Soil management and predator activity
in organic crop rotations

Gabor L. Lövei, Danish Institute of Agricultural Sciences, Søren Toft University of Aarhus & Jørgen A. Axelsen, National Environment Research institute

Cultivated areas are routinely characterised as "of poor diversity", but even a heavily cultivated crop field in Denmark provides habitat for thousands of species of invertebrates. They include herbivores, predators, pollen feeders, decomposers, fungal feeders, etc. There are also hundreds of species that feed on plants and are thus potential pests of crops. Most of these do not reach densities that would make them pests. This is not always due to their low reproduction rate. Natural enemies play a significant role in keeping them at low densities, and prevent them from becoming pests. This is an important ecosystem service provided by the different types of natural enemies. These natural enemies include beetles (esp. ground beetles), spiders, ants, predatory bugs, parasitoid flies and wasps.

The life of a predator, however, is spent mostly in hunger, and consequently, many of them would prey on whatever they can find, kill, eat and digest. These polyphagous predators are very useful for biocontrol, because they can reside in cultivated fields all their life, and their presence does not necessarily depend on high pest insects densities. Further, the largest impact of predators is early in the season, when prey densities are not yet high. A major task for improved biocontrol by agricultural management is how to keep the polyphagous predators "at hand", in the fields where we want to use their beneficial effects?

A current project, running on the DARCOF organic crop rotation plots at Foulum and Flakkebjerg, is looking at the impact, soil nutrient enhancement can indirectly have on natural enemies. The argument goes like this: if the soil nutritional status is high, this allows not only better plant growth, but also a more rich soil fauna to develop. Several polyphagous predators can feed on soil-developing insects, such as springtails, flies, mites and caterpillars. This type of prey is especially important when aboveground prey is still not abundant - in early spring. A rich soil-based fauna can provide polyphagous predators with food early in spring, and thus keep them in the fields until pest insects start to arrive and reproduce.

Our DARCOF-project is testing this theory. We study wheat grown with different soil nutrient manipulation methods: undersowing, manure, manure + undersowing, and compare the soil and surface-active fauna with non-fertilised control plots. Samples are taken to evaluate the number of springtails, mites, and flying arthropods as soil-born food for predators, as well as ground beetles and spiders as polyphagous predators. The numbers of the grain aphid, Sitobion avenae serve as a reference for biological control effect.

The first year, evaluation on ground beetles has recently been completed. At Foulum, there were a total of 46 species, some 12-15 species/ pitfall trap. There were fewer (only 22) species in Flakkebjerg, 3- 10 species / trap. There were differences between the two regions, but in both, ground beetle assemblages responded to manure but not to undersowing. The composition of the assemblages showed large location-specific differences, probably influenced by soil differences.

Most of the species did not seem to numerically respond to the soil treatments, but the predator Pterostichus melanarius showed a significant treatment * time interaction effect. Further evaluation would show if this is repeated in other years, and whether this correlates with soil prey abundance and/or biological control impact.