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Using horticultural oil to control insects
Horticultural oil is a great way to control many insects on plants. The oil acts to smother the adults or egg stages of several different species of pests. Horticultural oils are refined petroleum oils combined with an emulsifying agent. Some plant-derived oils also are used.  Depending on th
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e weight of the oil, you can use them all year or just in winter.  Be sure to read the labels carefully.  Horticultural oils can damage some sensitive plants.  If you use the oil that is made for use when plants are dormant, during the growing season, you can cause severe damage or even death to your plant.
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 the oils can help treat and prevent:
  • Powdery mildew
  • Some aphid-transmitted viruses

The following plants are sensitive to the oils, use with caution.

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)
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