SIGNALIZATION OF THE OCCURRENCE AND DAMAG E BY THE BRASSICA SEED MIDGE (DASINEURA-BRASSICAE WINN)

Experiments were conducted in the Central Bohemia in three winter rape varieties in 1985-1987. In the three years of study, the second gener ation of the midge larvae were by 59-1 00 % more numerous than those o f the first generation. In the cold year of 1987, the third generation larvae were the most numerous. Their numbers per three varieties were by 135-190 % higher than those of the second generation larvae. No co rrelation was found between the number of adults captured in yellow di shes and intensity of pod damage. Catches in yellow dishes indicate ad ult flight activity only. The use of the sum of effective temperatures , based on the lower threshold of development of the pest, in determin ing the time of occurrence of adult midges was inaccurate as the maxim um occurrences of adults of the overwintered population differed in th e years of study and they did not correlate with the temperature condi tions in the respective years (Tab. I). A suitable method to study the occurrence of adults of the successive generations is used in cages c ontaining infested pods, placed at the edges of fields (Fig. 1 ). No c orrelation was found between the number of mature larvae captured in d ishes placed on the soil in winter rape stands and intensity of pod in festation. Comparing the cumulative mean numbers of larvae captured pe r dish in the three years of study, it appeared that the characteristi cs were not comparable. In 1987, with the highest percentage of destro yed pods (26.8 %), the number of larvae caught per dish (160-173) was half to that captured in 1986 in which 8.5 % of pods was damaged and a n average of 308 larvae was captured per dish (Fig. 2). The brassica s eed midge damages winter rape pods all along the fruit-body. In the th ree varieties under study, infested pods were found in tiers 1 to 14 f rom below. The percentage of infested pods in the head inflorescence, found in the three years of study, varied from 26 to 43.3 % depending on the variety. The most suitable timing of control measures is in the oviposition period of the first generation, that is, in the winter ra pe growing stages from the development of the first pods until full bl oom (Figs 3 to 5).