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 pe sticides, large fields) and two conventional farms, one with large fields ( M), the other with small fields and lots of marginal structures such as hed ges (G). No synthetic pyrethroids (mostly harmful to spiders) were sprayed in any of the fields during or before the study period. Ten pitfall traps p er farm and year were set into the centre of wheat fields and emptied weekl y 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 trappi ng period). The frequency and density of spiders did not show general diffe rences between the sites. The average numbers caught per 10 pitfall traps p er 12 weeks were 5157 at M, 4943 at D and 8355 at G. On the other hand, lar ge 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 tou t of nine families only) at M, 72 at D and 73 at G. Thirty-nine species wer e 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 c omposition of spider species between sites D and G by the differences in mi croclimate of the fields. Richness in spider species seems to be strongly d ependent on the structure of the agricultural area, in addition to the effe cts of insecticides and of crop rotation. The results obtained are compared with the results of published spider studies conducted in other agricultur al areas in Germany and in other European countries, to derive information on the average and maximal numbers of spider species in wheat fields.