THE LONG-TERM IMPACT OF PLANT-PROTECTION AGENTS ON DIPTERA WITH SOIL-DWELLING DEVELOPMENT STAGES

In the years 1989 to 1992 the Diptera (flies and midges) in four parce ls of a 12-hectare plot of land in Ahlum (approx. 15 km south-east of Brunswick) were caught using photo-eclectors. Plane protection and fer tilising agents were applied in different concentrations to parcels I- 0-I-3. The crops sown were sugar beer (1989 and 1992), winter wheat (1 990) and winter barley (1991), with rape as a catch crop (1991/92). In addition to Diptera caught with photo-eclectors, soil samples were ta ken from which larvae were also extracted. We were able to study the b iology of certain Diptera species in our laboratory. It emerged that t he Diptera present in the agricultural ecosystem under study were most ly saprophagous species, which help to sustain the fertility of the so il by breaking down necrotic organic material. The second commonest gr oup were the predators. Only avery small number of species harmful to cultivated crops were identified during our survey. Neither did the ga ll midges (Ceidomyiidae) produce any significant number of species det rimental to plants, most species being mycophagous. The majority of th e more commonly found Diptera species reacted to: increased cropping i ntensity in at least one crop, with a clear reduction in abundance. Di rect effects of the individual plant protection treatments were detect ed at the most only in individual cases; hence any differences must be primarily due to the long-term cropping at different intensities. The dominance pattern was also influenced by the cropping intensity Moreo ver, it was discovered that the impact oi cropping measures on Diptera varied with the crop. During this project more was learnt about the b iology of certain Diptera species. Differences in biology (e. g. pheno logy, preference for crops) were also revealed in closely related spec ies. Taken overall, it may be concluded that the cropping intensity I- 1 of the field experiments assessed here, which closely approached the aims of modern integrated plant protection policy, is considerably le ss hard on the Diptera biocenosis than very intensive crop management.