Dormant Oil For IPM

magnolia scale We’ve got milder temperatures starting and yes, it’s still a good time to get some dormant oil applications on in many areas of southern Ontario (Photo: overwintering nymphs of magnolia scale).

As long as there is no sensitive vegetative or flower tissue emerging, you can still make dormant rate, horticultural oil applications for overwintering insect pests.

In fact, the warmer it is outside, the more active the overwintering insects will be (e.g. respiration) and therefore, the more susceptible they are to a smothering agent such as dormant oil.

Insects do not breathe through their mouths, they breathe through pores on the sides of their abdomens, called “spiracles”.  This is why smothering agents work so well.

Some good examples of overwintering pests that can be managed with dormant oil


  • Magnolia scale (Magnolia)
  • Fletcher scale on evergreens (Thuja, Taxus)
  • Cottony maple scale (Acer, Viburnum, Prunus)
  • European fruit lecanium scale (Acer, Quercus, Fraxinus)
  • European elm scale nymphs (Ulmus)
  • San Jose Scale (Fruit Trees, Prunus, Acer, Salix)
  • Tuliptree scale (Liriodendron)


  • European red mite eggs (Malus, Pyrus)
  • Maple spider mite eggs (Acer, especially reds and silver-red hybrids)
  • Spruce spider mite eggs (Abies, Picea)

Some plant health managers are also experimenting with dormant applications of horticultural oil as a smothering agent for egg masses (e.g. eastern tent caterpillar, viburnum leaf beetle).  Not only could the oil layer help reduce oxygen flow to the maturing eggs but the residual oil may also provide a barrier to impede the hatching larvae from successfully moving out to developing foliage.

When using horticultural oils, agitation and rapid drying important.

Remember, although there are emulsion agents in the oil, you are still mixing oil and water to get a uniform solution.  Anything you can do to keep the solution mixed well during the application process will result in a more even application layer.

Always spray horticultural oils under conditions that will support rapid drying.  It is prolonged wet conditions that can lead to phytotoxicity and unsatisfactory management of pests.  Never apply during wet conditions or conditions where relative humidity is > 90%.

Horticultural oils are a lot more refined than they used to be.  Impurities associated with phytotoxicity are nearly completely removed through extra filtration, distillation and dewaxing.  If the horticultural oil product you are using also has summer uses on the label, then it’s likely a highly refined horticultural oil.

Always read the label before applying any pest control product.  Here is the most common warning statement on many horticultural oil labels.

Dormant oils should not be applied if the temperature is below 4oC or if the temperature will approach freezing soon after application of the dormant oil spray.  The freezing temperatures can mess with the emulsion of the oil mixture and cause uneven application.

Do not apply oil when temperatures are excessively high (>30oC).  High temperatures can be associated with drought stress and those plants may be damaged.

Never spray sulphur with (or within 30 days) a horticultural oil application as the combination can be phytotoxic.

Horticultural Oil – Plant Sensitivity – WARNING




Japanese Maple, Japanese Holly, Sugar Maple and Silver Maple may be sensitive to oil sprays. Non-woody plants such as ferns may be damaged. Bark injury may occur on Red Delicious, Empire and Mutsu apples. Do not apply to apples or pears after greentip. Only 1 application per season for peaches. Do not apply to broadleaved evergreens or palms when freezing temperature may be expected within 3 weeks after application. Avoid spraying during or immediately prior to hot weather (over 30°C), hot dry winds or rain. Do not apply if frost is expected before spray dries

All Canadian pesticide labels can be found by clicking here

About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
This entry was posted in Insects, IPM and tagged , , . Bookmark the permalink.

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