Forurening og formering af mitter i Egeløkke Lung

English Summary

Field site and investigation period

From May 1996 to June 1997 the ecology of biting midges (Culicoides: Ceratopogonidae) of the organic polluted marsh Egeløkke Lung, Langeland was studied. The density of larvae and the larval growth were monitored in mud-samples; the hatching and flight activity of adults were recorded by means of emergence traps and a light trap respectively.

Results of field investigation

In the light trap 28 Culicoides species were recorded, including 3 species new to Denmark. C. punctatus, C. duddingstoni and C. obsoletus were predominant, constituting > 80 % of the catch.

In May, prior to the main emergence of adult midges, the average larval density was > 13000 m². Four distinct summer maxima in larval density were found and the larval size varied correspondingly. In autumn, prior to hibernation, the larval density in the mud was high. The variation in larval density and growth reflects the population dynamics af the larval generations, pupation and hatching of adults and the recruitment of newly hatched larvae originating from the oviposition of successive summer generations. In the ligt trap biting midges were recorded from early June to late October, however, the seasonal activity pattern of the species differed considerably.

Simulation model

A mathematical simulation model calculating the daily number of adult midges showed a fine coincidence between simulated and observed activity maxima of C. punctatus. After adjustment the model is expected to predict adult population maxima, i.e. periods when pastured cattle are liable to a heavy load of biting midges.

Conclusion

In the early seventies C. nubeculosus, i.e. a species associated with eutrophic, bare mud flats, was predominant in the site. Since then this species has declined rapidly and the actual fauna is dominated by species mainly breeding in habitats covered by vegetation. The change in species composition reflects the gradual overgrowing of Egeløkke Lung. Further, the outlet of not purified sewage has ceased. Still several of the species recorded are tolerant of organic pollution.

To-day Egeløkke Lung is the breeding site of a Culicoides fauna rich in species, some of them occurring in very high densities, e.g. notorious cattle-biting species. Actually, the larval densities recorded are considerably higher than those found in the early seventies.