What is dormant oil?

Dormant oil is an oil based, low-impact insecticide, that is sprayed on vulnerable woody plants during their dormant season, to kill the overwintering stage of pest insects. The oil used is typically a highly refined mineral oil to which an emulsifying agent has been added, which allows the oil to mix with water. Sometimes cottonseed oil or soybean oil are used instead, with good result. Collectively, oils products that are used in horticulture are called horticultural oils.

How does dormant oil work?

When horticultural oil is sprayed on an insect it covers their air holes (spiracles) there by suffocating the insect. When sprayed on overwintering insect eggs the oil penetrates the eggs and impedes the respiratory and metabolic functions of the insect.

What insects are controlled by dormant oil?

• Aphids (overwintering eggs)
• Caterpillars (overwintering eggs)
• Mites (some species overwinter as eggs, others, such as the two spotted spider mite, over winter as adults)
• Scale Insects (many species overwinter as eggs beneath the dead females other species over winter as immature nymph).
• Adelgids (overwinter usually as immature nymphs but they can overwinter at any stage)
• Psyllid (over winter as eggs)
• Leaf hoppers (most species overwinter as eggs, a few species overwinter as adults).
• Plant bugs (eg: honey lotus plant bug) (overwinter as eggs)
• Thrips (overwinter as adults)
• Dormant oil also helps to control some viral diseases, by controlling the pests that vector them.

When to apply dormant oil?

As the name suggests dormant oil sprays are applied before woody plants break dormancy in the spring. Temperatures must be at least 5^oC with no risk of frost for 24 hours. Overly low temperatures cause the emulsifying agent (which allows the oil to mix with water) to break down resulting in an uneven coverage. Temperatures should also be less than 21^oC to prevent plant injury (ideal temperature is between 12-15^oC). Do not spray immediately after rain, allow leaves to dry first. Do not spray if there is rain in the forecast in the next 24 hours. Also make sure plants are not drought stressed before spraying.

How to apply dormant oil?

Dormant oil is sprayed onto woody plants using a pressurized tank sprayer. The wood is thoroughly coated, to the point of dripping, to ensure every crack and crevice has been filled.

What plants are sensitive to dormant oil?

While dormant oil sprays are a safe and low risk way to control the overwintering stages of some insect pests, some plants are sensitive to it and may experience damage to their bark. Read the product label for full instructions, in some cases the product can be used but in a lesser concentration for certain species and for other species not at all. Remember also that dormant oil sprays are to be applied before the buds begin to crack open otherwise the oil can cause transpiration problems and new leaves may be damaged. Once buds crack open switch to summer oil applications.

• Beech (Fagus)
• Butternut (Juglans cinerea)
• Cedar (Thuja, Arborvitae)
• Douglasfir (Pseudotsuga menziesii)
• Eastern redbud (Cercis canadensis)
• Eastern white pine (Pinus strobus)
• Hickory (Carya)
• Japanese holly (Ilex crenata)
• Junipers (Juniperus)…especially blue varieties
• Maples (Acer) especially Japanese and Red
• Red oak (Quercus rubra)
• Smokebush (Cotinus)
• Spruce (Picea) especially dwarf Alberta and blue varieties
• Walnut (Juglans)
• Yew (Taxus)

(The bolded names on the list are considered the most sensitive)

Can dormant oil be combined with lime sulfur?

Dormant oil is often combined lime sulfur, (a low-impact fungicide) that serves to kill overwintering fungal spores as well as enhancing the effects of the oil insecticide. This combination spray must be used in accordance with the manufacturer’s guidelines. During dormant season the concentration is typically higher. Once growth begins lower concentrations are applied usually with a 30 day gap between oil application and lime sulfur application. This 30-day gap goes for all sulfur-based products including elemental sulfur.

What is the difference between dormant oil and summer oil?

As already stated, dormant oils are applied when the woody plant in in dormancy. Summer oils are applied when the plants are in leaf; they are typically at a much less concentration. The summer oils are particularly useful for the crawler stage of scale insects as well as all the same insects targeted by dormant oil. It is also useful on many insects that overwintered in the soil rather than on woody plants (such as flea beetles and cucumber beetles), and thus missed the dormant oil application. Summer oil may not necessarily kill the insect, but it will disrupt it’s feeding

Photo credit: photo by the author.

References:

Cranshaw W.S. and Baxendale B. (1999). Revised 12/13. Insect Control: Horticultural Oils. https://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/

Stephen Westcott-Gratton, (2017). Updated: October 2, 2020. Dos and don’ts of dormant oils. https://gardenmaking.com/dos-donts-dormant-oils/

University of Nevada, Reno, (n.d.). Horticultural Oils – What a Gardener Needs to Know. https://extension.unr.edu/publication.aspx?PubID=3029

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