Population density of Theridion impressum L. Koch (Araneae, Theridiidae) in sugar beet fields in Germany, and its possible effects on numbers of Myzus persicae (Sulzer) (Hom., Aphididae)

Theridion impressum was found in summer with an average of 1.6 females per m 2 in fields of sugar beets near Frankfurt/Main (Germany). No significant differences between insecticidal treatments occurred, while no synthetic pyrethroids were used. In the laboratory, female T. impressum exerted a significant effect on the growth rate of colonies of Myzus persicae. On average, one female spider eliminated 10.4 aphids per day initially, and 14.4 when the spiderlings had hatched. But although significant, the reduction (9.5 to 20.5%) of aphid density by T. impressum is not to be regarded as sufficient for the natural control of aphids in fields of sugar beets. In the fields, where there might be alternative prey, the percentage of aphids preyed upon might even be lower than in the laboratory exeriments presented here. Suggestions about the annual life cycle of T. impressum, which has not been described exactly before (hibernation as juveniles in field margins and other undisturbed areas), are given.