The effect of stand size and vegetational background on the colonization of cruciferous plants by herbivorous insects.
The colonisation of collard plants by herbivorous insects was studied in New York in stands of 1, 10 and 100 plants. The stands were cultivated in some experiments and not in others, and the plants were grown in pots to assure uniformity under different experimental conditions. Colonisation was assessed by direct counts in the field. The fauna did not respond as a unit to the spatial distribution of the plants, and each of the major herbivore species reached its maximum colonisation potential under different conditions of stand size. When the three most important species occurred together, Pieris rapae (L.) was most abundant in the one-plant plots, Phyllotreta striolata (F.) in the ten-plant plots and P. cruciferae (Goeze) in the 100-plant plots. Aphid alatae were abundant but did not establish large colonies and showed an inconsistent response to plot size. The vegetational background of the plots had a significant effect on colonisation. In uncultivated plots, only Pieris rapae was a successful invader; the other species that occurred in cultivated plots were infrequent and not abundant. The findings indicated that the diversity of the crucifer fauna is maintained by each species having a distinct pattern of colonisation behaviour. Furthermore, it appears that many potential pest species preferentially colonise large pure stands of their food-plants, a condition characteristic of cultivated plots.