Structural features of field boundaries which influence the overwintering densities of beneficial arthropod predators.
Vegetation cover was manipulated in enclosures in a field boundary in southern England to test experimentally the effect on overwintering by the predatory beetles Tachyporus hypnorum and Demetrias atricapillus, which use grassy boundaries as winter refugia. Winter survival was lowest for beetles enclosed on bare earth and highest for those enclosed on tussocks of Dactylis glomeratus. The contrast in structural complexity of the experimental treatments caused a 44, 43 and 36% variation in the densities of beetles in 3 successive winters, resp. Highest winter densities of T. hypnorum were found from boundaries with deeper soil and greater vegetation height, with an east to west orientation, warmer mean daytime temperature and lower soil moisture. A quadrat survey was carried out in Norway on the equivalent group to T. hypnorum (T. chrysomelinus, T. obtusus and T. hypnorum). With multiple regression more individuals of Tachyporus spp. and other beneficial arthropods occurred in sampling units from boundaries raised higher above the field level that comprised grass cover with a high proportion of tussock grass.