Horticultural Oils

Scott Clark, Cornell Cooperative Extension and Daniel Gilrein, Cornell University

Horticultural oil has gained wide acceptance in recent years in pest management programs due to its plant and environmental safety and effectiveness in controlling many types of insect and mite pests. Dormant and verdant or summer oil applications have been used safely on many types of plants. As a benefit to the applicator, horticultural oil is also a relatively "safe" material to use, and leaves no toxic residue.

MODE OF ACTION

Horticultural oil interferes with both respiration and membrane function and disrupts feeding activities of virus-transmitting aphids and leafhoppers. For oil to be effective, the material must coat the pest or egg, thus complete coverage is essential for optimum results.

PHYTOTOXICITY

READ THE LABEL to become familiar with all the precautions and application information. Some oil labels caution the applicator about using oil in combination with or immediately before or after a spray containing certain chemicals. Concern exists over the potential phytotoxicity from oil applications on green and growing plants. Many recent research studies have shown no phytotoxicity except to a few sensitive species under the most adverse spraying conditions of high heat and relative humidity. Some species which may be sensitive under certain conditions include Acer rubrum October Glory, Acer saccharum Green Mountain, Quercus rubra, Juglans nigra, and some cultivars of Juglans ailanthifolia and J. cinerea. Phytotoxic symptoms may appear as twig dieback, leaf burn, and killing of new growth. Oil removes the blue coloration from the current season's growth on plants such as Picea pungens var. glauca and Juniperus horizontalis Wiltoni. New, untreated foliage is not affected by previous applications.

APPLICATION CONSIDERATIONS

Oil mixtures left to sit for short periods without agitation will not stay in suspension at high temperatures. Globules of oil will form in the hoses which, due to the concentration, may cause necrotic spots even when sprayed on resistant plants. Applicators should frequently agitate the mixture and avoid having the mixture remain in the hose for any period.

SPECIFICATIONS

Several grades of horticultural oil are available. Some are strictly limited to dormant use, while others can also be safely applied to actively growing plants in spring and summer. Despite labeling inconsistencies, the terms unsulfonated residue (U.R.), gravity <198>API (minimum), and distillation at 10mm Hg in <198>F will always be found in the ingredient statement on up-to-date labels. These tell the user the type of oil purchased. The unsulfonated residue provides the degree of purity in percent of saturated hydrocarbons. A U.R. rating of 92% or above indicates a highly saturated oil which is necessary to minimize the potential for phytotoxicity to growing plants. A gravity reading of 30<198> or greater indicates a highly paraffinic oil, important both in insecticidal effectiveness and plant safety. Oil refined at a distillation range from 400<198> to 435<198>F (under a 10mm Hg vacuum) yields a product that persists long enough to kill pests, yet evaporates before plants can sustain injury. Oils distilled above 435<198>F are only safe for dormant application.

DORMANT APPLICATION

Timed for late winter and early spring when plants are dormant, this preventive application can save valuable hours later in the season when time is at a premium. Applied to aboveground plant parts, horticultural oil is effective against the overwintering stages (including eggs) of pests such as scales, adelgids, and mites.
For dormant treatments, the oil should be applied at a 2-4% solution (3% is commonly used) uncombined with any other insecticides. Heavier grades of oil (those with a viscosity exceeding 100 seconds or a distillation temperature of 435<198>F or higher) may improve efficacy for dormant applications. If signs of growth are evident, a 2% solution will increase the material's margin of safety.
Oils should not be applied to plants when there is a danger of freezing.

SUMMER (VERDANT) APPLICATION

Horticultural oils applied during spring and summer have been shown to be safe on a wide variety of plants when used properly. Plants should not be under moisture stress or phytotoxicity may result. Although research has shown 3% solutions to be safe, 2% work as well and serves to reduce pesticide use and application costs. Only oils with a distillation range of 400<198> to 435<198>F should be used for summer treatments. Up to four treatments per season can be applied safely. These oils may also be used for dormant applications at the higher (3-4%) rates.

Note: Much of the information in this fact sheet was adapted from publications by Dr. Warren T. Johnson, Cornell University.

Bonide's All Seasons Oil may be safely applied through the summer according to label directions.

Following is a listing of insect pests that can be controlled by a horticultural oil. Timing either dormant or summer is dependent upon the biological stage of the pest.

Common Name
Aphids
Balsam twig aphid
Beech scale
Boxwood mite
Cooley spruce gall adelgid
Cottony maple scale
Cottony taxus scale
Eastern spruce gall adelgid
Elongate hemlock scale
Euonymus scale
European elm scale
European fruit lecanium
European red mite
Fletcher scale
Golden oak scale
Hemlock eriophyid mite
Hemlock scale
Hemlock woolly adelgid
Honeylocust mite
Honeysuckle aphid
Juniper scale
Kermes oak scales
Magnolia scale
Maple bladdergall mite
Oak spider mite
Oystershell scale
Pine bark adelgid
Pine eriophyid mites
Pine needle scale
Pine tortoise scale
Rust mites
Southern red mite
Spruce bud scale
Spruce spider mite
Taxus mealybug
Tuliptree scale
Twospotted spider mite
White pine aphid
White prunicola scale

This publication contains pesticide recommendations. Changes in pesticide regulation occur constantly and human errors are still possible. Some materials mentioned may no longer be available, and some uses may no longer be legal.

Read the label before applying any pesticide.

Revised 1/92