Get Ready for Fall Dormant Horticultural Oil

In mid-late autumn, we have an opportunity to manage some of our plant pests, such as mite eggs and scale insect nymphs, before they go completely dormant.  Now that trees are starting to drop their leaves and the weather has turned mild for a bit, horticultural professionals have a great window for managing some overwintering plant pests! The full, dormant rate of horticultural oil is excellent for smothering juvenile stages of insects this time of year as long as the application isn’t followed too closely by a significant freeze event.  Magnolia scale (photo above: overwintering nymphs in autumn J. Llewellyn) populations were…significant in some hot spots around southern Ontario this year, making infested magnolias prime candidates for dormant oil applications.  Several professionals have reported that for magnolia scale especially, fall dormant-rate horticultural oil applications are very effective reducing populations of the overwintering scale nymphs.  Be certain to aim the spray on the UNDERSIDES of twigs to maximize coverage and efficacy.  In heavy infestations, it is still advisable to follow up with dormant oil applications in the early spring.

Something we’ve noticed in the last 10 years is how Magnolia scale doesn’t seem to be developing in a synchronous manner anymore.  If you take a close look at multiple twigs on infested trees, you will notice that this scale insect can quite often be found in varying stages of development.  In the photo above, we can see the reddish-grey crawlers, then the larger dark-grey newly settled nymphs and then even larger, light-grey more mature nymphs.

Here is another photo of the various stages of Magnolia scale nymphs, with newly settled nymphs on the left and larger, more mature light grey nymphs on the right (that likely emerged as crawlers back in early August).

In fact, just last week I was picking off some adult females and noticed that some were still a little fleshy underneath…..harbouring a few crawlers still!  This is very late for magnolia scale crawler emergence, meaning that insect development has become so staggered that crawlers are emerging from August to late September now.  This makes fall dormant oil applications an even more important tool for IPM.  Applying dormant rate horticultural oil by mid-October should give effective management of the overwintering scale 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 the our horticultural oils.   Horticultural oils may be phytotoxic in extreme temperatures so CONSULT THE WEATHER FORECAST BEFORE YOU MAKE ANY APPLICATION.

ALWAYS CONSULT THE PESTICIDE LABEL BEFORE APPLYING ANY REGISTERED PEST CONTROL PRODUCT.  The PMRA Label Database can be found HERE.

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 horticultural 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 horticultural 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)}

Pear Leaf Blister mite (Eriophyes pyri) (photo above) was found in high numbers again this year on some of our ornamental pears (e.g. Pyrus calleryana ‘Chanticleer’).  The tiny mites overwinter under the outer bud scales, where they feed on the developing buds.  When pear buds swell and start to emerge in the spring, the mites will lay their eggs into leaf tissue.  Mite feeding and egg-laying causes brown, unsightly blisters to form on the leaf surface.  Management of pear leaf blister mite is often timed for the AUTUMN to treat the mites while they are hiding among the outer bud scales.

Some mite species that overwinter as unprotected eggs on the host plant and ARE SUSCEPTIBLE to fall dormant horticultural oil applications include:
European red mite eggs (Malus, Pyrus)
Maple spider mite eggs (Acer, especially reds and silver-red hybrids)                                                    Oak Spider mite (Quercus)                                                                                                                      Pear Leaf Blister Mite (Pyrus)

{Note : the following mites do NOT overwinter as exposed eggs on host plants are NOT SUSCEPTIBLE to dormant oil applications: Two Spotted Spider Mites.  TSSM overwinters as adults in debris on various surfaces. }

Dormant horticultural applications may cause some injury on evergreen foliage during freezing temperatures.  Although spruce bud scale (on Picea), spruce spider mite eggs (on Abies, Picea, Thuja) and Fletcher scale (on 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 injuring the foliage if temperatures drop down below freezing.

spruce-spider-mite-black-kt-2                                    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.  October miticide applications can be very effective against spruce spider mite (on Thuja, Picea, Abies).  Dormant rate horticultural oil is not usually as effective in the fall and is more appropriate in the spring before egg hatch (which begins around bud swell of the conifer host).

Brunches, Trees, Autumn, Winter, Sky    Good coverage is really important for any pesticide application.  For a smothering agent like horticultural oil, it is especially important.  Wait until all the leaves have dropped to help ensure good coverage of stems and buds, and the susceptible insect and mite pests on them.  And if you can, try to time your applications for mild, sunny days where insect and mite pests are actively respiring (breathing), making them more susceptible to this smothering agent.

About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
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