You may have seen pyrethrum spray in your local garden centre. It is a commonly used and effective insecticide derived from the pyrethrum daisy. In fact you can make some yourself. The easiest way is to soak pyrethrum flowers in water in a dark place for 12-48 hours. A more effective but time consuming way is to suspend pyrethrum flowers in water for 24 hours then dry the flowers. Once dried, crush the flowers (you can do it in a blender) and either use the powder by scattering it over plants that are to be protected or you can soak a couple of teaspoons of the powder in warm water (2-3 litres or 0.5-0.8 gallons) for a couple of hours and then use it as a spray. This spray will be 2-4 times more potent than if you make an infusion with fresh flowers. Either way, the spray will only last a few days so it needs to be made up regularly and growing your own pyrethrum flowers is thus very useful if you want to make your own spray. You can also add small amounts of detergent to help the spray 'stick' to foliage.
Pyrethrum sprays area available commercially but be very careful as many of them contain non-organic additives.
Pyrethrum is effective against a variety of soft and hard bodied pests but apply it in the morning or evening as it breaks down faster when it's warm. Pyrethrum is also harmful to ladybirds and a variety of predators that control aquatic pests so even though it can be sprayed or sprinkled on plant foliage, it is better to only use it directly on pests. Never use it near ponds or waterways.