Many horticultural professionals are gearing up for fall applications of horticultural oil. But the warm weather is causing trees to retain foliage later than they usually would, delaying dormant 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 (photo above, overwintering nymphs) populations were quite high again in the GTA 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 early spring.
In mid-late 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)}
Pear Leaf Blister mite (Eriophyes pyri) was found in higher numbers this year on 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 fall 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 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 NOTSUSCEPTIBLE 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.
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 insect and mite pests on them.
