Natural enemies and their food sources in an organic crop rotation with undersown crops and animal manure

Summary

The abundance and species composition of natural enemies (spiders and carabid beetles) and some of their important arthropod food sources (microarthropods, dipterans and hymenopterans) were sampled by soil extraction, pitfall traps and sticky traps in winter wheat placed in an organic crop with undersown catch crops and animal manure (slurry). The working hypothesis was that the undersown crops and animal manure would stimulate the microarthropods and other food sources, thereby enhancing the abundance of natural enemies.
The ANOVA analyses revealed no impact of manuring and undersown catch crops on the microarthropods, but there was a larger abundance of most predator groups in the manured plots. There was not found any impact of undersown crops on the arthropod fauna elements. In stead analyses with the statistical package PRIMER revealed correlations between certain species of the Collembola community and the natural enemies (carabids and spiders), correlations which due to knowledge on food quality relationships are likely to be cause – effect relationships.
The results suggest that when attempting to boost abundance of natural enemies in agricultural fields the working hypotheses needs a higher level of detail. It seems not to be enough to boost the microarthropod fauna in general, but it is important to distinguish between species that are beneficial or unfortunate for the natural enemies.