The effect of uncultivated land on the distribution of cabbage aphid (Brevicoryne brassicae) on an adjacent crop.
The numbers of Brevicoryne brassicae and its parasites and predators in a crop of brussels sprouts were counted during a season. The results were grouped so that the centre, 2 unsheltered edges and an edge of the field sheltered by trees could be compared to demonstrate the operation of edge-growth effects on a natural infestation of the aphid. The deposition of álate aphids was increased by shelter to windward, resulting in the heaviest initial infestation at the sheltered edge of the crop. The edge-growth crucifers became heavily infested at flowering, when numbers rose from less than 10 to 400 aphids per 90 plants within 2 weeks. Plants in unsheltered parts of the crop produced over 50 leaves during the season and carried more leaves than plants at the sheltered edge. Plants in this area produced on average only 38 leaves, which contained more water (87.2%) than the leaves of unsheltered plants (86.1 %). The aphids at the sheltered edge reproduced more slowly than at the centre of the crop and appeared to suffer heavier mortality from rainfall, with the result that aphid numbers were about half those found at the centre of the crop. Losses of aphids caused by predation could be linked with the observed distribution of predators; aphids at the open edges suffered most from predation. [From author's summary].-Univ. Reading.