Get Ready for Fall Dormant Oil Applications

Gray Treefrog

The Grey Treefrog is a carnelian, it can change colour from green to grey, depending on their environment. (Photo: Joe Crowley)

Today marks the first day of Autumn.  And with it another interesting calendar event, the Autumn Equinox.  Like the Spring Equinox (March 21), the Autumn Equinox marks a unique period of time where the length of daylight and night time hours are pretty much equal.  So as you can imagine, the wildlife can get a little confused this time of year.  If you spend time outdoors you might notice the odd mating song of birds and frogs.  I have heard a lot of male Grey Tree Frogs singing in the last week or so.  How cool is that? 


Magnolia scale NYMPHS on the undersides of twigs (J. Llewellyn)

Many horticultural professionals are gearing up for fall applications of horticultural oil. The full, dormant rate of horticultural oil is excellent for smothering juvenile stages of insects this time of year.  Magnolia scale populations were quite high in the GTA again this year, making these trees prime candidates for dormant oil applications.  Several professionals have reported that for magnolia scale especially, fall dormant-rate horticultural oil applications are very effective at managing this pest.  Be certain to aim the spray on the undersides of twigs to maximize coverage.  In heavy infestations, it is still advisable to follow up with dormant oil applications in the spring.

In October we will 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.

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).

{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)                                  Oak Spider mite (Quercus)

{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.

There are several cricket, grasshopper and katydid species still calling this time of year and I’m one of those people who needs to know who’s doing the calling.  Katydids are one of my favorites.  But there is this one insect I’ve been chasing down for a couple of years now.  I always hear it in wet, marshy areas in August and September.  It reminds me of the sound that in impact irrigation sprinkler makes, followed by a long buzz.  They seem to only sing during the day but they stop calling and hide any time they see me coming.  Steve Paiero, of the University of Guelph Insect Collection has solved the mystery!  It is the mating call of the Black-legged Meadow Katydid (Orchelimum nigripes).  Click HERE to listen to this beautiful Katydid. Thank you Steve 🙂


Jen Llewellyn and small mouth bass

Summer is over, thank goodness, and September is an awesome time to get out of cellular range and go fishing!  It’s now or never guys.  All work and no play makes deprived fishermen difficult to be around.

About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
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