Integrated Pest Management for woody ornamentals

If the last growing season had a name, it would be “Treepocalypse of 2023.” If a nice day was thwarted by a concerning tree observation, rest assured, you were not alone.

There are many reasons why a tree may appear unhealthy. Most of the time it is due to abiotic (environmental) reasons such as drought stress, winter damage, or sunscald. If you have followed the mantra of “Right Plant, Right Place” and taken all the steps you could to maintain the health and vigor of trees and shrubs, you may have fewer issues related to pests and disease.

Inevitably, some years are worse than others for trees, and stressed trees make inviting homes for pests (such as insects and other arthropods). If you see pests or signs of insect or mite damage, bring a sample to your local MSU Extension agent for diagnosis. When a proper sample is submitted for diagnosis, the MSU Extension agent or Schutter Diagnostic Lab can identify the disease or pest and give a few management options.

close-up of a pine branch with white spots and brown tips indicating a pine needle scale infestation.

Pine needle scale infestation.
Photo: USDA Forest Service – Coeur d’Alene Field Office,

Integrated Pest Management (IPM) includes the use of environmentally responsible pest management strategies (including prevention, mechanical control, biological control, and chemical control). Even when you try your best to prevent pest-related issues by maintaining plant health, sometimes pest problems are inevitable. If other IPM strategies don’t work, management using pesticides may be necessary. One of the least toxic options for chemical treatment of many pests is dormant oil. However, there is a fair amount of confusion over dormant oil. What is it? How do I make sure I purchase the right one? How do I apply it? Is it safe? What about beneficial insects?

Dormant oils are petroleum-based products like other horticultural oils. The term “dormant oil” refers to the time of year when they are effectively used, late fall-early spring, before bud break. Dormant oils have a low unsulfonated residue (U.R.), meaning that there are limited plant-toxic compounds remaining after distillation and refining. When at your local garden center or hardware store, look for dormant oils with a U.R. value less than 92%. For example, the label might say “Pet. Oil-Min. U.R. 91%-Class Light-Med.” under the Active Ingredients section.

a green serrated leaf with bright pink striations due to Eriophyid mites

Eriophyid mites on Viburnum species.
Photo: Steven Katovich,

Dormant oils target pests that are found on plants during the dormant season. Late winter and early spring applications of dormant oils can be helpful for controlling overwintering insects and insect eggs in fruit trees and woody ornamentals. They can be used to treat pests including aphids (eggs), adult mites and eggs, scales (overwintering females), caterpillars overwintering as eggs on foliage, etc. Dormant oils are contact insecticides and work mechanically by blocking the breathing pores (called “spiracles”) or by penetrating and disrupting fatty acids in cell membranes of arthropods. They essentially suffocate the pests. Dormant oils degrade rapidly and don’t leave a toxic residue. Reapplication may be necessary for effective control (as per label instructions). As such, dormant oils can also be a way to minimize negative effects on non-target insects, and proper use can result in reduced need for further insecticide application in the growing season.

Pest Life stage to target
with dormant oil
Eriophyid mites Overwintering adults
Cottony maple scale Adult females
Aphids Eggs
Ash flower gall mite Adult females
European elm scale Overwintering nymph
Oystershell scale Eggs
Pine needle scale Crawler stage
Spruce bud scale Scale nymph
Spruce spider mite Eggs
Blister mites Adult mites


Dormant oils can be a less toxic approach than some other conventional insecticides for managing insect pests when used properly. When applied in late winter/early spring before bud break, dormant oils can reduce insect populations that would cause severe damage in the spring/summer. They can also reduce injury to non-target arthropods (including beneficial insects such as pollinators that aren’t active during the dormant season). As with any pesticide product, which includes dormant oils, always read and follow label directions, and contact your local MSU Extension office if you have additional questions.

Note: These recommendations are provided only as a guide. It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. The authors and Montana State University assume no liability resulting from the use of these recommendations.


  • Have a proper insect diagnosis. This can be done through your local MSU Extension office. When dropping off a sample, it’s helpful to include many specimens of the insect especially if there are different life cycles present. You’ll want to include a fragment of the vegetation, with some damaged and undamaged material in the container or plastic bag. You can also email photos of the landscape and fill in details, like the age of the planting, where the plant was purchased, and if you’ve added any soil amendments.
  • Read the label before purchasing dormant oils. Dormant oils can be purchased at a local garden center or hardware supply store. In addition to the U.R. value, the label will give important details like timing of application, frequency, rate, target species, and personal protective equipment.
  • Follow label instructions (every time you apply). Following instructions will lead to better control of the pest, reduce injury to non-target insects, prevent discoloration or damage to sensitive plants, and keep yourself, pets, and children safe.
  • Monitor the plant before applying dormant oil. Success when using dormant oils depends on timing. Usually only one life cycle of the pest will be susceptible. Monitor the plant to see if the pest is in the susceptible life stage (“instar”) and see where the pest is located (underside of foliage, bark, leaf buds, to name a few).
  • Give yourself time to achieve excellent coverage and contact. Dormant oils will only be effective if they are in direct contact with the pest at the correct life stage.
  • Check the weather. Only apply dormant oils when temperatures will remain above freezing for at least 24 hours.
  • Drag out the hose and water plants before application. Drought-stressed plants may show symptoms of phytotoxicity.


  • Avoid “blanket” treatments. After monitoring, you should know the specific spots where the pest is located. Spraying the entire plant does not mean better control, complete coverage of susceptible life stages of the pest is the most important factor. Depending on the species and life stage, the mite or scale may overwinter on the bark or underside of leaves, so make sure to target locations where the pests are located.
  • Do not apply dormant oil on sensitive plants. Phytotoxicity effects have been observed on new transplants of spruce, juniper, arborvitae ‘cedars’, and maple. Most commonly, phytotoxicity is a result of improper application rate or timing – either the rate is above 4% OR application occurs after budbreak/before dormancy. Before treating the entire plant, spray dormant oil on just one section to test how the plant will respond (and look for any symptoms of damage).
  • Do not apply or mix with sulfur-based fungicides.
  • Do not apply in windy conditions. High winds at time of application can turn evergreen needles brown.

a person wearng rubber gloves and eye protection points a spraybottle onto a tree branch

Read the label before purchasing dormant oils and always wear personal protective equipment.

Photo: Jackie Pondolfino, MSU Extension

Jackie Pondolfino is an MSU Extension Agent in Park County.