I use isopropyl alcohol regularly to kill mealybugs on asclepiads, and
cactus and have never had ill effects. The person who told me about it
however warned me never to apply alcohol to a plant in the sun, but rather
to remove it to a shady place before applying, and to make
sure that all was evaporated before the plant was placed back into the
sun, otherwise the plant would suffer severe burns.