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First published 2nd November 2021

I’d never be without this classic remedy for arthropod* pests. Simple and dirt-cheap to make, lasts forever, and with the added bonus that arthropods* can never develop resistance to it.

First, the recipe:

Half cup detergent to two cups any vegetable oil (don’t waste your good stuff — the cheapest no-name you can buy does just as good a job!)
Shake well until solution turns milk-white, as seen in the 500 mL bottle at left in above photo.

This is your concentrate and will last a very long time.

How to use:

Add one tablespoon to 500 mL water in a spray bottle like the one at right in the above photo.
Spray all over pests such as aphids, mites, scale, mealybugs, stinkbugs…

How it works:

Arthropods* exchange gases through spiracles, or small holes, along their abdomens. Suffocation occurs when these spiracles are blocked — something an arthropod cannot evolve a countermeasure to.

White oil acts by blocking these holes — the oil does the blocking and the detergent is the ‘glue’ by which the oil sticks to the body long enough to cause suffocation.

Important:

It is best to spray after the heat of the day has passed, or when temperatures are below 25–30 °C, as this prevents foliage overheating.

Overheating occurs when an oil drop acts like a lens by concentrating light energy onto a small piece of plant tissue.

It doesn’t matter how late in the day you spray, as the arthropods* aren’t going anywhere and are always breathing! White oil will work throughout the night just as well as in daylight.

*I keep saying ‘arthropod’ whereas others would say ‘insect’. But mites are arachnids, and related to spiders, scorpions and ticks. Insects are arthropods, and arachnids are arthropods, therefore ‘arthropod’ is just a more accurate word to use!
(Fun fact: ‘Arthropod’ comes from Ancient Greek and means ‘jointed foot’, and lobsters and crabs too are arthropods!)