Rubbing alcohol is known to be effective against a wide range of plant pests including aphids, thrips, whiteflies, mealybugs, spider mites, etc.
It is most effective when used in a spray bottle. Mix 1 part rubbing alcohol (70% Isopropyl Alcohol) with 7 parts of water. Spray the solution thoroughly on infected plants, covering all surfaces like stems, leaf axils, the underside of leaves, and leaf surfaces.
The alcohol spray will also draw insects out from their hiding, so be ready to spray for another time.
There is no exact formula for diluting the alcohol solution. You can use a 50/50 mixture when treating a more hardy plant. Use a 1:7 mixture if the plant has soft, sensitive leaves.
Some gardeners even use undiluted alcohol to treat more serious pest infestations.
If you're unsure, test it out by spraying on a few leaves and waiting for 24 hours. If they don't show signs of damages, continue treating the whole plant.
Diluted alcohol does not leave residuals as it evaporates thus does not require rinsing. If you are using undiluted alcohol, rinse the plant to prevent any damages.
When done thoroughly, often a single treatment is adequate. Repeat the treatment if you still notice bugs.
Cotton Swab Treatment
Alcohol-dipped cotton swabs are often recommended from many online outlets to spot treat plants that are infected with mealy bugs.
It certainly does kill the insects, but you will only be treating visible bugs while leaving those in hiding soon to come back again for another infestation.
Spraying with alcohol is more effective and likely to get to all the insects.
Is Rubbing Alcohol Safe to Use on Plants
Isopropyl alcohol at 70% is commonly sold for antiseptic use.
At this concentration, it can be sprayed to safely kill pests without doing damage to plants. There are, however, some more sensitive plants, so diluting the solution can be a safer approach.
Alcohol should only be applied to spraying for pest treatment. Do not water rubbing alcohol down into the soil, which will damage or kill the plant. Normal spraying will not drench the soil.
Even though rubbing alcohol is commonly used in hospitals and as the main ingredient in most hand sanitizers, there is a risk that you can get intoxicated by the fumes if you use it in an enclosed space.
Make sure to only use the spray in a well-ventilated area, wear a mask and goggles to avoid getting them into your eyes.
Most importantly, alcohol is flammable, never use them near open flames.