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Horticultural
oils applied during the dormant season have shown to be effective
against overwintering scale insects, mites, mite eggs, and aphid
eggs. They are applied when the target insect is most vulnerable,
but when the host plant is least exposed.
Horticultural
oils kill insects, insect eggs, and mites and mite eggs by contact
and only if it is present at the time of application. A pest migrating
to a recently sprayed plant will not be affected. Oils kill immatures
and adults mainly by suffocation as a result of blocking the spiracles
(openings through which insect receive oxygen) resulting in a reduction
of oxygen and preventing the exchange of gases for metabolic activity.
Death usually occurs within 24 hours. Oils also kill insect eggs
by suffocation as well and can penetrate the egg membrane interfering
with embryonic development. Oils also may kill beneficial insects,
but only if the insect is covered with oil.
When
using dormant oils, certain guidelines must be followed in order
to insure effective pest control and to prevent plant injury.
- When
selecting a spray oil, the unsulfonated residue (UR) rating (level
of purity) should be 92% or greater and the distillation (boiling)
midpoint should be 412-440°F. (see Table 1)
-
Oils should not be applied if temperatures are below 40°F because
of freezing and the potential for excessive oil deposition.
-
Apply the oil according label rates. Do not overapply.
-
Do not spray when buds are fully opened and shoot elongation is
occurring.
-
Take into consideration plant genetic make-up and variability.
- Finally,
and probably most important, be aware and alert to plants that
are sensitive to oil sprays. Be sure to read the label for specific
plant species and varieties. If in doubt, do not spray! For
example, do not spray Colorado blue spruce with oil as it will
remove the glaucus (blue) bloom! If you experiment on several
plants, be sure to make notes on weather conditions, plant variety
and/or species, and application rate. A partial listing of oil
sensitive plants and plants with a tendency for sensitivity is
shown in Table 2.
Advantages
of Horticulture Oils
- Oils are very safe environmentally and are degradable by evaporation.
-
Oils quickly dissipate by evaporation shortly after application
with little pesticide residue left behind.
-
Oils are compatible with an IPM program and are less harmful to
nontarget organisms.
-
Oils are virtually nonpoisonous to the applicator.
-
Oils are noncorrosive to spray equipment.
-
Oils can provide a wide range of pest control without plant injury.
Disadvantages of Horticulture Oils 1
- Oils, if improperly applied during the dormant season, can cause
terminal and/or branch dieback which may be apparent in the spring.
| Table
1. Uses and Dosages for Horticulture Oils |
| Distillation
Temperature |
Primary
Use |
Dosage
per 100 Gallons |
| 412°F |
Summer |
2-3
gallons |
| 435°F |
Summer
Dormant
|
2
gallons for summer
4 gallons for dormant |
| 438°F |
Dormant
|
2-3
gallons |
| 1Taken
from Johnson, W.T. 1985. Horticulture Oil. J. of Environmental
Horticulture 3:188-191. |
| Table
2. Plants Sensitive to Horticultural Oils1 |
|
Oil
Sensitive Plants
|
Tendency
Toward Sensitivity
|
| Maples |
Dormant |
Beech |
Dormant |
| Hickories |
Dormant |
Japanese
Holly |
Dormant/Summer |
| Black
Walnut |
Anytime |
Redbud |
Dormant |
| Cryptomeria |
Anytime |
Savin
Junipers |
Summer |
| Smoketree |
Summer |
Photinia
sp. |
Summer |
| Azaleas
(Limited) |
Summer |
Spruce |
Dormant |
| |
|
Douglas-Fir |
Dormant |
| 1Taken
from Johnson, W.T. 1985. Horticulture Oil. J. of Environmental
Horticulture 3:188-191. |
THE
PLANT HEALTH CARE PROGRAM
THE MORTON ARBORETUM
MARCH 22, 1999
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