Abstract
The relation between the degree of the soil coverage by weeds and the occurrence of aphidophagous syrphid larvae in colonies of black bean aphid on red beet cultivation was analysed over the period 1997-2000. The different degree of the weediness was obtained by varying the frequency of weeding on particular plots. In combination A the plots were continuously kept weed-free during the whole vegetation season, in B weeds were removed three times, in C weeds were removed twice and in combination D weeds were not removed. It was found that the greatest number of predatory syrphid larvae was noticed in the combination kept weed-free and weeded three times, where also the most intensive occurrence of Aphis fabae Scop. was noticed. The combinations weeded three times and two times created the best conditions for the effectiveness of the predatory Syrphidae which larvae feeding in the colonies of Aphis fabae Scop.