Flora og insekter i hegn på økologiske og konventionelle bedrifter

Summary

The aim of the project was to compare flora and insect fauna of organic and conventional hedgerows and to study whether the drift of herbicides into hedgerows alone or in combination with differences in fertiliser application may explain any differences. The project consequently consisted of two parts, viz. collection of flora and insect data in existing hedgerows (multi-row hedgerows, age 10-15 years) on two soil types and an experiment in which a sown grassland vegetation was treated with combinations of glyphosate (0-25% label rate) and nitrogen (0-100 kg N/ha/year) as a simulation of the most important agricultural conditions having an effect on flora and insect fauna in different agricultural systems. In the experiment flora and insect fauna were studied for three years. In the hedgerows clear differences in the floral composition were found, with more plant species in hedgerows at organically grown fields than at conventionally grown fields, on both sandy and loamy soils. The insect fauna was correlated with the flora, but no clear differences were found between the two agricultural systems. Apart from the hedgerow flora, also the type of crop grown on the adjacent fields affected the abundance of herbivorous insects, especially bugs and weevils.
In the experiment the effects of glyphosate and nitrogen treatments interacted strongly. In unfertilised plots the number of plant species decreased at increasing glyphosate dosages, whereas plant biomass was virtually unaffected, and litter biomass decreased. For plant and litter biomass the glyphosate effect increased at increasing fertiliser levels, i.e. there was a severe decrease in both plant and litter biomass as a consequence of glyphosate treatment. For numbers of plant species the interactive effect was opposite, as the glyphosate effect decreased at increasing fertiliser levels. Insect abundance and species numbers followed the picture seen for plant biomass, but with differences between insect groups.