Horticultural Oil is a pesticide used primarily for controlling insects and mites. The oils are manufactured either from petroleum or vegetable oils, and are very lightweight. Until the late 20th Century, heavier "dormant oils" were used for the dame purpose, but they could only be used in winter as they were too heavy to be used in warmer conditions.
Horticultural oils work by smothering the target pests, either clogging their stomatae or in the case of eggs simply preventing oxygen from moving through the oily film. Oils need to be applied directly to the pest, they have no residual effect and do not harm arthropods if ingested.
Horticultural oils are mixed with water, and sprayed directly onto the pest organisms.