THE INFLUENCE OF SOWN HERB STRIPS AND SPONTANEOUS WEEDS ON THE LARVALSTAGES OF APHIDOPHAGOUS HOVERFLIES (DIPT., SYRPHIDAE)

The influence of sown herb strips on larval stages of aphidophagous sy rphids was studied in different strips and adjacent winter wheat field s near Berne (Switzerland) from 1993 to 1995. The aim of this study wa s to learn whether these sown herb strips offer a supply of aphids suf ficient enough to sustain the first generation of syrphids in spring o r a further generation after crop harvest. Aphid infestation of the st rips occurred at about the same time as in winter wheat, between the e nd of May and the beginning of August. A first syrphid generation in s pring can therefore not develop on weeds in strips. Furthermore, after harvest no further generations will appear in strips. The highest num bers of syrphid pre-imaginal stages in 1993 and 1994 were found on spo ntaneously growing weeds (Rumex obtusifolius, Cirsium arvense and C. v ulgare). Among the sown species, Centaurea jacea and Pastinaca sativa showed the highest syrphid larvae densities. The densities in herb str ips (max. 0.6 larvae/m(2)) were manyfold lower than in wheat (10 pupae /m(2)). In herb strips the same syrphid species as in winter wheat wer e identified. In both habitats Episyrphus balteatus was the most abund ant species. Epistrophe spp. were only found in strips. No dependence of oviposition within wheat fields on the distance to the strips could be demonstrated. Due to their great mobility, it is easy for adult sy rphids to find aphid colonies within a crop field. The strips are neit her significant for an early development of the first syrphid generati on nor for an additional generation after wheat harvest. The importanc e of herb strips for hoverflies lies in their quality to supply them w ith pollen and nectar, which increases the fitness of the adults.