INFLUENCE OF THE SYNTHETIC PYRETHROID INSECTICIDE ALPHA-CYPERMETHRIN ON THE STRUCTURE OF THE SYRPHID COENOSIS (SYRPHIDAE, DIPTERA) IN A POTATO ECOSYSTEM

The influence of the synthetic pyrethroid insecticide alpha-cypermethr in on the structure of syrphid coenosis was assessed in a field experi ment. A significant influence on the number of species was noticed and a lesser influence upon the species composition of the syrphid coenos is. The effect of the chemical mixture on the population density of th e distinct species differed in intensity but only on the first day cou ld the effect of the insecticide be shown with regard to number comple x before and after treatment. The total average density, defined as th e result of all densities of the distinct species of syrphid flies was most highly affected on the first day after the treatment. At that ti me the estimated density was half than the level before the applicatio n of the insecticide. The effect of the insecticide treatment was mani fested upon both the composition of complexes of dominant species and the general dominant structure of the syrphid coenosis. The parameters examined for species structure: Simpson's index of concentration of d omination (D), Shannon-Weaver's index of total diversity ((H) over bar ) and Margalef's index of species abundance (d) showed that the insect icide treatment exerts an unfavourable but not strong influence upon t he syrphid coenosis.