Semi-tilling maintains the arthropod food web structure but decreases biological pest control in olive groves

Abstract

Semi-tilling is an extended practice in olive groves aiming to manage the water competition between ground cover and olive trees while maintaining the soil structure in the crop. However, the effect of semi-tilling on the arthropod community and biological pest control is still uncertain. This study assesses the impact of semi-tilling on the arthropod community and biological pest control in olive groves. It was approached by analysing the arthropod food web composition and structure as well as the arthropod distribution among the three strata of the crop (tree canopy, ground cover and epigeal soil). Stable isotope (δN15 and δC13) analysis was used to predict the trophic position of arthropods in the food web and to establish trophic links between predators and prey. The food web structure was measured by estimating the unweighted and node-weighted quantitative descriptors in both cases (mowing and semi-tilling). In addition, we evaluated the effect of ground cover management on the abundance of arthropods in the three strata of the crop. Results showed that although the structure of the food web is maintained in semi-tilled crops in summer, mowing enhances the predation pressure on the olive grove pests Euphyllura olivina (Hemiptera: Psyllidae) and Prays oleae (Lepidoptera: Praydidae). Thus, we observed a lower abundance of pests potentially related to the migration of predators to the olive tree canopy facilitated by mowing. Furthermore, isotopic distances showed that Anthocoris nemoralis from the tree and the ground cover (Hemiptera: Anthocoridae) preys on E. olivina in the mowed olive grove, while in the semi-tilled olive grove, this trophic link disappears in the ground cover. The overall results showed that semi-tillage during late spring–early summer negatively affects arthropod biological pest control in organic olive groves.