Abstract

Understanding the full diet of natural enemies is necessary for evaluating their role as biocontrol agents, because many enemy species do not only feed on pests but also on other natural enemies. Such intraguild predation can compromise pest control if the consumed enemies are actually better for pest control than their predators. In this study, we used gut metabarcoding to quantify diets of all common arachnid species in Swedish and Spanish apple orchards. For this purpose, we designed new primers that reduce amplification of arachnid predators while retaining high amplification of all prey groups. Results suggest that most arachnids consume a large range of putative pest species on apple but also a high proportion of other natural enemies, where the latter constitute almost a third of all prey sequences. Intraguild predation also varied between regions, with a larger content of heteropteran bugs in arachnid guts from Spanish orchards, but not between orchard types. There was also a tendency for cursorial spiders to have more intraguild prey in the gut than web spiders. Two groups that may be overlooked as important biocontrol agents in apple orchards seem to be theridiid web spiders and opilionids, where the latter had several small-bodied pest species in the gut. These results thus provide important guidance for what arachnid groups should be targets of management actions, even though additional information is needed to quantify all direct and indirect interactions occurring in the complex arthropod food webs in fruit orchards.

Highlights

  • The reality is that we often lack information on the efficiency or even diet of natural enemies in agricultural fields, what species significantly feed on pests and whether negative side effects through indirect interactions in food webs, such as intraguild (IG) predation on other natural enemies, outweigh positive effects from the consumption of pest species (Garcia, Olimpi, Karp, & Gonthier, 2020; Grass, Lehmann, Thies, & Tscharntke, 2017; Martin, Reineking, Seo, & Steffan-Dewenter, 2013; Rosenheim, Kaya, Ehler, Marois, & Jaffee, 1995; Straub, Finke, & Snyder, 2008)

  • In Spain, sampled predator groups were dominated by web spiders: Theridiidae and Araneidae, followed by cursorial spiders: Thomisidae, Salticidae, Philodromus and Opiliones

  • The age structure of Opiliones and Philodromus differed between regions, with adult Opiliones (65%) and juvenile Philodromus (96%) dominating in Swedish samples and juvenile Opiliones (100%) and adult Philodromus (88%) dominating in Spanish samples

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Summary

Introduction

Hamb€ack et al / Basic and Applied Ecology 57 (2021) 1À13 crops This focus on non-chemical means to reduce pest damage is well motivated, both because negative side effects from chemical pesticides are common and because of the high loss rates in many crops caused by pest damage if unchecked (Cross, Fountain, Marko, & Nagy, 2015; Oerke, 2006; Pimentel, 2005; Sharma et al, 2020). Spiders may avoid attacking other spiders just because they pose a risk for the attacker (Polis, Myers, & Holt, 1989) For these reasons, we analyzed predator diets from the perspective of prey taxonomy separately from the prey functional role

Materials and methods
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