Apple Pest Update: April 3, 2020

If you’re thinking of applying dormant oil, now is the time to start planning. Spring is not taking it’s time this year and green tip is already beginning in early regions of the province. Thankfully, things have slowed down this week compared to last which buys some time for preparation. Looking at the 14-day forecast, you may not want to jump on this spray right away as it looks like we’re in for another dip in temperatures late next week. Insect development and activity is driven by temperature; the milder it is, the faster the insect matures. This improves the efficacy of how oil works as well ensures a lower risk of phytotoxicity concerns.

How Does It Work?

Oil sprays work mainly by suffocation. Coating the insect – which means in a high volume spray to reach all the cracks in the tree bark – prevents normal respiration from the air holes (spiracles) where they breathe. This works best on the immobile and immature stages where: 1) the insect can’t move away to avoid the spray, 2) the scale coverings have still not hardened and oil can penetrate, and 3) respiration rate is the highest. However, oil can also interfere with egg development, prevent settling of scale crawlers and deter feeding by pests such as aphids.

If applied properly, the use of dormant or delayed dormant oil can sufficiently reduce the amount of miticides or insecticides that may be used for scale or mites later in the season. Early season oil sprays also fit well into IPM programs because the product is applied before predatory mites and other beneficial insects are present.

What Pests Does It Target?

Overwintering San Jose scale on limb of apple tree

There are several species of scale insects affecting apples; San Jose scale (SJS) is the most common in Ontario orchards. This insect overwinters as an immature scale under bark and emerges just prior to bud break. As the immature scales feed, they exude a waxy substance that forms a protective layer. Dormant oil sprays are the best timing for this pest before they develop that waxy covering.

Without the foliage to block the spray, dormant oil applications can get reasonable coverage of limbs and trunk where the overwintering SJS population is located. Targeting individuals at this stage will help reduce the population that will produce the summer generation crawlers. Postbloom management targets these crawlers which move from the infested area to maturing fruit. These sprays can be very effective at reducing the amount of fruit damage; however, they do not always provide good control of the crawlers that move elsewhere such as to new branches, a different spot on the trunk or to an adjacent tree. In other words, you could find yourself in a continuous cycle of managing fruit damage if the SJS population is not suppressed. While it may be hard to find time and good weather early season, an oil application is well worth it.


European red mite_eggs
European red mite eggs under bark of apple tree


European red mites overwinter as eggs on roughened bark around the bases of buds and spurs, or in the inner parts of the tree close to the main trunk and branches. Oil sprays should be applied before egg hatch, between half-inch green and tight cluster. Some growers in Ontario, who have used dormant oil with good water volumes and good coverage, have achieved excellent mite control with few summer escapes and no need for summer miticides.


While delayed dormant oil applications primarily target scale and mites, you may see some additional efficacy against other pests at this timing including impeding egg hatch and movement of aphids and some spring feeding caterpillars, interfering with egg development of apple leafcurling midge and preventing release of overwintering powdery mildew spores as infected buds open.

Dormant vs Delayed Dormant

Unfortunately, optimal timing for scale is not necessarily the same for mites. If monitoring indicates scale is a bigger issue in the orchard, oils need to be applied before or shortly after bud break. This efficacy against scale is significantly reduced with later oil applications for European red mite as they develop a waxy protective layer. However, if European red mite populations are the problem, sprays can be delayed to half-inch green to tight cluster.


Original precautions around the use of dormant oil were developed prior to the refinement processes that are carried out now with the commonly registered products. Most impurities that were associated with phytotoxic effects with some of the older “heavy” horticultural oils are removed through extra filtration and distillation. If you have concerns with using oil, especially with sensitive varieties like Red Delicious, Empire, Mutsu and Ambrosia, consider a product such as Purespray Green Spray Oil 13E as it is registered for both dormant and summer use.

However, even highly refined “summer” oils can cause crop injury when they are applied:

  • when temperatures are consistently below 4C
  • within 48 hours before or after a freezing event
  • in slow drying or prolonged wet conditions
  • with or too close to products containing sulphur or captan
    • do not apply oil within 14 days before or after these products
  • above label rate
    • 2% solution (20L/1,000L) for dormant sprays
    • 1% solution (10L/1,000L) for summer sprays
    • High water volumes are essential for good coverage
  • to plants are under moisture stress
  • when temperatures are very high (above 25°C)

Always read the product label for additional instructions and precautions.

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