An Introduction to Horticultural Oils

Dormant and horticultural oils are effective against exposed tree insect eggs, larvae, pupae, and adults. Dormant oils are sprayed on trees in the winter before their buds open in the spring. In Denver, these oils can be used in the winter to control tree aphids, mites, and scales. 

Pest control has been around since humans discovered agriculture in Mesopotamia 10,000 to 12,000 years ago. Farmers as far back as 2500 B.C. applied sulfur compounds to crops. In 350 B.C., the Romans mixed oil with ash for pest control. A thousand years ago, the Chinese invented insecticide soaps. In the U.S., some of the first insecticides were oils used in the fruit industry in the 1880s. Their use migrated to control tree insects in the city as an alternative to chemical-based control products. 


tree insects overwintering as eggs

What Are Dormant Oils?

Distilling petroleum to 92 to 99 percent purity creates oils and removes any phytotoxic compounds such as aromatic compounds and compounds containing sulfur, nitrogen, or oxygen. Since oil and water do not mix, adding an emulsifier allows mixing horticultural oils with water for application. Typical dormant oil solutions are at a two percent oil concentration. Below are various terms for oils used to control insects currently on the market.

  • Dormant Oils – These less refined oils are sprayed on trees in the winter to control insects but not safe to use on plants after leaf-out.
  • Spray Oils – A oil with emulsifiers mixed with water to be applied to trees for insect control.
  • Summer Oils – Horticultural oil sprayed on trees during the spring or summer. These oils are formulated not to burn or harm foliage. 
  • Supreme or Superior Oils – These highly distilled oils contain paraffinic hydrocarbons, allowing use on trees and plants with foliage. Sometimes these products are called narrow range or summer oils. 
  • Vegetable Oils – Some horticultural oils come from Soybean, Cottonseed, and Sesame seeds. Cottonseed oil is the most insecticidal of the vegetable oils. However, plant-based oils are less refined and may have some phytotoxicity. 

How Do Horticultural Oils Work?

The oil suffocates tree insects’ eggs, larvae, pupae, and adults by clogging their breathing tubes. Dormant oils dissipate quickly by evaporation, leaving little residue or toxicity. In the winter, dormant sprays kill overwintering insects and their exposed eggs, larvae, and pupae. 

In Denver, dormant oils are used to control these common tree pests.

  1. Leaf Curl Aphids

    Aphids feed on the sap of trees. Large infestations cause wilt and dieback of shoots and buds. Aphids also produce honeydew, which coats sidewalks, driveways, furniture. Dormant oils kill aphid eggs during the winter.
    leaf curling aphids

  2. Leafrollers

    Leafrollers infest almost all hardwood trees growing in Denver, but the most common hosts are Apple, Crabapple, and Lindens. Leafroller larvae chew leaves, causing significant defoliation during outbreaks. Dormant oils sprayed on twigs effectively kill overwintering egg stages of this insect.
    oak leaf roller moth

  3. Tent Caterpillars

    There are several tent caterpillars native to Colorado and attack most hardwoods found in Denver. The Forest Tent Caterpillar feeds on fruit trees, Elm, Aspen, and Ash, and do not build tents but instead makes silken resting mats. The Sonoran Tent Caterpillar attacks Oaks, and they do build tents. The Western Tent Caterpillar feeds on Aspens, fruit trees, Willows and build large silk tents. Dormant oils effectively kill overwintering eggs of all these tent caterpillars.
    forest tent caterpillars

  4. Spruce Spider Mites

    Spider mites, a common insect found in the Denver area, attack Conifers, particularly Spruce and Douglas Firs. The mites feed on tree sap, and infested trees become brownish gray and become defoliated. Dormant oils used in the winter kill the mite’s eggs. 
    spruce spider mites

  5. Kermes Scale

    Kermes overwinters as nymphs on the branches of oak trees and gather around leaf buds. In the late summer, the adult female lays eggs that hatch in September and October migrating to their overwintering sites. Dormant oil applications smother the insects before they can turn into an adult. 
    oak kermes scale

  6. Conifer Scales  

    Conifer scales feed on bark and needles. They remove sap and damage cells leading to decreased vigor, needle drop, and increased susceptibility to other insects or diseases. Some scales also excrete sticky honeydew, which is a nuisance.
    striped pine scale denver

  7. Oystershell Scale

    The Oystershell Scale grows on the bark of trunks and limbs of Aspen, Ash, Poplars, and Willow trees. The oyster shaped scales grow to about 1/8-inch in length. The scale blends in with the underlying bark and goes unseen until they cause limb dieback and other tree damage. Oystershell Scale also weakens trees making them more susceptible to pathogens. In Aspen and Popular trees, Oystershell Scale infestations can lead to cankers formed by Cytospora fungi. Check out our popular guide on Aspen Care in Denver. 
    oystershell scale

  8. Cottony Maple Scale

    Cottony Maple Scales can reach epidemic numbers on silver maples in Denver. During the summer months, females produce white egg sacs, called an ovisac, that look like cotton balls. Heavy infestations cause branch dieback and, in rare instances, tree death. Dormant oils control Cottony Maple Scales, but application timing is critical because of Maples’ sensitivity to these oils.
    cottony scale on silver maples denver

The Advantages and Disadvantages of Using Dormant Oils

Dormant oils show low toxicity to humans and wildlife.

  • Since the oils evaporate quickly, they are less disruptive to beneficial insect populations than chemical insecticides.
  • Since these oils suffocate insects, they cannot develop resistance.
  • Do not apply when below freezing because the emulsion breaks down, causing uneven coverage.
  • Do not apply oils in wet conditions because the additional moisture inhibits oil evaporation.
  • Do not apply dormant oils until winter hardening has occurred. Fall treatments sometimes increase susceptibility to winter injury.
  • Do not apply oils in combination with sulfur-containing pesticides because they form phytotoxic compounds. 

The following trees are sensitive to dormant oil applications.

  • Black walnut
  • Dwarf Conifers
  • Douglas Firs
  • Junipers and Cedars
  • Some Maples
  • Redbud
  • Smoke tree
  • Spruce

Neem Oil

Neem oil is not a dormant oil but has become popular in Denver to control Japanese Beetles. The highest quality Neem oils are cold-pressed from the nut of the Neem tree, not chemically extracted. The active insecticidal component of Neem oil is Azadirachtin, which reduces insect feeding, growth, and egg-laying. The oil also repels insects. The oil is non-toxic to birds and mammals but is slightly toxic to aquatic animals. The oil should only be used when bees are not active.

In conclusion, dormant oil spraying in the winter kills overwintering eggs of aphids, mites, and scales. These oils are less toxic than chemical insecticides. 

Want to control tree insects overwintering in your yard trees? Why not call a local tree company in business since 1978. We have the equipment to apply dormant oils in high places and into thick canopies. Please fill out this tree service request form, or call us at 303-871-9121.