cacti dont seem to like oils much, i have used neem on cacti before, and it works ok if they are left in the shade for a day or two.. always spot test oil on new plants! most oils just smother insects/fungi's. lots of folks add a drop of soap to oil sprays to spread it out, also, a good time to foliar feed your nutrient loving plants (ie-mix oils with foliar nutes) if you have serious issues, adding insecticide to the oil will help. or just use neem(most neem oil has some active in it, others are an extract of neem, containing oil and a %of pesticide) for "knock-down" oil foliar, add pyrythiens(sp) if fungicide is the reason oil is applied, washing it off is counter productive if for mites, give it time to smother eggs before washing off!