Pesticides in pure oil? I guess you mean pesticides dissolved in some kind of volatile organic solvent, like white spirit. Spraycans with this stuff are safe to use on cacti as long as the can is held at some distance from the plant (at least 2-3 dm). If held too close the plant can get terrible damages (I know that for sure ...).

The other form is emulsions of pesticide/solvent in water, the type where you add a few ml of a concentrated solution to 1 l of water and mix well. They are safer to use for the inexperienced user. Soap (with or without oil) solutions/emulsions work too, as long as the bugs are sensitive to it, of course.

I can't remember which is it - Malathion or Diazinon? - that are harmful to all Crassulaceae member plants....

There are a number of insecticides (the organophosphates, for one) that are particularly nasty, so do be careful when you do use them.

I think what the original poster is eluding to is the unsightly effects of certain chemicals to the skins, spines, and waxes found on succulents.

Yes, oily residues can leave ugly spots and markings on the surfaces of some species. I am not aware of any harmful systemic chemicals. Only certain topical sprays can blemish the plants. Avoid using sprays on anything with waxy or powdery leaves (Echeveria, Crassulas, etc) or on hairy cactuses (Mamm plumosa, Ariocarpus fuzz, etc) or cactuses with waxy skins (like Copiapoa). Then there are plants like Adenium that have plain leaves which become discolored easily with certain chemicals. If you're unsure the only way to know is trial and error.

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