Spiders can suppress populations of some important crop pests. Although dispersal is essential to their survival in the disturbed farmland environment, accounts of their dispersal activity over several seasons are few. Spiders dispersing across a landscape of mixed farmland were sampled over an 18-month period using Stick, Net and Bottle traps (SNB traps). Traps were located in a two year old grass ley where ground densities of spiders and wind speed data were also recorded. SNB traps were effective at sampling large numbers of dispersers; Linyphiidae were the most abundant family sampled (93%). Numbers of adult linyphiids dispersing were found to increase in autumn and winter with dispersal activity occurring frequently throughout the study period. Dispersal patterns were similar for congeners (Erigone spp., Oedothorax spp.) although differences were evident between common agrobiont species. Weather conditions associated with stable high-pressure systems appeared important for stimulating mass dispersal in Oedothorax Bertkau, 1883 species. Erigone atra Blackwall, 1833 dispersed more frequently and under more variable conditions. Winter and spring dispersal was low for adult Tenuiphantes tenuis (Blackwall, 1852) and Bathyphantes gracilis (Blackwall, 1841) compared to common erigonids. Ground populations correlated positively with dispersing spiders for some species indicating that dispersal activity was in part a function of population size. For Oedothorax fuscus (Blackwall, 1834) it is suggested that life history traits and weather conditions may interact to influence the sex ratio of dispersers over time.
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Vol. 44 • No. 3