Landscape and Nursery - More on Horticultural Oils

The following is more information on the use of Horticultural oils in landscapes and nurseries.

Some plant pests controlled by horticultural oils.

Dormant Season Applications

Aphids that curl leaves in spring
Caterpillars that winter as eggs on the plant (leafrollers, tent caterpillars)
Mites that winter on the plant (e.g., conifer-infesting species)
Scale Insects (e.g., pine needle scale, striped pine scale, Kermes scale, cottony maple scale)

Summer/Foliar Applications

Insects and Mites
-Adelgids
-Aphids
-Eriophyid mites
-Leafhoppers
-Scale Insects
-Spider mites
-Whiteflies

Diseases
-Powdery mildew
-Some aphid-transmitted viruses

Precautions

The following precautions are recommended whenever using an oil on a woody plant:

-Avoid using oils on plants that tend to be oil-sensitive. Avoid drift onto sensitive plants.
-Do not apply when temperatures are excessively high (above 100 degrees F) or low (below freezing). High temperature limitations are primarily related to the drought-stress status of the plant. Plants under stress may be damaged. Those not stressed are much less likely to be damaged by an oil application. Dry conditions without plant stress generally reduce risk of injury by oil, because evaporation is more rapid.
-Do not apply oils during freezing weather. This can cause the emulsion to break down and produce uneven coverage.
-Do not apply oils if plant tissues are wet or rain is likely. These conditions inhibit oil evaporation. High humidity (above 90 percent) also may contribute to injury risk, while low humidity generally reduces it.
-Do not spray when shoots are growing.
-Avoid treating plants during the fall until after winter hardening has occurred. Fall treatments have sometimes caused increased susceptibility to winter injury.
-Do not apply oils in combination with sulfur or sulfur-containing pesticides such as Captan or Karathane. They can react with oils to form phytotoxic compounds. Because elemental sulfur can persist for long periods, label directions on most oils prohibit their use within 30 days of a sulfur application.

Plants that tend to be sensitive to oils.

Black walnut
Cryptomeria
Douglas-fir
Hickories
Junipers and cedars
Maples (particularly Japanese and red maple)
Redbud
Smoke tree
Spruce (particularly dwarf Alberta spruce)

The neem oil insecticides (Trilogy®) have been most widely used on greenhouse-grown ornamentals. They have shown good plant safety, but there are some precautions for use on impatiens, fuschia, hibiscus, some roses, ornamental olive and some carnation varieties.

Information from "Insect Control: Horticultural Oils" by W.S. Cranshaw, Colorado State University Extension entomologist and professor, bioagricultural sciences and pest management; and B. Baxendale, Teikyo Loretto Heights University professor, botany, Denver from Colorado State University