Dormant Horticultural Oil App’s This Week!



Some milder day and night temperatures are forecasted this week, making it a good time for dormant horticultural oil applications in many areas of southern Ontario.   (Photo by:  P. J. Jentsch_Cornell. Susceptible, overwintering nymphs of San Jose scale on Malus).  I have also seen San Jose scale on Prunus x cistena, purple-leaf sandcherry).

Depending on your location, host dormancy and pest biology, you may even wait a little bit until the temperatures become consistently milder. 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 horticultural oil.

When can I apply Horticultural Oil? Dormant horticultural oils should not be applied if the temperature is below 4C or if the temperature will approach freezing soon after application of the dormant hort 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 (>30C).  High temperatures can be associated with drought stress and those plants may be damaged.

How late can I apply Horticultural Oil? As long as there is no sensitive vegetative or flower tissue emerging (see product label for host plant tolerance), you can still make dormant rate, horticultural oil applications for overwintering insect pests.

How do Horticultural Oils work?  Insects breathe through pores on the sides of their abdomens, called “spiracles”.  Horticultural oils cover the spiracles, preventing respiration.  The treated insects suffocate.

What pests can be managed with dormant applications of Horticultural Oil?  Some 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)
grey magnolia scale nymphs on magnolia twig

Overwintering nymphs of Magnolia Scale. The nymphs are usually found on the undersides of twigs and are very susceptible to horticultural oil applications in the fall and spring (J. Llewellyn)


  • European red mite eggs (Malus, Pyrus)
  • Maple spider mite eggs (Acer, especially reds and silver-red hybrids)
  • Spruce spider mite eggs (Abies, Picea)
Spring is Spring-ging - ONnurserycrops BlogONnurserycrops Blog

Maple spider mite eggs (orange-red) near the leaf scar on Acer freemanii (red x silver hybrid).  Clear-looking eggs are empty egg shells that have already hatched.

Some plant health managers use 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.

What are some important points to remember when using Horticultural Oil as a dormant treatment?

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.

I read the pest control product label last year, should I read it again? Always read the label before applying any pest control product.  Be aware of the common warning statements on horticultural oil labels, they can change from year to year.

What are some caution statements I should be aware of?  Never spray sulphur with (or within 30 days of) 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 green tip. Only 1 application per season for peaches. Do not apply to broad-leaved 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 –

Take a look at the date of the pesticide label (top left corner) and see if it has been revised since you last checked the most recent label online.  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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