Arable weeds in organically managed wheat fields foster carabid beetles by resource- and structure-mediated effects

Abstract

Arable weeds in organically managed fields may foster arthropod generalist predators by the provision of shelter and favorable microclimate (structure-mediated effects) and the provision of additional animal and floral food resources (resource-mediated effects). In three organically managed winter wheat fields in Central Germany, we investigated the impact of weed removal and introduction of artificial weed-like structure on the activity density and species richness of carabid beetles with respect to trophic groups, microclimatic conditions, and densities of potential prey. Removal of weeds reduced both carabid activity density and species richness but did not affect trophic group composition. The decline in carabid activity density was dampened by the addition of artificial structure. Mean daily surface temperature and light intensity were significantly lower under weeds and artificial plants than under wheat plants alone. Weed removal reduced the abundance of leafhoppers and true bugs, but the response was inconsistent across fields. We conclude that the presence of arable weeds in organically managed wheat fields fosters carabid activity density and species richness via resource-mediated effects, such as a higher availability of weed-borne resources (e.g. seeds and pollen) and herbivorous prey. Structure-mediated effects (altering the microclimate) add to this positive effect. The presence of weeds in organically managed wheat fields enhances carabid activity density and diversity and needs to be integrated into future management strategies for natural enemy conservation.

Impacts of Dryland Farm Management Systems on Weeds and Ground Beetles (Carabidae) in the Northern Great Plains

“…Alternatively, generalist seed predators like ground beetles are impacted by management systems and, in turn, could impose top down impacts on weed communities [10]. Overall, our study showed an increased abundance of weeds and ground beetles in organically managed wheat fields and posed an interesting question for future studies that these two contrasting components of associated biodiversity could be directly (positively or negatively) associated with each other [66]. Additionally, a greater activity density and diversity of carabid beetles in organic fields than in conventional fields may provide increased biological control of weed seeds in these systems [47,67].…”

The potential of crop management practices to reduce pollen beetle damage in oilseed rape

“…Similarly, the direct effects of weed management on pollen beetle regulation by natural enemies has been little studied, however, generalist predators such as carabids and spiders may benefit from increased weed cover through provision of shelter and alternative sources of food/prey (Speight and Lawton 1976; Holland et al 1999;Schmidt et al 2005;Sunderland and Samu 2000). In particular, omnivorous carabid species that consume weed seeds as well as prey have been shown to be associated with increased weed cover (Kulkarni et al 2017;Diehl et al 2012). Potentially, therefore, less intensive weed management regimes may enhance pollen beetle regulation by these natural enemies.…”

“…Potentially, therefore, less intensive weed management regimes may enhance pollen beetle regulation by these natural enemies. However, more work is needed in this area as positive relationships between weed density and generalist predators may not necessarily lead to better pest suppression; alternative weed-associated resources may distract generalist enemies from target pests (Diehl et al 2012). …”

Additive effects of predator diversity on pest control caused by few interactions among predator species

“…This effect of weeds was confirmed by Diehl et al . (), who found an increased activity density of carabids in weedy plots but not in plots were only an additional structure was provided. They conclude that weeds can benefit predator communities in the long term through the promotion of predator establishment and increased predator fitness.…”