As you already know, scale are hard to get rid of. The nurseryman who pointed you toward the horticultural oil was right on course. It may be that you did not apply it enough times. Apply it at least three times on a seven-day interval. There are also systemic insecticides that can be applied to the soil that will help control sucking insects such as scale and mites. Isolate this plant from any other houseplants to avoid transfer of these insects.

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