What is a natural insect repellent?

For insects, the best thing you can do is get a house gecko to eat them. Especially roaches and termites, the house gecko is the way to go.
It depends. There are a hundreds of different folk-lore type deterrents, some have been used for centuries. Most of these have not been scientifically tested to see how effective they are but many individuals swear by them. Of course some people say they don't work so well. It all depends on which one you use, on what and how well you use it. Naturally, if you don't apply them right they won't work so great, if at all. In more recent years the scientific community has also come up with many forms of biological and natural deterrents. There have been cases were government agencies have used biological pest control methods with great success. Such as here in Northern MN. There is a caterpillar up here called the army worm that was a major problem back in the 50s. They developed a fly that lays eggs on the worm and for about the last 50 years, to my knowledge, the army worm has not been much of a problem. If they had gone around spraying all the woods and forests with insecticide it would have cost millions, been an ecological disaster and would likely have killed several people in the process. Unless you kill every single one throughout thousands and thousands of square miles, they will just keep repopulating! Personally, I believe in natural pest controls because chemical sprays can be hazardous to the environment and of course to you and your family. The chemicals also kill the pests natural enemies which can actually cause them to increase! The fire ant down south is a good example of this. They used tons of chemical insecticides to try to eradicate it and in the process killed several people and livestock but instead of wiping it out, the fire ant spread like wildfire because their natural enemies had been pretty much wiped out by the insecticides.