Which beneficial insects help against aphids and how do I protect them?

Several natural enemies feed heavily on aphids. The main ones are ladybird beetles, both adults and their alligator shaped larvae, green lacewing larvae, hoverfly maggots, and tiny parasitic wasps that lay eggs inside aphids and leave swollen brown mummies. The most powerful and free thing you can do is conserve the ones already in your field. That means not spraying broad spectrum chemical insecticides at the first sight of a few aphids, because those sprays kill the helpers faster than the pest and then the aphids bounce back worse. Grow flowering border plants and let some flowering weeds remain to give the adults nectar. In greenhouses and some open crops you can also buy and release lacewing eggs or ladybirds, but releases work only if you stop using insecticides that would kill them. If you must spray, choose a selective option like a soft soap or neem spray that is gentler on natural enemies, and spray only the bad patches. Healthy populations of these predators can keep light aphid attacks in check on their own. Ask your KVK or a bio control lab about purchasing releases.