Host plants - Swallowtail (Papilio Machaon)
The below descriptions applies to Swedish nature conditions
The two main host plants in the wild are for the butterflies living at the coast line, Garden Angelica, and for the butterflies living on the wet lands inland, Milk Parsley.
Garden Angelica, is an about 0.5-1 m high very robust plant. It carries a lot a lot of big leaves and is therefore maybe the best host plant for the larvae.
Milk Parsley is a slender, about 1 m high, wetland plant which the larvae love too.
Both the above host plants are also excellent egg laying plants for breeding purpose. For egg laying smaller, own grown, plants are best since big plants requires big cages which is not practical. After hatching, the larvae can carefully be moved with a small brush to a bigger plant (which is also best to grow by yourself to avoid lot of searching and digging in the wild). Or even better, you place your egg laying plant besides and in contact with the bigger plants and the larvae will move over themselves to he bigger plant after eating some on the smaller plant.
Both Garden Angelica and Milk Parsley plants are easy to cultivate. Best germination result is achieved if seeds are planted in the late summer or autumn and kept outside during winter. The best soil is a mixture between normal plant soil and peat. Both the plants prefers half shade and the soil should be kept moist especially for the wet land plant Milk Parsley which prefers wet soil.
Fennel, which can be bought in most shops selling seeds, is also an excellent host plant for larvae and for egg laying and is also very easy to grow.