Slugs and snails can be a major pest in some gardens and though a polytunnel can help you to control the pests somewhat, some are bound to slip through. If you discover that you have a bit of a slug problem in your polytunnel never fear – there is plenty that you can do to return the natural balance and keep slug numbers within bounds.
Far and away the best thing to do to keep pests in check is to do what you can to keep nature's balance intact. When the balance is intact, slugs and snails tend to remain in small numbers only, making any problems small ones. There may always be a few slugs in your garden, but having a few is not usually a massive problem. Problems really only arise when there are so many slugs and snails that you end up losing a number of valuable plants or a good portion of the crops you are trying to grow in your garden or polytunnel.
To make sure the balance in your garden is established and maintained, gardener's must do what they can to garden organically, to help protect predatory wildlife, which will help to keep slug and snail problems at bay. Some birds, frogs and toads and some small mammals can help to keep slug and snail numbers down. (If you keep chickens, ducks or geese, these will also dispose of some slugs for you if left to roam around, and perhaps at certain times of year inside, your polytunnel. To keep slug and snail numbers down, then, it is most important to do what you can to encourage their predators.
If you notice slugs in your polytunnel, you could, for example, place a pond close by so that your polytunnel will be 'guarded' by frogs or toads, get some chickens, or place feeders, a bird bath and nesting boxes close by to encourage specific bird species that eat slugs, such as thrushes. Of course, you should also encourage birds and other wildlife by planting things, within and around your polytunnel, that will attract them throughout the year. Plants with winter berries, for example, and a wide range of native plants used in a range of ways by native wildlife. Improving biodiversity inside and outside your polytunnel will slowly help to balance the ecosystem and keep slug and snail populations within more acceptable levels.
In the short term, you can trap and remove slugs from your polytunnel by using old tricks like half a grapefruit or a container of beer. You can plant chives, fennel, anise or rosemary, which are said to repel slugs and may help manage them to a degree (though they are not infallible). Collars and cloches will help protect precious plants. Go out with a torch at night and you will be able to see and remove the culprits and also be better able to gauge the scale of the problem.