Be very careful with the homemade pesticide recipes that circulate in amateur gardening circles. Yes, they can sometimes be effective, but often they are nearly worthless. Worse, there are cases where the pesticide is so toxic that the person who applies it becomes sick or can even die.
A telling case: nicotine insecticides. These were withdrawn from the market in most countries some 30 years ago because of their toxicity to humans, but ever since, recipes for nicotine pesticides have been abundantly seen in print and on the Internet. Usually they involve making a decoction by soaking cigarettes, cigarette stubs, or tobacco leaves in water for a given length of time, then spraying this product onto plants to kill insects. The same people who kindly share this information ought to also mention that a few spoonfuls of such a decoction can kill a child!
I’m not trying to prevent you from using or making homemade pesticides, but anytime you use a product because you believe it is toxic to insects or kills diseases, it is quite possibly also toxic for humans and should be used accordingly.
Never forget that biological pesticides are often as toxic chemical pesticides: a poison remains a poison, whatever its origins. Strychnine is derived from a plant, for example, and yet it is one of the most virulent poisons known.
One way of reasoning about a homemade pesticide’s possible toxicity is to ask yourself if the ingredients are normally toxic to humans. Homemade pesticides made from garlic, soap, hot peppers, baking soda, milk, etc. probably won’t not be too toxic (at least when they are used fresh and well diluted) because their ingredients are normally harmless to humans when used adequately. Still, they can be irritating to the skin and the eyes, so protective clothing is always recommended during application. Decoctions made from rhubarb, tomato or potato leaves, on the contrary, are indeed derived from natural products, but ones that are known to be toxic to humans: they should be used with caution and kept out of the reach of children and pets.
Personally, I try not to use pesticides, whether chemical or biological. My main treatments against insects and diseases are:
1. Rinse plant with clean water to (hopefully) reduce the number of pests;
2. Pull out and destroy the infested plant infested before it infects its neighbors;
3. Plant varieties that are naturally resistant to insects, slugs and diseases.
That’s mighty simple advice, but usually surprisingly effective.