Studies of the ecology of the cabbage aphid ( Brevicoryne brassicae ) on brassica field crops in Canterbury, New Zealand

Abstract This paper is the first of a series dealing with field population studies of Brevicoryne brassicae on brassica field crops. In 1958–59 field trials at three sites of Calder swede, medium‐stemmed chou moellier, and broad‐leafed Essex rape were sampled at approximately weekly intervals and records taken of numbers of aphids and plant weights. Flying aphids were also trapped at crop height. Major peaks in aphid numbers on the plants occurred at all sites in late summer and in spring. The former coincided with peak vegetative growth and the latter with flowering and seeding of the biennial host plants. Peak numbers of flying aphids more or less coincided with maximum numbers on the host plants. Differences in aphid numbers on the various host varieties were not consistent at the different sites. Thus significant differences in aphid numbers were observed between sites and varieties, and in about half the samples the site x variety interaction was highly significant. Over the whole season greatest aphid numbers were found at Annat, and lowest aphid numbers were found on swede plants, irrespective of site. The spring migration of aphids from seeding plants resulted in the infestation of newly sown crops. The intensity of initial aphid infestation appeared to be largely determined by proximity of seeding crops, but the ultimate intensity of aphid infestation was not necessarily determined by this factor. Detailed interpretation of these results will be considered in a future paper.