The role of crop density, seed and aphid presence in diversification of field communities of Carabidae (Coleoptera)

In 1995-1997, we studied the factors which may influence the ground activity density of Carabidae using pitfall traps placed in winter wheat, winter rape and pea stands (1995 only) grown within a 1 km(2) area with uniform physical conditions. The traps were placed in plots of bare ground established within the crops and under surrounding intact plant stands. The communities were similar between crops within years (Pearson's correlation coefficient r = 0.60-0.81), and between years within crops (r = 0.89-0.91), except for the poor winter rape stand in 1997. Factors influencing carabid activity density were: (i) Density of cropstand. The carabids preferred crop-shaded ground as long as crop density was low or medium but moved to bare ground plots when crop density became high. Under moderate crop density the preference differed between beetle species, most of which preferred crop-shaded ground while a few ones preferred bare ground. Carabid preferences were probably determined by microclimatic differences caused by presence and density of crop cover. (ii) Presence of seeds dropped on the ground. In rape stands, presence of crop and weed seeds increased the activity density of seed predators (species of genera Amara, Harpalus, Ophonus and Pseudoophonus). Scattering of rape seeds significantly increased local activity density of Harpalus affinis and H, distinguendus in the wheat stand. (iii) Presence of aphids. Activity density of Bembidion lampros and Trechus quadristriatus and between-year variation in pooled abundance of the five species recognised as aphid predators was associated with variation in aphid abundance.