Boletín de la AeE

Fragmentation of natural ecosystems is one of the most evident consequences of modern agricultura leading to species loss and dismption of food webs. Pest outbreaks can also be expected to occur more often in cleared agricultura] landscapes than in diverse áreas.

We tested the influence of differentially sown and naturally developed field margin strips and fallows on insect comrnunities of oilseed rape in landscapes of southern Lower Saxony (Germany). Analysis included the herbivore-parasitoid communities of inflorescence feeders, seed feeders and stem borers. Identification of the species is based on plant dissection and subsequent rearing of the insect larvae found.

While parasitism of the seed weevil Ceittorhynchus assimilis (Col. Curculionidae), pod gall midge Dasyneitra brassicae (Dipt. Cecidomyiidae) and stem boring weevils Ceutorhynchus pallldactylus (Col. Curculionidae), Cei4torhynchiis napi (Col. Curculionidae) and Psylliodes chrysocephola (Col. Chrysomelidae) indicates no significance of field margin infiuence, distinct effects could be detected on parasitism of rape pollen beetle.

Comparision of types of field margin strips showed that the parasitism of the pollen beetle M. aeneus in oilseed rape stands increased only when strips were naturally developed and oíd. Results from manually established oilseed rape plots in different habitats emphasized significance of area-effect. Rate of parasitism in rape crop was 26.5% in rape crop fields, 33.0% in field margins and 49.3% in large oíd fallows that had the same vegetation like the field margins (six years oíd and naturally developed).

In conclusión, diversificatión of the landscape by oíd strips or fallows adjacent to crop fields does not only promote biodiversity but may often improve biológica! control.