Weed species, not mulching, affect web-building spiders and their prey in organic fruit orchards in South Africa

Weed infestation affects economically relevant orchard properties, including tree performance, yield, and fruit quality negatively, and weeds are therefore often controlled by herbicide application in conventional farming. The addition of organic mulch below tree canopies has been proposed as an alternative reliable practice to suppress weeds and preserve soil moisture in organic farming. Mulching, however, may also affect arthropod pest and natural enemy populations, which highlights the need for simultaneously assessing weed, natural enemy, and animal pest communities in mulch experiments. This study addresses the limited knowledge about nonchemical ground cover management strategies for the control of plant and animal pests in orchards as a major constraint for organic growers. Here, we hypothesize that decisions about ground cover management practices in organic temperate fruit orchards affect the composition of web-building spider communities and their functional role as natural enemies of pest arthropods through effects on weed and insect pest communities. We studied weed, prey, and spider communities, as well as spider diet composition, in four temperate fruit types (apricot, peach, plum, and quince) on a single farm in the Western Cape, South Africa. We established experimental plots with and without addition of dead organic mulch under fruit tree canopies. Addition of organic mulch did not significantly affect weed cover under trees or the taxonomic composition of weed or spider communities over the eight-month study period. However, independent of mulching, the taxonomic composition of weed communities was significantly related to the composition of potential prey and spider communities. These relationships indirectly affected the prey composition of web-building spiders. These results suggest that the identity of weed species in the study orchards had a pronounced effect on the diet composition and functional role of web-building spiders. Future research should focus on the value of individual plant species for the promotion of pest control services provided by spiders across larger spatial scales and with higher levels of replication to allow for wider generalizations. The expected results would not only be relevant for weed control but could also be considered during the development of future flower strips in orchards.