Is there a DIY formula for protecting plants from insects and disease?

Cayenne pepper

1 tbsp dr. Bronner's Castile soap soap to a large spray bottle of water for controlling insects. Spray on the underside of the leaf

There are all kinds of recipes for specific remedies. Too many to list. You can do a Google search.

Depending on the problem, i've used peppermint oil. I just put it in a spray bottle (about 10 drops) with water and a little alcohol.

First of all you need to identify your insects, they could possibly be beneficial insects and you don't want to get rid of them. If your plants have other problems such as fungus or bacteria, try 100% organic Neem Oil Concentrate, it's made from the seed of the Neem tree which grows in India, it acts as a insecticide, fungicide and bactericide. Just mix according to package directions and it can be used as a foliar spray or water the plants with it for systemic treatment. It's so safe the women of India use it on their faces for beautiful skin.