One of the most common questions I get from people who visit my garden is "If you don't spray, why aren't your plants all eaten up by bugs? Are you sure you don't spray?" (yes, many do actually ask that).
When you spray, you tend to kill ALL the bugs - both good and bad. Yup, most people realize that. What they don't realize is how long it takes for the good bugs to come back.
The bugs that eat your plants are like the rabbits of the bug world - they live short lives, and reproduce fast and in large numbers. Take for example the aphid. According to entomologist Stephen A. Marshall: in optimal environmental conditions and lacking any predators, parasites, or disease, a single aphid could produce 600 billion descendants in one season. That's a lot of aphids!
The bugs that feed on them, on the other hand - the predatory bugs - are like the wolves and lions of the bug world. Longer lived, slower to reproduce, and they don't reproduce in the same kind of numbers. Each praying mantis will only reproduce once in its life time, the male being cannibalized during mating, and the female dying after laying eggs. The egg cases hold around 200 eggs each, and many of those won't hatch due to exposure and predation.
So when you spray, the destructive bugs come back quite quickly, but the predatory ones don't, which leaves your garden very vulnerable. It can take a few years (and patience and frustration) of not spraying to establish a bug-based pest control system.
Meanwhile, it is important to ensure there are food supplies for the good bugs in early spring and late autumn. Wild fennel is a fantastic plant to have tucked away in a corner of your garden. It flowers early in spring, and late in autumn, and provides nectar, when little else is available, for the lady bug (eater of aphids), and the parasitic wasp (killer of white butterfly caterpillars). This will ensure they are in your garden before the aphids and white butterflies are.
It is also important to provide shelter for over-wintering - someplace that isn't going to be disturbed by your yard and garden activities - to keep them there, so they are in your garden before the pests arrive. Pinterest and the internet in general are full of ideas and instructions for building 'bug hotels', from the very simple to the very elaborate.
Of course, it doesn't mean the good bugs will do all the work - although they are more efficient than spraying (my neighbour sprays profusely, and still complain bitterly about the bug problem). You still need to do your morning rounds - picking the stink bugs out and dropping them in a bucket of soapy water, collecting the snail and slugs you find and putting them out on the pavement or drive for the thrushes to eat, wiping off any white butterfly eggs you find and killing any of the caterpillars you see, and pulling out anything too heavily infested with aphids.