Functional diversity of predators and parasitoids does not explain aphid biocontrol efficiency

Abstract

Many studies demonstrate an important role of natural enemy biodiversity in the regulation of agricultural pests, but the role of different aspects of biodiversity in influencing this crucial ecosystem service remain controversial. We hypothesised that the functional diversity generated by combining divergent consumer groups (roaming coccinellid predators and parasitoid wasps) fosters complementarity, enhancing aphid biocontrol. We tested this using experimental mesocosms containing plants, aphids and natural enemies located in a greenhouse. We compared the aphid control efficiency (final aphid abundance) of low functional diversity treatments (two parasitoid species, or two predator species) with high functional diversity treatments (all four possible predator-parasitoid combinations). We also included all four enemies as single species treatments to allow calculation of the non-additive effects of combining natural enemies. Results showed that biocontrol (final aphid abundance) was driven by the species identity of natural enemies and positive non-additive effects in two treatments in which the most efficient predator species was combined with a parasitoid species and the other predator species, respectively. Functional diversity did not consistently influence biocontrol or non-additive effects. In conclusion, functional diversity, as defined by differences between roaming predator and parasitoid functional groups, failed to consistently explain biocontrol efficiency in our study. This calls for consideration of finer-scale functional traits and how they govern natural enemy interactions and cascading effects across ecosystems.

The effects of pollinator diversity on pollination function

“…Failure to account for saturating function could help to explain-at least in part-why BEF relationships other than the sampling effect are sometimes not detected in experimental studies of predation and animal seed dispersal (Alhadidi et al, 2019;Tim oteo et al, 2016), potentially contributing to the mixed evidence for BEF relationships in these fields. Diversity-function relationships in single-trophic-level ecosystem functions also saturate, but it can be unclear whether these limits are extrinsic to the community and due to resources being exhausted (analogous to fertilization of all accessible ovules in pollinators) or intrinsic to a community's ability to sequester more of the available resources (analogous to saturation of a pollinator's functional niche).…”

The combined foraging of a parasitoid and a predatory bug enhances mortality of Plutella xylostella larvae

“…Such ecological processes are complex and may involve a great variety of coexisting species from different trophic levels, which interact directly and/or indirectly (Price et al, 1980; Rosenheim et al, 1995, 1999). Understanding the interactions of multiple natural enemies, and their influence on herbivore populations is crucial for both population ecology and IPM (Losey & Denno, 1998a; Alhadidi et al, 2019), as depending on the prevailing intraguild interactions, the simultaneous presence of natural enemies in a crop can have neutral, negative, or positive effects on prey suppression (Alhadidi et al, 2018; Culshaw‐Maurer et al, 2020). Some potentially negative interactions might be outweighed by positive interactions (Rocca & Messelink, 2016), and because a positive effect is a preferred outcome, it is important to understand how this may be achieved.…”

“…This may happen when they differ in their resource use, as this reduces competition for resources such as food or space (Symondson et al, 2002) and/or prevents prey escape (Losey & Denno, 1998c), and also when predator facilitation occurs (Losey & Denno, 1998a; Sih et al, 1998), i.e., the presence of one natural enemy facilitates the capture of the prey by other natural enemies (Rocca & Messelink, 2016; Greenop et al, 2018). This occurs because the presence of one predator drives the prey from the habitat avoiding being predated upon, becoming more exposed or vulnerable to the other natural enemy, which hunts the fleeing or displaced prey (Losey & Denno, 1998a,b; Nelson & Rosenheim, 2006; Paull et al, 2012; Alhadidi et al, 2019; Culshaw‐Maurer et al, 2020). For instance, Losey & Denno (1998a) observed that if the foliar‐foraging predator Coccinella septempunctata L. and the ground‐foraging predator Harpalus pennsylvanicus DeGeer, both natural enemies of A. pisum , were present in a system, the combined predation rate of both predators nearly doubled the sum of their individual predation rates, due to the aphid's ‘dropping’ behaviour elicited by C. septempunctata on the foliage, making aphids susceptible to predation by H. pennsylvanicus on the ground.…”

The positive association between natural vegetation, native coccinellids and functional diversity of aphidophagous coccinellid communities in alfalfa

“…Increasing evenness of natural enemies may also result in lower pest density and higher yield in some crops; although the dominance of a single highly effective predator could also be highly efficient for pest suppression (Crowder et al ., 2010, 2012; Alhadidi et al ., 2019; Snyder, 2019). Even when some studies have analysed the role of the diversity of natural enemy functional groups on biological control (e.g., Cardinale et al ., 2003; Alhadidi et al ., 2019), functional diversity of natural enemies has been scarcely analysed in agroecosystems (Crowder & Jabbour 2014; Wood et al ., 2015; but see Gagic et al ., 2015; Martínez‐Salinas et al ., 2016; Greenop et al ., 2018). Recently, it has been suggested that increasing functional diversity of predatory invertebrates would maximise pest control in agricultural ecosystems because of niche complementarity (Greenop et al ., 2018).…”

“…Cardinale et al ., 2003), with greater natural enemy species richness generally increasing pest suppression (Letourneau et al ., 2009; Snyder, 2019; Dainese et al ., 2019). Increasing evenness of natural enemies may also result in lower pest density and higher yield in some crops; although the dominance of a single highly effective predator could also be highly efficient for pest suppression (Crowder et al ., 2010, 2012; Alhadidi et al ., 2019; Snyder, 2019). Even when some studies have analysed the role of the diversity of natural enemy functional groups on biological control (e.g., Cardinale et al ., 2003; Alhadidi et al ., 2019), functional diversity of natural enemies has been scarcely analysed in agroecosystems (Crowder & Jabbour 2014; Wood et al ., 2015; but see Gagic et al ., 2015; Martínez‐Salinas et al ., 2016; Greenop et al ., 2018).…”