Resistance to attack by Brevicoryne brassicae among plants of Brussels sprouts

Because they remained almost uncolonized by the cabbage aphid (Brevicoryne brassicae (L.)) throughout the growing season, plants of Brussels sprouts were singled out in each of 4 years, from plots heavily infested with the aphid, as possibly being resistant to attack. Clones of these plants were established from cuttings and tested in a controlled environment by inoculation with B. brassicae and later, in the field, by natural infestation. The tests confirmed that some of the plants were resistant to the aphid, and the most resistant of those from the first year of the work proved at least as resistant as any subsequently found. The resistance was expressed as antibiosis, but in the field host non-preference was also shown by incoming winged aphids. The possibility that biotypes of B. brassicae might exist, to which the resistant sprout clones were not necessarily resistant, was investigated using B. brassicae collected from sprouts from each of several areas in England. Eight sprout clones, seven of which were known to be resistant, and the other susceptible, to B. brassicae from Wellesbourne were tested with these other B. brassicae. The results showed that biotypes of the aphid, with differing abilities to colonize respective sprout clones, existed in each area, and of the seven sprout clones resistant to the Wellesbourne aphid, only one appeared never to be fully susceptible to one or more of the other biotypes of B. brassicae.