Organic Pesticide for Fungus Gnats by: Countryfarm Lifestyles
From what you have said, it sounds as if the gnats that are causing the problem are fungus gnats. There are several ways in which you can eradicate this pest from your houseplants.
First of all, they flourish in moist soil. Therefore to minimize the risk of infestation allow the soil of your plant to dry out in between watering. Secondly, put a half inch layer of fine sand over the top of the soil to prevent the flies laying their eggs. Lastly, pyrethrum or nicotine sprays will get rid of them.
HOMEMADE NICOTINE SPRAY
1 cup liquid dish soap
1 cup antiseptic mouthwash
1 cup chewing tobacco juice (Place 3 fingers of chewing tobacco in an old nylon stocking and soak in a gallon of hot water until mixture turns dark brown.)
Put mixture into a 20 gallon sprayer and fill the rest of the container with warm water. Spray on vegetables every alternate week.
HOMEMADE PYRETHRUM SPRAY
Pyrethrum is by far the most effective natural insecticide and miticide there is and kills most insects including fungus gnats.
As soon as insects come into contact with the spray, they will die, attacking the nervous system. However, because it is not systemic and is destroyed by UV light, it is not deadly to mammals and can be safely sprayed on vegetables and fruit when picked 12-48 hours after application. Unfortunately, it is also deadly to good bugs as well, so use with care and only spray on heavily infested plants as a last resort.
Spray both the upper and lower surfaces of the leaves, because spray must directly contact the insects that hide in leaf sheaths and crevices. Spray in cooler temperatures as it is more effective then, evening or late afternoon being ideal. Never use pyrethrin products around waterways and ponds. And for those of you who have dermatitis, asthma and sinus you may have an allergic reaction when using pyrethrum.
Mix 10 gm of pyrethrum powder to 3 litres of water. Let stand for 3 hours in a dark place. Add 1 teaspoon of dish washing liquid as a fixative just before use.
You can also steep the pyrethrum flowers in a jar filled with boiling water, and apply after it has cooled down.
However, the best way is to cover the flowers in a container with brandy, although kerosene and mineral oil will also work but not as well. Leave this mixture overnight keeping it away from any light. Strain the liquid and use 1 part liquid to 6 parts water.
To store your flowers place them in a freezer up to 6 months and use accordingly.