Horticultural oils control different pests at different times of the year.
Dormant season (March to April across most of Canada): aphids that cause leaf curl in spring, overwintering caterpillar eggs (tent caterpillars, leaf rollers), overwintering mites (especially on conifers) and overwintering scale insects (especially on magnolias and euonymus).
Growing season (June to August): aphids (all species), mites and spider mites, leafhoppers, scale insects, whitefly; and many insect-borne viruses, as well as powdery mildew.
SENSITIVE TREE TYPES
Some plant species are naturally sensitive to oil sprays of any kind. Don't use horticultural oils on:
Maple (Acer spp., especially Japanese and red maple)
Japanese laurel (Aucuba japonica)
Hickory (Carya spp.)
Redbud (Cercis canadensis)
Smoke tree (Cotinus coggygria)
Japanese cedar (Cryptomeria japonica)
Butternut (Juglans cinerea)
Black walnut (Juglans nigra)
Juniper (Juniperus spp.)
Spruce (Picea spp., especially dwarf Alberta spruce)
Douglas fir (Pseudotsuga menziesii)
Cedar (Thuja spp.)