Clip 'N Keep: Dormant Oil

By Anne Marie Van Nest, Instructor, Niagara Parks Botanical Gardens Photographs by: Anne Marie Van Nest and Liz Klose, Instructors, Niagara Parks Botanical Gardens

Dormant oils have been used since the late 1800s to control a wide range of insect pests on fruit and shade trees. They have a faithful following because they are safe to apply and degrade quickly in the environment. Additionally, they are valuable because of their compatibility with many other IPM controls and particularly because they are less harmful, than other pesticides, to non-target beneficial insects. Dormant oils are even non-corrosive to spray equipment. Dormant oils have many different names. Some are called superior oils, supreme oils, petroleum oils, narrow range oils or horticultural oils. All are a light-weight oil that is pale amber in colour with little odour.

When should dormant oils be applied?
The window for dormant oil spraying is in early spring prior to bud break or in late fall after leaf drop. Some products can also be used in the summer during active growth at a diluted rate. Higher viscosity oils (heavier) are used for dormant spraying. Lighter oils are used for summer applications. The application time for many junipers is after pruning and until mid-May. For magnolias, the application window is relatively small; dormant oil should be sprayed in the spring when temperatures rise above freezing and before the buds break.

What is being applied?
The dormant oils are comprised of a long chain of hydrocarbon oils and are used as contact insecticides, acaricides and ovicides. The toxicity rating of LD50(oral) of between 5,000 and 15,000 mg/kg puts this product in the least hazardous category.

Why use it?
Dormant oils are used to control overwintering and exposed soft-bodied insects and eggs of aphids, mealybugs, mites and scales (Fig 1). It is most effective when pests can be smothered by the oil and has no effect on insects that are inside a plant such as the eggs of Viburnum leaf beetle.
For example, dormant oil can be used on mites attacking dwarf Alberta spruce, roses (Fig. 2) or junipers, scale on Mugho pines, magnolia, euonymus or junipers.
The low toxicity of this product makes it safe to use for humans and most wildlife. Prolonged contact with the skin can lead to the removal of natural oils and fats which will cause irritation for some people. Care should be taken so that it does not come into contact with the eyes. Excessive inhalation can cause irritation of the nose, throat or eyes.

How does it work?
Several theories exist about the mode of action for dormant oil. One theory is that the oils smother insects by clogging their breathing openings. It is also believed that the oil disrupts some cell membranes and interferes with metabolic activities. Still others believe that oils provide residual protection against certain sucking insect pests and disrupt their feeding habits.

How is dormant oil applied?
Read the instructions thoroughly. Traditionally, it is put on as a dormant application before deciduous plants leaf out but after buds swell. (Fig. 3, 4 & 5). Do not apply dormant oil when buds are fully open and shoot elongation is occurring (Fig. 6 & 7). If dormant oil is being used on deciduous trees in the fall - don't rely entirely on leaf drop as a sign to begin. Ensure that the plant is fully dormant, or damage may occur.

To apply a dormant oil spray:
  • The spray should be thoroughly dry before temperatures drop below freezing.
  • Use when temperatures are between 6 to 8 degrees Celcius (45F). Spray all foliage and bark until the solution starts to drip. Generally, a two per cent solution is used (read the product label for the rates). Dormant oils are applied at rates that are up to three times more concentrated than summer oils. This is required because of the small oxygen exchange of insects during dormancy. When mixing the dormant oil and water, note the uniformity of the solution. It should have a milky-white appearance when stirred vigorously.

To apply a summer oil spray:
Spray when the leaves are dry and when temperatures are between 0 - 35C (32 - 100 F). Do not use when humidity is expected to be over 90 per cent for the next 48 hours. High humidity reduces the rate of evaporation which increases the effectiveness against pests but this also might increase the chances of phytotoxicity damage to the plant.
For all situations, do not apply oil when plants are drought stressed or if windy conditions are present. Do not overapply. Your pesticide supplier may be able to give you more information about the products available and their degree of refinement. Look for a minimum unsulfonated residue (UR) of 92. The sulfonation rating is a percentage that indicates the degree of refinement. The higher the figure, the more refined the oil and the safer the product.

What plants should dormant oil NOT be used on?
Do not spray or allow drift to come in contact with beech (Fig. 8), dormant maples, dormant hickory, black walnuts, cryptomeria, smoke tree (summer), some azaleas (summer), Japanese holly, photinia (summer), savin junipers (summer), some arborvitae (summer), blue spruce, dormant spruce and dormant Douglas fir. Damage or death of the plant may occur. Avoid spraying buildings (Fig. 9), benches, fences or walks as the product may stain. Dormant oil is toxic to fish.

What are benefits of using oils?
The newer, more refined products have reduced the chance of phytotoxicity (burning of plant foliage) and have a valuable place in your IPM program because of their safety.

Fig 1: These soft bodied Gypsy moth eggs can be controlled with an application of dormant oil.
Fig 2: Roses benefit from dormant oil applications early in the season.
Fig 3 & 4: Dormant oil is applied before deciduous plants leaf out, but after buds, such as this Acer platanoides 'Globosum' have swollen.
Fig 5: Dormant buds of Magnolia soulangeana can be sprayed at this stage.
Fig 6 & 7: By the time the buds of Magnolia soulangeana have opened this far, it is too late to spray with dormant oil.
Fig 8: When this Magnolia was treated with dormant oil, extreme care was taken not to let the spray drift on to the nearby Fagus hedge. Fig 9: This euonymus can not be sprayed with dormant oil, as the oil will stain the wall behind the plant.