Abstract
ContextTritrophic interactions may be affected by local factors and the broader landscape context. At small spatial scales, carnivorous enemies of herbivorous insects use herbivore-induced plant volatiles (HIPVs) to find herbivores, but it is unknown whether variation in plant attractiveness due to differential HIPV emission can enhance recruitment of carnivores from the wider landscape.ObjectivesWe studied whether parasitism of caterpillars of the butterfly Pieris brassicae on white cabbage was influenced by landscape composition and cover with brassicaceous species that provide hosts for parasitoids of P. brassicae in 19 landscapes in the Netherlands. We also investigated whether differential attraction of parasitoids to herbivore-infested plants affected parasitism of P. brassicae caterpillars by using different accessions as proxies for HIPV emission.ResultsThe cabbage accession that is highly attractive to parasitic wasps recruited more wasps than a less attractive accession, but only when parasitism rates were high. Parasitism rate as proxy of wasp recruitment correlated positively with the cover of brassicaceous plants and area of arable land, suggesting that these habitats support hosts for the wasps and their population growth. In contrast, forest area was negatively associated with parasitism rates.ConclusionsOur study shows that the degree of attractiveness of plants to parasitoids, which is probably mediated by HIPVs, can be a useful predictor of parasitism, but needs to be considered within the landscape context. To understand the strength of tritrophic interactions it is crucial to consider local-scale processes driven by plant-trait variation in combination with landscape-scale processes that determine carnivore abundance.
Highlights
Species abundance and trophic interactions can be influenced by processes that extend beyond the local scale of a single patch or habitat (Thies et al 2003; Tscharntke and Brandl 2004)
The cabbage accession that is highly attractive to parasitic wasps recruited more wasps than a less attractive accession, but only when parasitism rates were high
Forest area was negatively associated with parasitism rates
Summary
Species abundance and trophic interactions can be influenced by processes that extend beyond the local scale of a single patch or habitat (Thies et al 2003; Tscharntke and Brandl 2004). There is a mature body of studies that focus on host-parasitoid dynamics in agricultural landscapes (Cronin and Reeve 2005; Chaplin-Kramer et al 2011; Karp et al 2018; Dainese et al 2019). Land use classes may not be a meaningful predictor of parasitoid abundance when they do not capture the function of the habitats, for instance in terms of resource distribution for parasitoids and their hosts (Tscharntke et al 2016; Karp et al 2018) In these cases a functional land cover approach may be advantageous and contribute to a more mechanistic classification of the habitats in terms of the life-support functions they provide (Fahrig et al 2010; Bianchi et al 2012). A metric capturing the abundance of wild plants that support host plants for the hosts of parasitoids might be a better predictor for parasitoid abundance than a general land use class such as ‘‘non-crop habitat’’ that may comprise a wide range of habitats with varying suitability for the parasitoid species
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