A comparison of naturally growing vegetation vs. border-planted companion plants for sustaining parasitoids in pomegranate orchards

Diversification of vegetation within and around agricultural habitats is an effective strategy to support populations of natural enemies of crops' pests. Such diversification can be achieved by conservation of natural vegetation that develops spontaneously around the plots, as well as by active introduction of companion plants to the crop. In this study we compared these two approaches in pomegranate orchards in Mediterranean climate. First, we evaluated ten candidate companion plant species for their potential to attract parasitoids of pomegranate pests. We then planted a combination of the two leading species − celery and Syrian oregano − along the perimeter of five orchards. In five additional, paired orchards, no plants were added. Arthropods were sampled from added and naturally growing companion plants throughout the pomegranate fruit growth season. Parasitoids were the most common natural enemies in our samples, and their overall abundance was similar in both treatments. Pest levels did not differ between treatments either. However, the distribution of some parasitoids (Neochrysocharis and Telenomus) and pests (leafhoppers and dipteran leafminers) within the orchards was affected by the margin vegetation type: these insects were more abundant in the margins than in the centers of the orchards with companion plants (suggesting a role as trap plants), whereas the opposite was observed in orchards with natural vegetation. We conclude that introduction of companion plants and conservation of local natural vegetation were equally effective in sustaining parasitoid numbers and diversity, but that planting attracted some parasitoids away from the orchards towards their margins. This possibly provides these natural enemies with a refuge from agricultural disturbances, but might reduce their contribution to pest control.