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Horticultural Oils (Petroleum Base) 
Beyond Pesticides Rating: Toxic

Oils are hydrocarbons used as contact insecticides, acaricides, and ovicides. Kerosene was the first petroleum oil to be used for insect control in the early 1900’s. Most horticulture oils used today are petroleum based (Grossman 1990), yet a growing number of horticulture oils are being made with vegetable oils, which are considered a least toxic pesticide. Carefully read the label or ask your pest control service provider to determine if the horticulture oil is vegetable or petroleum based. 

Horticulture oils are effective in controlling aphids, adelgids, spider mites, mealy bugs, sawfly larvae, whiteflies, plant bugs, caterpillars, scales, and some plant diseases like rusts and mildews (Olkowski 1991). They flood insects breathing pores which lead to prompt asphyxiation and suffocation. Oils also kills an insect when it touches the outer body, or cuticle, of an insect leading to dehydration and death of the pest. 

Horticulture oil sprays are formulated with a detergent or soap surfactant in order to keep the product from separating. Adding soaps to horticulture oil products also increase the effectiveness of its insecticidal properties. There are several types of formulations of oils: dormant oils, summer oils, emulsifiable oils and stock emulsions. Dormant oils are extremely phytotoxic with varying degrees of susceptibility between varieties and species. Dormant oils tend to contain a higher level of impurities, but modern refining techniques have removed a high percentage of phytotoxic impurities (Grossman 1990). 

Horticulture oils have not been found to lead to insect resistance (Thomson 1983). 

Toxicity

Because of horticulture oils mode of action, they do not pose the high exposure risk that chemical pesticides do. Horticulture oils have relatively low mammalian toxicity (Grossman 1990). Petroleum based horticulture oils can cause skin and eye irritation (Olkowski 1991). 

Horticulture Oils, Vegetable Based

Horticulture oils that are vegetable based are equally as effective in killing certain insects as oils that are petroleum based (Grossman 1990). Vegetable based oils are similar in mode of action, application method, and phytotoxicity. Vegetable based horticulture oils do not leave residues behind like petroleum based oils.  

References: 

Grossman, J. 1990. “Horticultural Oils: New Summer Uses on Ornamental Plant Pests.” The IPM Practitioner 12(8): 1-10. 

Olkowski, W. 1991. Common-Sense Pest Control: Least-toxic solutions for your home, garden, pets and community. The Taunton Press. Newtown, CT. 

Thomson, W.T. 1983. Agricultural Chemicals - Book I Insecticides. Thomson Publications: CA.