Hi Rosie –
The following tips for treating scale on your cactus are taken from the following page:
https://www.planetnatural.com/pest-problem-solver/houseplant-pests/scale-control/.
To get rid of scale insects prune and dispose of infested branches, twigs and leaves.
When scale numbers are low they may be rubbed or picked off of plants by hand.
Dabbing individual pests with an alcohol-soaked cotton swab or neem-based leaf shine will also work when infestations are light.
Azamax contains azadirachtin, the key insecticidal ingredient found in neem oil. This concentrated spray is approved for organic use and offers multiple modes of action, making it virtually impossible for pest resistance to develop. Best of all, itโs non-toxic to honey bees and many other beneficial insects.
Horticultural oilsย and other safe, oil-based insecticides work by smothering insects and will control all pest stages, including adults which are protected from most other insecticides by their armor coverings. I have checked the label on the All Seasons Horticultural Spray Oil and found no restrictions for cactus. As always, test a small area first before spraying the entire plant.
Hope it helps!