Zur Befallsverteilung der Kohlschotengallmücke Dasyneura brassicae Winnertz in großflächigen Beständen (Diptera: Itonididae).

Abstract

In the years 1963 to 1965 an attempt was made to gain an insight into the distribution of the attack of Dasyneura brassicae Winnertz on large fields of winter rape. For this purpose the control areas were divided into zones and several procedures were used in collecting and measuring. Both yellow trap basins and ring net catching proved to be unsuitable for collecting imagos. Only rarely could a small number of individuals be caught, which was entirely useless for an evaluation. A far better characteristic indicating the distribution of the attack was the number of pods infested by larvae of the midget. Investigations made according to a certain system showed a clearly differentiated distribution of the attack; the density of the population was perceptibly diminishing from the borders of the field to its centre. As the greatest number of infested pods was chiefly concentrated in the zone along the borders of the field, there was in almost all cases a particularly abrupt drop from this zone to the next. The statistics showing the differences of the attack on the various zones in comparison with the centre of the field confirmed on almost all days a difference of the 25 m zone which was sometimes highly significant. These results suggest that with still larger fields the possibility of limiting control measures to the treatment of a proportionately wide zone on the borders of the field will gain practical importance. This assumption is as yet a working hypothesis, and further studies are necessary to assess the practicability of this procedure.