If you don't like the idea of the backpack sprayer, you can always get a 2 gallon pump model and carry it on a garden cart secured by bungee cords. I typically use a garden cart as it takes a lot of walking to go back to the barn to retrieve anything, so I outfit a cart with various items that may be needed and it trails around behind me. That included a sprayer secured in one corner for spot spraying. Of course the mix is generally something like Neem as I do my bulk spraying with a sprayer on a tractor, by sometimes you need to pop those aphids (or whatever). This is just another idea, so you can have options. I have thought of building a robotic cart that would follow a few feet behind.