Daily per capita birth rates of the cabbage aphid,Brevicoryne brassicae(L.), increased 40–50% as numbers of adult female aphids in artificially created groups on kale plants increased from three or fewer to six or more adults. Reproduction was enhanced at a declining rate such that the maximal effect was observed at 10 females per group, with no further increase up to 27 females per group. Increasing the number of females per group enhanced per capita birth rates on potted kale plants under greenhouse conditions and on field-grown kale plants. Enhancement under field conditions was observed in warm (22°C) but not cool (15°C) periods.