I'm rather fond of using eucalyptus oil in the garden - it's derived from plant materials and is very effective whether used as is, or diluted in water (with a very small amount of detergent (something environmentally friendly and low in phosphates) to allow the oil and water to mix). There's something very satisfying about finding the snails responsible for destroying a newly planted or sprouted seedling and spraying them with undiluted eucalyptus oil - sweet revenge in the form of bubbling snails.
More often than not however, diluted eucalyptus oil is more than adequate for pest control - I recommend experimenting with amounts - I usually use about 5 drops in 500 mL of water with a drop of detergent. It is effective at killing bugs such as harlequin beetles and other sap sucking insects and it is even more effective on soft bodied pests. It will also harm beneficial insects however, so you must only use it on insects that you can actually see and you must be sure there aren't any beneficial insects in the immediate area that might get accidentally sprayed.