Dormant oil is a spray or liquid application that is applied to trees and shrubs when they are dormant to control over-wintering insects, their eggs and their larvae.
Many harmful insects will winter over in the crotches of branches and on the underside, or leeward side of branches. Dormant oil actually coats these insects with a viscous oil that prevents them from breathing. It will also kill insect eggs and larvae.
Dormant oil can be purchased at nurseries, garden supply stores, and online.
There are even organic (non-petroleum-based) dormant oil available.