Horticultural oils are insecticides. The ultra-fine types are just more highly refined, therefore lighter weight and so can be used year round rather than just in the dormant season. Often, they are combined with a fungicide for dormant season spraying as they have little effect on disease control.

They do not prevent insects. Their mode of action is to smother soft bodied insects and in some cases, disrupt feeding patterns and alter digestive processes. They must make contact with the insect in some form to accomplish this. To my knowledge, they have no effectiveness against beetles although could have a negative impact on beetle larvae should they come into contact with that.