The influence of sown herb strips and spontaneous weeds on the larval stages of aphidophagous hoverflies (Dipt., Syrphidae)

Abstract

Abstract: The influence of sown herb strips on larval stages of aphidophagous syrphids was studied in different strips and adjacent winter wheat fields near Berne (Switzerland) from 1993 to 1995. The aim of this study was to learn whether these sown herb strips offer a supply of aphids sufficient enough to sustain the first generation of syrphids in spring or a further generation after crop harvest. Aphid infestation of the strips occurred at about the same time as in winter wheat, between the end of May and the beginning of August. A first syrphid generation in spring can therefore not develop on weeds in strips. Furthermore, after harvest no further generations will appear in strips. The highest numbers of syrphid pre‐imaginal stages in 1993 and 1994 were found on spontaneously growing weeds (Rumex obtusifolius, Cirsium arvense and C. vulgare). Among the sown species, Centaurea jacea and Pastinaca saliva showed the highest syrphid larvae densities. The densities in herb strips (max. 0.6 larvae/m2) were manyfold lower than in wheat (10 pupae/nr). In herb strips the same syrphid species as in winter wheat were identified. In both habitats Episyrphus balteatus was the most abundant 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 syrphids to find aphid colonies within a crop field. The strips are neither significant for an early development of the first syrphid generation nor for an additional generation after wheat harvest. The importance of herb strips for hoverflies lies in their quality to supply them with pollen and nectar, which increases the fitness of the adults.