Horticultural oil products smother pests such as overwintering scales, mites, aphids, and peach twig borer. The active ingredient in most horticultural oil (also called dormant oil, insecticidal oil, narrow-range oil, or superior oil) is paraffinic or petroleum oil. Horticultural oils can be used alone as dormant oils or growing-season sprays. They can also be added to fungicides or bactericides as “spray adjuvants” to help those ingredients adhere to the tree bark. Dormant oil is an insecticide and miticide used to kill soft bodied insects such as aphids, thrips, scales, and mites. It is also a preventative insectide because it can smother insect eggs (including codling moth eggs). The oil must be sprayed to immediately contact the insects, mites or eggs; the oil is not effective after it dries.
Dormant spraying is a key step in preventing damage from insect pests, fungus, and disease. Peach Leaf Curl is a common disease of peaches and nectarines in California. Applying Horticultural Oil and Fungicide/Bactericide three times during the dormant season will greatly reduce the chance of infestation.
You can use the following guidelines and holidays as a reminder for spraying:
- The first spray should be just after most of the leaves have dropped (Thanksgiving)
- The second spray should occur at full dormancy (New Year)
- The third spray should occur at bud swell (Valentine’s Day)
The holidays generally correspond with the life cycle of deciduous plants, but always use your own observation to decide when to spray. Additionally, you should research the specific species of fruit or nut trees to learn about their particular needs and any precautions you should use when dormant spraying them.