Some effects of weed control on the numbers of the small cabbage white (Pieris rapae L.) on brussels sprouts.
The presence of weeds within the crop provided a habitat which was more suitable for some of the arthropod predators of P. rapae. Harpalus rufipes was particularly dependent on ground-cover. With the higher number of predators, survival of Pieris caterpillars was significantly poorer on weedy than on hoed plots and fewer Pieris eggs were laid on plots with many flowering weeds. In 1968 pest numbers were low and the advantageous effects of weeds were outweighted by their harmful competitive effects. Yields were highest on plots which were kept free of weeds.- Monks Wood Exp. Stat., Hunts.