Spiders (Araneae) form abundant and diverse assemblages in agroecosystems such as fruit orchards, and thus might have an important role as natural enemies of orchard pests. Although spiders are polyphagous and opportunistic predators in general, limited information exists on their natural prey at both species and community levels. Thus, the aim of this study was to assess the natural prey (realized trophic niche) of arboreal hunting spiders, their role in trophic webs and their biological control potential with direct observation of predation events in apple orchards. Hunting spiders with prey in their chelicerae were collected in the canopy of apple trees in organic apple orchards in Hungary during the growing seasons between 2013 and 2019 and both spiders and their prey were identified and measured. Among others, the composition of the actual (captured by spiders) and the potential (available in the canopy) prey was compared, trophic niche and food web metrics were calculated, and some morphological, dimensional data of the spider-prey pairs were analyzed. Species-specific differences in prey composition or pest control ability were also discussed. By analyzing a total of 878 prey items captured by spiders, we concluded that arboreal hunting spiders forage selectively and consume a large number of apple pests; however, spiders’ beneficial effects are greatly reduced by their high levels of intraguild predation and by a propensity to switch from pests to alternative prey. In this study, arboreal hunting spiders showed negative selectivity for pests, no selectivity for natural enemies and positive selectivity for neutral species. In the trophic web, the dominant hunting spider taxa/groups (Carrhotus xanthogramma, Philodromus cespitum, Clubiona spp., Ebrechtella tricuspidata, Xysticus spp. and ‘Other salticids’) exhibit different levels of predation on different prey groups and the trophic web’s structure changes depending on the time of year. Hunting spiders show a high functional redundancy in their predation, but contrary to their polyphagous nature, the examined spider taxa showed differences in their natural diet, exhibited a certain degree of prey specialization and selected prey by size and taxonomic identity. Guilds (such as stalkers, ambushers and foliage runners) did not consistently predict either prey composition or predation selectivity of arboreal hunting spider species. From the economic standpoint, Ph. cespitum and Clubiona spp. were found to be the most effective natural enemies of apple pests, especially of aphids. Finally, the trophic niche width of C. xanthogramma and Ph. cespitum increased during ontogeny, resulting in a shift in their predation. These results demonstrate how specific generalist predators can differ from each other in aspects of their predation ecology even within a relatively narrow taxonomic group.

Urbanization can affect arthropod abundance in different ways. While species with narrow habitat range and low dispersal ability often respond negatively to urban environments, many habitat generalist species with good dispersal ability reach high densities in city centers. This filtering effect of urban habitats can strongly influence predator-prey-mutualist interactions and may therefore affect the abundance of predatory and phytophagous species both directly and indirectly. Here, we assessed the effect of urbanization on aphids, predatory arthropods, and ants on field maple (Acer campestre) trees in and around the city of Budapest, Hungary. We used the percentage of impervious surfaces within a 500 m radius of each site as an index of the degree of urbanization. We found that the abundance of aphids increased with increasing level of urbanization. However, abundance of predatory arthropods and occurrence of poorly dispersing species within the predator community were negatively related to urbanization, and we identified these two independent factors as significant predictors of aphid abundances. The abundance of ants decreased with urbanization, and contrary to our expectations, did not affect the abundance pattern of aphids. Our results suggest that urbanization, by altering the abundance and composition of predator communities, can disrupt biological control of aphid populations, and thus may contribute to the aphid outbreaks on urban trees.

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