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Oil sprays have been used since the late 1800's for the control of various insect pests of fruit and shade trees. As early as 1880, kerosene-soap emulsions were used for control of aphids and other soft-bodied insects. By 1923 the successful use of a 2% light lubricating oil against the San Jose scale was reported. Since then more highly refined oils with most of the sulfur compounds removed have been developed and field tested with more effective results.
The winter season is the time to apply dormant oil sprays to plants, for the control of insects and disease. Covering the dormant stems of a plant with a highly refined oil such as Lilly Miller Spray Oil suffocates overwintering insect eggs and fungal spores. Before you spray make sure to clean up the fallen leaves around your plants and remove any leftover fruit that may be still hanging on your trees. Roses can also benefit from the use of horticultural oil sprays in the control of black spot, powdery mildew and rust. Powdery mildew can be effectively controlled by using a mixture of 2 Tsp. of Baking Soda with 2 Tbs. of Horticultural Oil in 1 Gallon of Water.
More recently summer oil spray rates have been developed for growing-season applications. Control of mites and scale on citrus, and powdery mildew on roses are two of the most common applications. Be sure and read the label on your dormant oil for warnings of phytotoxicity on selected plants. Ferns are particularly sensitive to oil applications and conifers can be burned by an improper application. Spraying on hot or humid days can also cause adverse reactions especially if the plants are under water stress.
Horticultural Oil sprays are one of the weapons that Integrated Pest Management (IPM) practitioners use in the control of harmful insects and diseases. Other IPM strategies include natural biological controls, such as lacewings for aphids or Creptolaemus montrouzieri for mealybugs; cultural controls such as the proper timing of soil cultivation and debris cleanup; the health maintenance of plants including the proper amounts and timing of irrigation and fertilizers; and when necessary the application of botanical pesticides on target pests at the proper time and dosage. All of these strategies help minimize damage to the environment, maximize pest control and can reduce the cost of pest control.