The distribution of insect pests on crops of oil-seed rape (Brassica napus L.) and the damage they cause

Abstract

SummaryDuring 3 years the distribution of pests on ten crops of winter rape and two of spring rape, and the damage they caused, has been determined.During immigration the adults of all pests infested crop edges more than crop centres, particularly on large crops, irrespective of the presence of wind breaks. The proportion of M. aeneus and G. assimilis at the crop edge gradually diminished as they moved further into the crop, and their larvae and pods with C. assimilis emergence holes were more evenly distributed over the crops than the adults. Damage by M. aeneus was not the only cause of podless stalks. The larvae of D. brassicae were largely confined to the edges of crops; split pods were largely but not solely confined to the edges.In winter rape crops correlations were found between the percentage of podless stalks per plant and the number of M. aeneus present, and the percentage of pods infested with C. assimilis larvae and the adult C. assimilis population. In two fields infested with D. brassicae the diminution in the proportion of split pods from crop edge to crop centre was positively correlated with diminution in the number of adult D. brassicae.