One homemade insecticide that can really do the job is 70% rubbing alcohol (isopropyl alcohol). Mix 1 part rubbing alcohol to 7 parts water and spray it on plants affected by aphids, mealybugs, thrips, whiteflies, etc. The alcohol will melt the protective wax that covers certain insects and dries the soft body parts of others, leading to their death. Furthermore, alcohol spray tends draw mobile insects out of their hiding places, making them easier to control. Simply spray the solution to saturation, covering all surfaces, including stems, both sides of leaves, and especially leaf axils where so many pests tend to hide.
You can also add a few tablespoons of rubbing alcohol to insecticidal soaps and to other homemade insecticides to increase their effectiveness.
This treatment is most effective against nymphs and adults, but, depending on the species being treated, doesn’t always work on eggs and pupae. If so, they’ll soon awake to try and retake control of your plant, so you’ll have to spray again every week or so until you no longer see any pests.
A Tip to Ignore: Cotton Swab Versus Mealybug
On other sites, you’ll see the recommendation you can control mealybugs by touching each with a cotton swab dipped in rubbing alcohol… but I’m not going to waste your time. That simply doesn’t work. Yes, directly touching the insect with alcohol will kill it, but you’ll only be treating the most visible pests. Others are always hiding in places where you can’t see them and soon the infestation is back again, as bad as ever. Spraying with alcohol is more likely to get to all the insects and thus to be effective.
Beware of Intoxication
Yes, I know rubbing alcohol is a pharmaceutical product widely used to in hospitals to rub down bedridden patients. It is also the main ingredient of many hand sanitizers. Even so, it is poisonous and you can become intoxicated by rubbing alcohol fumes if you use it in an enclosed area. There is no problem with using it outdoors, but always ventilate the room when you use it indoors.
Whisky Versus Pests
Of course, you can replace rubbing alcohol as an insecticide with just about any other type of hard liquor… but I’m not sure that killing a few aphids is really a good use of 20-year-old Irish whiskey ! Rubbing alcohol is much more affordable.