Blossom beetle (Meligethes aeneus Fab.) as a yield factor in turnip rape (Brassica campestris L.)

Over the period from 1972—1982 the incidence of blossom beetles and their effect on yield formation in rape were studied infield trials and cage experiments at the Agricultural Research Centre, Vantaa, near Helsinki. The beetles migrated to the winter rape stands in May, when maximum daily temperatures rose above + 15°C, and the migration to the spring rape stands occurred about three weeks later. The population peak in both winter and spring rapes occurred just before flowering. During the 1970’s the diminishing cultivation of winter turnip rape, the chief host of the blossom beetle, together with an increase in the extent of spring rapes resulted in the reduction of beetle populations to such low levels that intensive control measures were not required until 1981. It appears that the blossom beetles are gradually adapting themselves to the developmental rhytm of spring turnip rape. In the cage experiments beetles were maintained on the plants a) from the seedling stage to flowering, b) from flowering to maturity and c) from the seedling stage to maturity, starting with populations of 5, 15 or 25 beetles/plant. In the first treatment the beetles had no significant effect upon yields, as the plants were able to compensate for the injuries. In the other treatments even the lowest population density reduced yields. The larvae, too, contributed to the yield loss. Apparently a vigorous stand can tolerate large numbers of beetles up to a few days before flowering, provided they are promptly destroyed after that. In the field trials, yield losses caused by natural populations of beetles ranged from 0—34 %. Even lower incidences than 2 beetles/plant caused measurable losses in yield. The injured plants produced fewer siliquas than the control plants, but tended to compensate by increasing seed number and seed weight. The trial results confirm that the threshold level, 1 beetle/plant at the early bud stage, should be followed. On the other hand they clearly demonstrate that the threshold level is essentially dependent upon the growing conditions and time of injury. For fully effective control generally two applications of insecticide are needed. The best control strategy in the future will probably involve a comprehensive regional application eg. in two to three year sequences in order to prevent the beetle populations from building up to excessive levels.