and amounts recommended for the crops and type of application. Concentrated
sprayer applications are often at reduced rates.
6. Reduce rates on young trees or shrubs where the possibility of double spraying is likely.
7. Be wary of combinations of superior oil with other chemicals. Do not combine oil and sulfur
sprays, except for application to dormant deciduous plants. According to Johnson (1980), "The
label admonition, do not combine oil and sulfur sprays, is one that has no degree of freedom. It
means what it says. Likewise, there is danger of phytotoxicity with other fungicides such as
captan and karathane in combination with oil."
The addition of contact insecticides, usually organophosphorus but recently other classes of
insecticides, to oils to increase effectiveness should be mentioned. If wettable powders are used,
add them to a tank partially filled with water or add as a slurry to the water while agitation is
going on, then add the oil.
When a chemical is known to cause phytotoxic symptoms when used alone, we can assume that
when combined with oil the injury will be intensified.
On oil-sensitive plants, use "wettable powder" formulations rather than oils or emulsifiable
formulations of chemicals.
8. Test oil (especially oil held over from one year to the next) for emulsification before using.
Horticultural spray oils (Superior type oils) are formulated with a surfactant or emulsifying agent
so that the oil can be diluted with water. Recommendations for spraying with a 2% Superior oil
refer to a 2% concentration of the emulsified concentrated Superior oil in water. Do not use oil that
will not emulsify or if oil globules are floating on the water. A jar test is sufficient.
Table 1. Specification Standards for Superior Oil
Viscosity
Gravity (minimum)
Unsulfonated Residue MR, minimum %)
Distillation at 10 mm Hg (F degrees)
50% point
10-90% range (maximum)
Pour Point (F degrees, max)
*
30
92
412±8
80
20
* For technical reasons, a specific viscosity standard has not been made part of these
specifications. Oils otherwise defined here may fall within the 70-78-second viscosity range.
Literature cited:
Chater, Clifford S. 1980. The 1980-81 Insect and Disease Control Guide for
Massachusetts, Extension Service.
Johnson, Warren T. etal. 1980. 1980 Cornell Recommendations for Pest Control for
Commercial Production and Maintenance of Trees and Shrubs. This publication is revised biannually. Price per copy in 1980 was $1.75. Copies can be ordered from: Distribution
Center, 7 Research Park, Cornell University, Ithaca, New York 14850.
Johnson, Warren T. 1980. Spray oils as insecticides. Journal of Arboriculture 6(7):169-174.
Pesticide Use - Due to constantly changing laws and regulations, no liability for the suggested use of chemicals in
this Newsletter is assumed by the ONW Newsletter. Pesticides should be applied according to label directions on the
pesticide container.
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Criticism of products or equipment is neither intended nor implied.