Alcohol Pest Control
Alcohol work as a contact poison to insects by melting the protective wax on their body and drying out soft body parts.
Rubbing alcohol is effective against many 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 seven 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 unsure, test it by spraying on a few leaves and waiting 24 hours. If they don't show signs of damage, continue treating the whole plant.
Diluted alcohol does not leave residuals as it evaporates and thus does not require rinsing. If you are using undiluted alcohol, rinse the plant to prevent any damage.
When done thoroughly, often a single treatment is adequate. Repeat the treatment if you still notice bugs.
Cotton Swab Treatment
Many online outlets recommend alcohol-dipped cotton swabs to spot and treat plants infected with mealy bugs.
It certainly kills the insects, but you will only treat visible bugs while leaving those in hiding soon to return 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.
It can be sprayed at this concentration to safely kill pests without damaging 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.
Safety Reminder
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 only to 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.