Green bridges over the winter: consequences for Brassica pests
Abstract: In this study we analysed the population genetic structure of the cabbage aphid, Brevicoryne brassicae (L.) (Homoptera: Aphididae) using nine polymorphic microsatellites we have developed. The aim was to determine if the aphid populations are different in genetic structure in relation to the host plant species. Seventeen locations with cruciferous crops/species were sampled during the growing season in the polder Hoeksche Waard (province South Holland), an area covering approximately 400 ha with a high proportion of arable crops including Brussels sprouts and Savoy cabbage, with winter oilseed rape recently introduced and white mustard (Sinapis alba) as the dominant green manure, and several cruciferous crops in a wildlife mixture. Statistical analysis of the genetic structure at the level of the cabbage aphid subpopulations showed no differentiations among the various hosts. Pairwise comparison of the FST values for the aphid samples indicated no significant differentiation between the samples from hosts Brussels sprouts or winter rape in the field margin, sampled in March; oilseed rape and winter rape in the field margin, sampled in May; and Brussels sprouts and winter rape sampled in December. We conclude that green manure crops, field margins with wildlife mixtures containing cruciferous species, but especially oilseed rape can form ‘bridges’ during certain periods of the growing season for infestation of Brussels sprouts by cabbage aphids. In other words: during the period of absence of Brussels sprouts other cruciferous crops or cruciferous plants in margins can harbour populations of cabbage aphids that can infest Brussels sprouts. They function as a ‘seasonal bridge’ for infestation of Brussels sprouts.