Dormant trees and sunny skies (photo credit:

Listen, I know it’s raining. And by now you are thinking that you just can’t handle another rain day.
Believe me, I hear ya. And I’m there too. But I checked the weather forecast and they are predicting sunny skies for the next several days….
Do you know what this means?                                                                                                      You can start to get caught up on all that work you have to do and I promise, your mood will improve dramatically and people are going to start to be nicer to each other again.

With the nice fall weather is an opportunity to manage some of our plant pests, such as overwintering stages of mite eggs and scale insect nymphs.  Because daily temperatures and weather patterns can be more moderate in autumn compared to spring, the fall dormant period may be a less risky application period for our horticultural oil products. Horticultural oils may be phytotoxic in extreme temperatures.

<a href="/clm/species/oligonychus_aceris"><em>Oligonychus aceris</em></a> (Maple Spider Mite) eggs on maple.

Tiny red maple spider mite eggs overwintering next to bud scar on Freemanii maple (Photo: D. Cheung)

***Some mite species that overwinter as unprotected eggs on the host plant and ARE SUSCEPTIBLE to fall horticultural oil applications include:
European red mite eggs (Malus, Pyrus)
Maple spider mite eggs (Acer, especially reds and silver-red hybrids)
In the case of Spruce spider mites, the adults are still quite active in fall and are actually susceptible to miticides this time of year.
{Note : the following mites do not overwinter as exposed eggs on host plants are NOT SUSCEPTIBLE to dormant oil applications: Two Spotted Spider Mites }

<a href="/clm/species/neolecanium_cornuparvum"><em>Neolecanium cornuparvum</em></a> (Magnolia Scale) nymphs.

Nymphs of magnolia scale in early spring (photo: D. Cheung)

***Some of our soft scale insect species that overwinter as nymphs on the host plant and ARE SUSCEPTIBLE to fall horticultural oil applications include:
Cottony maple scale nymphs (Acer, Prunus, Viburnum)
European elm scale nymphs (Ulmus)
Fletcher scale nymphs (Thuja, Taxus)
Lecanium scale nymphs (Quercus, Fraxinus)
Magnolia scale nymphs (Magnolia)
Pine tortoise scale nymphs (Pinus)
San Jose Scale nymphs (Malus, Pyrus)                                                                                     {Note : many of our armored scale insects are NOT SUSCEPTIBLE to dormant oil because they overwinter as tolerant adults or eggs protected under the dead female scales: Euonymus scale (Euonymus, Pachysandra), Oystershell scale (Fraxinus, Salix and others), Pine needle scale (Pinus), Golden oak scale (Quercus) are not susceptible to dormant applications of horticultural oil.}

Dormant applications of horticultural oil may cause some injury on evergreen foliage during freezing temperatures. Although spruce bud scale (Picea), spruce spider mite eggs (Abies, Picea, Thuja) and Fletcher scale (Thuja, Taxus, Juniperus) are present in the susceptible juvenile stage in autumn, horticulturalists will often shy away from fall dormant oil applications on evergreens for fear of burning the foliage if temperatures drop down below freezing. Dormant oil will wash off in the rain and snow during the weeks following application.

Horticultural Oil – Plant Sensitivity – WARNING
To prevent injury to foliage, DO NOT APPLY ON: ARBORVITAE, BEECH, BUTTERNUT, HICKORY, WALNUT OR WHITE PINE. PERMANENT DISCOLOURATION OF FOLIAGE WILL OCCUR TO BLUE VARIETIES OF JUNIPER AND SPRUCE. 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 here

About Jen Llewellyn

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

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  1. Pingback: Control Next Year’s Insects Now - Toronto Master Gardeners

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