The species composition and frequency of spiders (Araneae) in fields of winter wheat grown under different conditions in Germany

Studies were conducted in winter wheat fields of three farms at Bad Vilbel near Frankfurt/M, during 1993-1995: one organic farm (D) (biodynamic; no pesticides, large fields) and two conventional farms, one with large fields (M), the other with small fields and lots of marginal structures such as hedges (G). No synthetic pyrethroids (mostly harmful to spiders) were sprayed in any of the fields during or before the study period. Ten pitfall traps per farm and year were set into the centre of wheat fields and emptied weekly from early May up to the end of July (harvest). In addition, the spiders were sampled by square flooding (1 m(2) per held per week during the trapping period). The frequency and density of spiders did not show general differences between the sites. The average numbers caught per 10 pitfall traps per 12 weeks were 5157 at M, 4943 at D and 8355 at G. On the other hand, large differences were proved to exist in species numbers: In all, 101 spider species out of 12 families were found in the fields of winter wheat, 55 tout of nine families only) at M, 72 at D and 73 at G. Thirty-nine species were common to all farms, four species occurred exclusively at M, 20 at D and 17 exclusively at G. It was not possible to explain the difference in the composition of spider species between sites D and G by the differences in microclimate of the fields. Richness in spider species seems to be strongly dependent on the structure of the agricultural area, in addition to the effects of insecticides and of crop rotation. The results obtained are compared with the results of published spider studies conducted in other agricultural areas in Germany and in other European countries, to derive information on the average and maximal numbers of spider species in wheat fields.