The hump-shaped effect of plant functional diversity on the biological control of a multi-species pest community

Abstract

Plant taxonomic and functional diversity promotes interactions at higher trophic levels, but the contribution of functional diversity effects to multitrophic interactions and ecosystem functioning remains unclear. We investigated this relationship in a factorial field experiment comparing the effect of contrasting plant communities on parasitism rates in five herbivore species. We used a mechanistic trait-matching approach between plant and parasitoids to determine the amount of nectar available and accessible to parasitoids. This trait-matching approach best explained the rates of parasitism of each herbivorous species, confirming the predominant role of mass-ratio effects. We found evidence for an effect of functional diversity only in analyses considering the ability of plant communities to support the parasitism of all herbivores simultaneously. Multi-species parasitism was maximal at intermediate levels of functional diversity. Plant specific richness had a negligible influence relative to functional metrics. Plant communities providing large amounts of accessible nectar and with intermediate levels of functional diversity were found to be the most likely to enhance the conservation biological control of diverse crop herbivores.

Flower strips, crop management and landscape composition effects on two aphid species and their natural enemies in faba bean

“…Table S3: Morphological measurements of the head and mouthpart size of the Syrphidae, retrieved from the literature (Gilbert, 1985a;Gilbert, 1985b). A decision tree (Gardarin et al, 2021) was built to take into account the three constraints limiting nectar accessibility: (1) ability of the insect to penetrate the flower, which is dependent on head size and flower opening, (2) ability to reach the nectar, which depends on proboscis length, nectar depth and corolla height, and (3) proboscis width and nectar tube diameter in the presence of nectar (Figure S2). Here, we hypothesized that there was no constraint related to the proboscis width (width of the labral sucking tube) with regards to nectar tube diameter, and that sucking was always possible, even in narrow tubes (Gilbert, 1981).…”

“…We determined the accessibility of floral nectar, with a mechanistic trait-based approach, by adapting the geometric model proposed by van Rijn et Wäckers (2016). Flower traits related to nectar availability (presence of floral and extrafloral nectar) and accessibility (flower opening diameter, corolla height, nectar depth and nectar tube diameter) were measured in a previous study (Gardarin, Pigot, et Valantin-Morison 2021). To determine the access of aphidophagous Syrphidae to the nectar provided by plants, we retrieved from the literature the size of their mouthparts and head, which limit corolla penetration.…”

“…Figure S2. Decision tree fromGardarin et al (2021) used to determine the accessibility of nectar to insects as a function of insect traits (radius of insect head r, proboscis length x and proboscis width z) and flower traits (width of flower opening w, corolla height h, nectar depth p, nectar tube diameter d).…”

Within-field floral resources have the potential to increase parasitism rates in winter oilseed rape pests more than resources at field margins

“…The parasitoids of stem flea beetles are active from February to March (Ulber et al, 2010), at least one month before those of pollen beetles, when nectar resources may be even more limited than later in the spring, due to the very small number of plants flowering during this period. In a recent field experiment, perennial wildflower strips increased the parasitism of P. chrysocephala, B. aeneus and of Ceutorhynchus pallidactylus, the magnitude of this effect depending on the amount of nectar available and accessible to parasitoids (Gardarin et al, 2021). However, the importance of the nectar resources provided by wildflower strips covering a small area at the edge of the field relative to those provided by weeds over the whole field area have not been investigated.…”

Initial assemblage characteristics determine the functional dynamics of flower‐strip plant communities

“…In particular, they support populations of pest predators, parasitoids, and pollinators, thereby contributing to conservation biological control by reducing pest abundance (Albrecht et al, 2020 ) and to entomophilous pollination. Their effects, particularly those on high trophic levels, are mediated by plant species composition and diversity (Gardarin et al, 2021 ; Haaland et al, 2011 ). Plant communities with high levels of taxonomic and functional diversity enhance diversity and multifunctionality across trophic levels (Lefcheck et al, 2015 ).…”