This is all about lifecycle management and good coverage. It doesn't matter whether you use Neem oil or soap or if you add isopropyl alcohol (dessicant) or mineral oil (surfactant).
Neem oil is described as
It reduces insect feeding and acts as a repellent. It also interferes
with insect hormone systems, making it harder for insects to grow and
lay eggs
Dish soap is described as
How soaps and detergents kill insects is still poorly understood. In
most cases, control results from disruption of the cell membranes of
the insect. Soaps and detergents may also remove the protective waxes
that cover the insect, causing death through excess loss of water.
So they act in a different manner on insects and this can work to your benefit. Scale insects can multiply rapidly with all stages of their life cycle found at the same time. You need to maximize coverage and effectiveness by alternating spraying with neem oil and then spraying with soap and water on a four or five day cycle depending on room temperature. The hotter is it the faster the scales will reproduce. The treatment plan will need to be continued for tough to reach infestations for several months.
Other tips:
- when using soap and water, wait a few minutes and then wash with water as the fatty acids in the soap can build up and burn plant tissue
- make sure ants are not farming the scales. Prevent access with diatomaceous earth around the base of the pot if this is likely
- qtips! dip them in your control agent and physically rub the scale off where you can see it.
- be prepared to get rid of some plants. If the plants have scale that is really deeply embedded you may not be able to remove them. Cut your losses and toss it.
