In all cases no matter the pest there is way to deal with the problem and not reach for the chemicals. With most pests you can introduce a biological control, in other words adding something to feed on the pest that will not have an adverse effect on the garden. Every year we buy ladybirds for our glasshouses just as we start to see the first signs of aphids. The ladybirds will make quick work of the aphids. Sadly, once the aphids have all been eaten the lady birds will move on. Spraying your plants with garlic will also deter aphids. For slugs use nematodes which are watered into the soil and will eat the eggs and larvae of the slugs. We have a very large pond in the garden and being in Lincolnshire have dykes around the nursery, this means we have lots of frogs. They help us keep slugs at bay and we keep little pockets of damp patches in the glasshouse to encourage them.