Gardening, like everything else in nature, is based on cycles that are all interdependent. If you want to get on top of insects in your garden you have to know why these pests get out of control; in short, know something about the dynamics of balance.
Perhaps your plants were not healthy enough? Did you overwater? Underwater? Or transfer disease when watering using sprinklers? Were there chemical residues in your soil? Did you plant fast growing herbs amongst your vegetables as companions as they may have sucked all the goodness out of the soil?
Have you been encouraging pests by planting large areas of just one crop instead of intercropping? Intercropping and rotation help keep some diseases at bay. Did you remove diseased plants or leaves at the first sign of a problem? Is your bug covered with a water-repellent waxy coating letting sprays bounce off?
You need to look at the season and what you are trying to grow. Are you planting cool season crops like brassicas in the middle of summer when they will be heat and water stressed and at a time when the cabbage white butterfly is busy procreating?
There is no better time than now to begin your insect lifecycle education. You will discover it is not the pupae or the eggs that cause damage, but the larval stage. Larvae are vulnerable to parasites and predators such as birds, spiders, wasps, praying mantis, and their soft skin is vulnerable to osmosis. Insects often act as vectors spreading viruses with leafhoppers, aphids, white flies and mealy bugs being some of the worst offenders.
It is true that plants have evolved chemical defenses against insects, making chemicals that are detrimental to some of them. Plants even produce insect hormones that can cause developmental abnormalities. But in nature everything evolves and changes, and insects have also evolved mechanisms to detect and avoid plants with defenses. Luckily for gardeners working with small gardens there is no bug that eats everything, and a diversity of plants encourages a good mix of insects to help the predator-prey cycle work.
Organic plant protection requires more than pest repelling plants or homemade sprays or for that matter commercial chemical sprays. Even organic sprays such as garlic can cause problems in the garden eco-system if used constantly.
However, at the end of the day you need to discover the science behind insect lifestyles and how minerals help keep plants healthy if you want to have better control of pests and diseases in your garden.