Dont spray anything until you know what kind of pest you are trying to control.
Anyways, an emulsion should work oils, fats, like soap or neem oils in water is an emulsion. Pests cant become resistant to it because it blocks the spiracles and suffocates them. You could use Doc Broners soap solution, couple teaspons to a gallon and it has peppermint in it to repel insects. And I like it for hand soap, leaves a bit of a film on my hands but it doesn't dry them out as bad.