My Hatiora salicornioides has a light infestation of soft brown scale. It reacted negatively in the past when it was sprayed with insecticidal soap. Is there a non-systemic insecticide that can be used safely with cacti? (I believe systemics are no longer available in my area.) For what it's worth the cactus does not appear to be very hospitable to the scale as they seem to die at young age. My guess is once the cactus skin hardens the scale can no longer feed and so dies. Nevertheless they eke out an existence on the younger growth.