Habitat management in winter wheat and evaluation of subsequent spider predation on insect pests

Density estimates, web cover and predatory importance of different spider families were investigated over one field season in relation to habitat manipulation by strip-management in a winter wheat field at Zollikofen 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 sheet webs of Linyphiidae in May and at the end of July. Qualitative assessment of spiders' prey revealed high percentages (up to 92%) of Diptera and Aphidina (including pests). Quantitative evaluation of spider predation in the orb webs of Araneidae/Tetragnathidae and the sheet webs of Linyphiidae showed very small prey turnover rates e.g. Linyphiidae 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 (including Theridiidae and Agelenidae) were calculated. Comparisons with the reduction of noxious insects by other polyphagous predators suggested that in the investigated field the importance of spiders as biological control agents is rather small.