THE IMPACT OF SPIDERS ON APHID POPULATIONS IN A STRIP-MANAGED APPLE ORCHARD

Aphids and their predators were studied in an apple orchard near Berne , Switzerland. To augment the density of the predators of aphids weed strips were sown in tramlines and along the border parallel to the fir st and the last row of trees in one half of the orchard (the other hal f served as control) in April 1992. Weekly visual observations, monthl y beating-tray samples, branch samples in winter, and assessment of th e density of spider webs and their trapping efficiency on randomly cho sen trees were carried out in 1993. During 1993, significant differenc es in the number of aphids, aphidophagous insects, and spiders were ob served on the apple trees within both the strip-managed area and the c ontrol area. The impact on aphids by the aphidophagous insects was imp ortant during the flowering time of weeds in spring and summer. In aut umn, however, spiders (i.e. Araniella spp.) were the dominant predator s of aphids on the trees; during this time of the year they were more abundant within the strip-managed area of the orchard. With a higher d ensity of webs, spiders significantly reduced the number of aphids abl e to return from their summer host plants. Consequently, less hatching aphids were found on branch samples of the strip-managed area compare d to the control area. The effects of the weed strips on spiders and a phidophagous insects, and the resulting impact of predation on the aph ids are discussed.