Organic farming and generalist predator communities

Organic farming systems rely on improved soil quality and biological control favored by reduced tillage and pesticide application, organic fertilizer application and crop rotation management. The first part of this study focused on the impact of organic and conventional long-term farming (DOK Trial, Switzerland), primarily characterized by fertilizer types, on major aboveground generalist predator taxa (spiders, rove beetles and ground beetles) in structurally simple (winter wheat) and complex (grass-clover) agroecosystem. Most analyzed animal taxa that improve internal nutrient cycling and biological control in agroecosystems benefited from organic farming and several taxa were negatively affected by inorganic fertilizer or herbicides application. Spiders (Araneae) benefited most from organic farming in wheat fields. Similar to results from wheat fields, spiders were more active and diverse in organically managed grass-clover fields. In contrast to wheat fields, only surface-active spiders responded positive, with strongest responses early and late in the season. Prey availability and vegetation structure caused a numerical response of spiders in organically managed fields; both factors were further analyzed in a field experiment in wheat. Increasing the structural complexity between wheat stands reduced the activity-density of cursorial spiders and ground beetles (Carabidae). Web-building spiders and rove beetles (Staphylinidae) responded positively to experimentally enhanced availability of alternative non-pest prey (Collembola), but cursorial spiders and ground beetles did not mirror prey responses. Aphid numbers increased with increasing activity-density of alternative prey. Such positive prey-prey relationship potentially reduces pest suppression by generalist predators, as non-pest prey may distract predators from aphid prey. The role of generalist predators as biological control agents is further affected by cannibalism, intraguild predation and competition between predators. Spatial patterns of web-building spiders in alfalfa did not resemble the aggregation of available pest prey (aphids). In contrast web-owners had fewer neighbors in the vicinity of their webs than expected under a random distribution of spider webs in times of highest spider activity. Spiders were most affected by organic management in the DOK trial and experiments in grass-clover and alfalfa suggest strong intraguild interactions at high densities. To evaluate the consequences of functional group identity for pest suppression, two major spider functional groups (web-builders and cursorial spiders) were manipulated in a cage experiment in wheat. Cursorial spiders slowed down the initial aphid population growth, but none of the treatments prevented an aphid outbreak. Molecular studies strengthen this result, as they indicate that cursorial species had the highest proportion of individuals that tested positive for aphid DNA. Results of this study suggest that cursorial spiders may be of particular importance as natural enemies under organic farming. They responded most consistent to organic farming across agroecosystems with different structural complexity and successfully slowed down aphid population growth during early colonization stages. Future studies need to analyze the factors responsible for the reaction of generalist predators to farming methods. Results may further identify conditions under which enhanced predator numbers result in improved pest suppression