Our analysis of the literature shows, not surprisingly,
that different organisms react in different ways to organic
farming. Weeds were more common when herbicides
were not used, which is what would be expected. More
importantly, the densities of predators, such as carabid
beetles and spiders, were usually higher in organic
farming systems than in conventional ones. Döring &
Kromp (2003) showed that the carabid species most
favoured by organic farming were open field species,
indicating that organic farming may enhance the abundance
and diversity of habitat specialists to a larger
extent than generalists.
On the other hand, non-predatory insects and pests did
not appear to be more common in organic agricultural
systems. Thus, our results, although based on a small
number of studies, suggest that natural enemies are
negatively affected by conventional management to a
much larger extent than other insects and pests. This is
supported by a study of biological control of the cereal
pest bird-cherry oat aphid Rhopalosiphum padi in a
mosaic landscape in central Sweden