I mush prefer using Eradicator for gnats, but on the topic of killing things you can make up White oil easily and cheaply.
Horticultural oils are among the safest and most effective ways of controlling a range of sap sucking and chewing pests in the garden. These days there are two options - traditional white oil, which is based on a vegetable oil, and modern horticultural oils, which are thinner and based on petroleum oil.
The lighter coloured, modern oils spread faster than the old fashioned vegetable oil. This is important because it means that the modern, less viscous, oil doesn't clog leaf pores when the temperature is over 30 degrees. So in hot weather the newer oil won't damage your plants.
The other difference is that the modern horticultural oils are based on petroleum and are expensive to buy. White oil is cheap and better still, you can make it yourself.
White oil recipe
- Use two cups of vegetable oil (I'm an organic gardener, so I use sunflower oil because there's no chance that it could be genetically modified) and half a cup of washing up liquid.
- Put those into a jar. Put the lid on and give it a good shake and you've got concentrated white oil.
- Label it and store this in a cool, dry place and it should last about three months. Remember to also label it with the dilution rate - two dessert spoons per litre of water.
Gardeners have been using oils for about 200 years so it's been thoroughly road tested. Use these oils on broad leaved trees and shrubs. Don't use them on hairy or soft leaved plants like lettuce, because it will burn the leaves. The best spraying action is to cover both sides of the leaves and the bark and it's best to spray in the cool of the morning.
Horticultural oils will control aphids, scale, mealy bug and citrus leaf miner as well as caterpillars (works on Mites to). It works by blocking their breathing pores and this suffocates the pest. There is no way that pests will ever become resistant to white oil - so it's good forever.