The effect of uncultivated land on the distribution of cabbage aphid (Brevicoryne brassicae) on an adjacent crop.
The following is based almost entirely on the author's summary of this account of investigations in southern England in 1957-58. The numbers of Brevicoryne brassicae (L.) and its parasites and predators in a crop of brussels sprouts were counted during one season [cf. R.A.E., A 51 591]. The results were grouped so that those for the centre of the field, two unsheltered edges bordered by grassland with herbaceous flowering plants and an edge sheltered by trees could be compared to demonstrate the operation of the effects of the bordering vegetation on a natural infestation of the aphid. The deposition of alates was found to be increased by shelter to windward, and in consequence the heaviest initial infestation was at the sheltered edge of the crop. Cruciferous plants in the bordering vegetation became heavily infested at flowering, when numbers rose from less than 10 to 400 aphids per 90 plants within two weeks. Brussels-sprout plants in unsheltered parts of the crop produced over 50 leaves during the season and carried more leaves than plants at the sheltered edge, which produced an average of only 38; the leaves of sheltered plants contained more water (87.2 per cent.) than those of unsheltered ones (86.1 per cent.). The aphids at the sheltered edge reproduced more slowly than those at the centre of the crop and appeared to suffer heavier mortality from rainfall, with the result that they were about half as numerous as those at the centre. Of the early predators, it seemed likely that Coccinellids had invaded the crop from the bordering vegetation. Syrphids were more important predators; their eggs were half as numerous again near flowers as at the centre of the crop or elsewhere at the open edges. Dissection of the females showed that they had fed on pollen. Losses of aphids caused by predation could be linked with the observed distribution of predators. Thus, aphids at the open edges suffered most from predation, which kept numbers rather below half the level at the centre of the crop. The distribution of parasitized aphids could not be correlated with factors related to the bordering vegetation. Differences in the numbers of alate aphids emigrating from the crop area largely reflected the size and number of colonies present. Thus dispersal often reduced aphid numbers at least 10 per cent. more at the centre of the crop than in other areas. This tended to reduce the differences in aphid numbers between the centre of the crop and the edges.