Always work with nature rather than against it. Good insects help you in the garden. Not all insects are bad guys, chewing holes in your leaves or sucking the sap from your favourite plants. In fact, the vast majority are on your side - pollinating flowers, feeding on less desirable insects and keeping your garden healthy through cleaning up plant debris. Why kill them off through indiscriminate spraying when it's far more effective to encourage and nurture the good insects to help control the pesky ones.....
For the moment, leave aside the soil borne troublemakers like slugs and snails, and the airborne ones like German and Asian paper Wasps - these all need more targeted intervention. Look instead at Leaf Rollers, Red Spider Mites, Psyllids, White Fly Aphids and Thrips. These can all be significantly reduced in numbers by encouraging the good insects that predate them - Ladybirds, Hoverflies, Lacewings, Predator Mites, Predator Wasps, Spiders and Stick Insects to name a few!
Just as plants have a cycle of dormancy over the cold winter months before their spring growth, so do insects. They're waiting until the days are warmer and the nights shorter, before exploding in numbers to ravage the new spring growth.
To support a population of beneficial insects that target the unwanted ones, it's important to have flowering plants to act as hosts. These provide your "good guys" with desirable nectar/pollen and a place to call home when they’re not munching on the bad guys. Phacelia, Buckwheat, Borage, Cosmos, Thyme, Rudbeckia, Anise Hyssop, European Poppies, Peony Poppies, Coriander, Alyssum, Candytuft, Echium and a wide range of Umbelliferae species (flower heads shaped like umbrellas) have been identified as being the best ones to achieve this. Bees and butterflies will thank you too, for having these in your garden as they’re attracted by the smell of nectar on the wind.
Here's a few handy tips to make your garden a better place to host these good insects:
Target the key time of late spring to late summer when insect damage is a real problem.
Just like you, beneficial insects prefer a mixed diet so sow a few of these preferred flowering plants every 3 to 4 weeks from early September right through to late April in most areas.
Hoverflies, Lacewings and Predator Wasps don’t like flying great distances during their working day so having some host plants strategically placed every 7 to 10 metres is better than them being further apart.
Sowing only a few plants regularly is better than sowing no plant at all.
If you were only to sow one variety, the best one would be Phacelia, sprinkle a pinch of seeds of 1 square metre and lightly rake to cover.
All this information has to do with managing problem insects above the ground, but if soil nematodes are damaging your plants, consider sowing Marigolds in the infected area. Marigolds exude ozone through their roots which encourages the nematodes to hatch and go through their life cycle without reproducing thus cleaning out the resident population in the surrounding root zone.