HABITAT MANAGEMENT IN WINTER-WHEAT AND EVALUATION OF SUBSEQUENT SPIDER PREDATION ON INSECT PESTS

Density estimates, web cover and predatory importance of different spi der families were investigated over one field season in relation to ha bitat manipulation by strip-management in a winter wheat field at Zoll ikofen near Berne, Switzerland. Spider densities and web cover were in most cases higher near the sown weed strips than they were away from them. The strips contained a significantly greater web cover for the s heet webs of Linyphiidae in May and at the end of July. Qualitative as sessment of spiders' prey revealed high percentages (up to 92%) of Dip tera and Aphidina (including pests). Quantitative evaluation of spider predation in the orb webs of Araneidae/Tetragnathidae and the sheet w ebs of Linyphiidae showed very small prey turnover rates e.g. Linyphii dae killed on average 1.5-1.7 aphids m(-2) 9 h daylight(-1) in wheat. Theoretical estimates of diurnal prey capture in all spider webs (incl uding Theridiidae and Agelenidae) were calculated. Comparisons with th e reduction of noxious insects by other polyphagous predators suggeste d that in the investigated field the importance of spiders as biologic al control agents is rather small.