Dormant Oil in the Autumn?

FletcherScale8

Fletcher Scale Nymphs getting ready to overwinter on Taxus (yew) (J. Llewellyn)

It’s been a warm month so far but the long-term forecast is calling for much cooler temperatures.  That means we will soon have an opportunity to manage some of our plant pests, such as mite eggs and scale insect nymphs, before they go completely dormant.
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 the our horticultural oils.   Horticultural oils may be phytotoxic in extreme temperatures.

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

Magnolia Scale Nymphs (D. Cheung)

–    Insect and mite pests that overwinter in a juvenile, unprotected life stage can be very susceptible to dormant horticultural oil applications in the fall.  Some scale insect species that overwinter as nymphs and ARE SUSCEPTIBLE to dormant oil applications include:
Cottony maple scale (Acer, Viburnum, Prunus)
European fruit lecanium scale (Acer, Quercus, Fraxinus)
European elm scale nymphs (Ulmus)
Magnolia scale (Magnolia),
San Jose Scale (Several hosts including Acer, Salix)
Tuliptree scale (Liriodendron).

European Fruit Lecanium

Overwintering European Fruit Lecanium scale Nymphs in May

{Note : the following 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)}
–    Some mite species that overwinter as unprotected eggs on the host plant and ARE SUSCEPTIBLE to fall dormant oil applications) include:
European red mite eggs (Malus, Pyrus)
Maple spider mite eggs (Acer, especially reds and silver-red hybrids)

{Note : the following mites do not overwinter as exposed eggs on host plants are NOT SUSCEPTIBLE to dormant oil applications: Two Spotted Spider Mites }
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 to for fear of burning the foliage if temperatures drop down below freezing.  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.  Dormant oil will wash off in the rain and snow during the weeks following application.

Horticultural Oil – Plant Sensitivity – WARNING
To prevent injury, 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: http://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php

 

Advertisements
This entry was posted in Insects, IPM, Mites and tagged , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s