On the Western Coast of Schleswig-Holstein 25686 ichneumon flies belonging to 251 species were caught in traps or reared from their hosts (43 species). They parasitise spider egg cocoons (45% of individuals), pupae of flies (36%) and larvae of moths (12%).Usually more than half (54%) of the parasites emerging from their hosts are females. The males, however, show greater activity and predominate in the breeding areas (25% ♀♀). On the other hand, the females spread out over a greater area and therefore predominate in localities far from the breeding areas, e.g. on light ships and on shoals (80% ♀♀).More than 20 species of ichneumon flies live in salt marshes. They parasitise egg cocoons of Micryphantidae and Linyphiidae, pupae of flies (Scatophagidae) and larvae of Microlepidoptera. All groups of Arthropoda that can act as hosts to ichneumon flies are attacked in the salt marshes too. Nearly all of those species live in polders too, but are less abundant.More than 70 species immigrate into the polders after the building of dykes. Most of them live on weeds. They spread out after the immigration of such plants and recede after cultivation.All common parasites of insect pests of agricultural plants have been observed in young polders since the beginning of the investigation. Some of them are able to immigrate into uncultivated polders since their hosts also live on halophytes.All common species spread very rapidly, especially the females. Thus, many species are able to find host populations within a short time.