Horticultural Oil, Slow Release Fertilizer and A Few Pests


Overwintering nymphs of magnolia scale are sensitive to horticultural oil applications

Applications of horticultural oil to dormant trees and shrubs will be starting soon (daily temperatures between 4C and 12C are ideal).   If you can wait for a few mild days to pass, overwintering insects will have a higher rate of respiration and will be more sensitive to smothering agents.  Timing of horticultural oil applications are critical since freezing temperatures, mixing with sulphur and applying at the dormant-rate on actively growing tissue may result in injury.

Apply dormant oil on clear mornings to facilitate rapid drying. The oil provides a barrier that restricts both respiration and movement of overwintering insects. It is quite effective for the management of overwintering scale nymphs (e.g. magnolia scale, San Jose scale) and mite eggs (European red mite, maple spider mite on silver/red hybrids). Keep in mind that most of these insects overwinter on the undersides of leaves and twigs.

Note that scale insects that overwinter as eggs protected under adult scale are not controlled by horticultural oils (e.g. dormant hort oil applications not effective against oystershell scale, pine needle scale, juniper scale or Euonymus scale).

Since the oil droplets come out of solution so easily, frequent agitation is VERY important during application. Some plants listed as sensitive to horticultural oil applications include: Japanese maple, red maple, sugar maple, hickory, walnut, blue junipers, Colorado spruce, white pine, red oak, and to a lesser extent: yew and cedar.

Slow or Controlled release fertilizer (especially those that are injected) can start to be applied after the soils dry up a bit.  We have just had a tremendous amount of rain in southern Ontario and it could take a few days for soils to dry up enough for us to work with. It is too early to be applying fast-release fertilizer in the landscape. Wait until soils warm up and leaves have emerged to facilitate maximize uptake of fertilizer nutrients (the first major active root growth period, usually in May).


Cone-like bags suspended from twigs contain overwintering females of the bagworm and each one houses 100’s of eggs that will hatch into hungry larvae as new foliage emerges.

For those of you in areas where bagworm has been found (e.g. Essex County, Northumberland County), now is the time to collect overwinter “bags”.  The overwintering bags contain adult females that will give rise to an incredible number of larvae as new foliage is emerging.  The most common hosts of bagworm we’ve seen include Eastern white cedar, spruce and honeylocust but they can also be found on pine, juniper, black locust and sycamore.  Remove, collect and DESTROY bags housing the females before foliage begins to emerge this spring.  Many arborist and landscape companies offer bagworm clean up as an IPM service in early spring.


Overwintering bags on Eastern white cedar


Empty overwintering bag (eggs have hatched and migrated to new foliage) on honeylocust

<a href="/clm/species/pyrrhalta_viburnicola"><em>Pyrrhalta viburnicola</em></a> (Viburnum Leaf Beetle) eggs on viburnum. Now is the perfect time to remove egg masses of Viburnum leaf beetle.  Look for rows of bumpy caps on the twigs of last year’s growth.  Prune out infested twigs, collect and destroy them to reduce populations of larvae on Viburnum species.

??????????????????????????????? Overwintering egg masses of Gypsy moth can also be collected and destroyed this time of year to reduce the severity of infestations of hungry larvae.  Gypsy moth eggs masses appear as brown, fuzzy, quarter-sized masses stuck to bark and other wooden objects.  They will lay their eggs on several species of deciduous trees but also have a sweet tooth for Colorado spruce.

<em><a href="/clm/species/malacosoma_americanum">Malacosoma americanum</em></a> (Eastern Tent Caterpillar) egg mass. And if you are monitoring in the landscape this time of year, be sure to be on the lookout for Eastern tent caterpillar eggs masses.  Monitoring cherry, hawthorn and crabapple twigs for thickened, shiny silver bands that completely encircle small twigs.  Prune out, collect and destroy egg masses to prevent 100’s of larvae from hatching and feeding on foliage of beloved landscape specimens.



About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
This entry was posted in Insects, Plant Nutrition. Bookmark the permalink.

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