Horticultural oil sprayed on fruit trees is normally done during the dormant season of fall and winter to kill overwintering scale, mites, tent caterpillars, aphids and leaf hoppers.  While "Horticultural" oil can be used during the growing season in certain situations, it's a good idea to spray your fruit trees during dormant time, as it is easier to cover the entire tree without the leaf coverage and this will help prevent pest outbreaks the following spring and summer months. 

The oil blocks the air holes which the insect breathes causing them to suffocate.  In some cases, the oil acts as a poison and interferes with the insects metabolism.

Horticultural oil can be combined with fungicides or bactericides such as Copper to treat Peach Leaf Curl in peaches and nectarines.  But don't mix with a sulfur containing pesticides which can react with oils and cause damage to newer growth.  Be sure to read all labels first!

Milder climates benefit from three sprayings during dormant season.  We do this as soon as the leaves drop in the fall, Mid-January and again when the first pink shows through on the buds, but not once the buds open.  Cold climates can spray once in the spring and that does a sufficient job. Saturate the tree thoroughly until it begins to drip, making sure to cover the undersides of the branches, trunks and especially the crotches of the tree that seem to provide a safe haven for pests and eggs. 

Some guidelines to use when spraying Horticultural Oil.  Do not spray oil on windy days or when temperatures are above 90 degrees or below freezing.  This can cause plants to stress or they could be damaged.  Spraying when plants are wet or rain is likely can inhibit oil evaporation.