Landscape‐scale interactions of spatial and temporal cropland heterogeneity drive biological control of cereal aphids

Abstract

1. Agricultural landscapes are characterised by dynamic crop mosaics changing in composition and configuration over space and time. Although semi-natural habitat has been often shown to contribute to pest biological control, the effects of increasing landscape heterogeneity with cropland have been disregarded. Here, we examine how cereal aphids, their enemies and biological control are affected by the composition and configuration of the crop mosaic and its inter-annual change due to crop rotation. 2. We studied the abundance of cereal aphids, natural enemies and aphid parasitism over 2 years on 51 winter wheat fields. Arthropods were monitored at three distances (0, 10, 30 m) from field borders. Fields were embedded in landscapes of 1-km diameter selected along orthogonal gradients of compositional crop heterogeneity (crop diversity), configurational heterogeneity (field border and grassy field boundary length) and inter-annual change in cover of aphid host habitats (cereal, maize and grassland). We aimed to disentangle spatial and temporal heterogeneity effects through these independent landscape gradients. 3. Aphid densities were lower in landscapes with smaller field size (more field borders) coupled with high amounts of grassy field boundaries. Aphid densities decreased also in landscapes with higher crop diversity when the cover of aphid host habitat had decreased from the year before. Aphid natural enemy densities decreased with smaller field size and high amounts of grassy field boundaries. Aphid parasitism decreased with the inter-annual expansion in aphid host habitat, but only in landscapes with small field sizes. 4. Synthesis and applications. Our study shows for the first time that cereal aphid numbers can be reduced by optimising the composition, configuration and temporal heterogeneity of the crop mosaic. We highlight the value of maintaining small field sizes in agricultural landscapes and high densities of grassy boundaries for reducing aphid abundance. Landscape-wide crop diversification can reduce aphid densities as well.

Landscape‐level crop diversity benefits biological pest control

“…To our knowledge, scale effects on overall biological control using comparable crop diversity indices and exclusion experiments have not previously been investigated. However, two recent studies report reduced aphid densities (1,000 m scale; Bosem Baillod et al., ) and increased levels of predation by epigeal predators (750 m scale; Holland et al., ) with crop diversification in similar cereal systems in Germany and the UK respectively. These effects occurred on slightly larger scales than in our system.…”

“…Yet, scale‐specific differences in the response to landscape variables are common and depend on study region and system. For example, non‐crop landscape complexity may influence overall or taxon‐specific pest suppression and predator densities either at small (<500 m; Holland et al., ; Tamburini et al., ), intermediate (>500 m–2,000 m; Bosem Baillod et al., ; Chaplin‐Kramer et al., ; Gardiner et al., ; Rusch et al., ) or large‐spatial scales (>2,000 m; Chaplin‐Kramer et al., ; Gardiner et al., ; Woltz, Isaacs, & Landis, ). Due to their specialist diet and greater dispersal limitation, specialist parasitoids may be less responsive to resource diversification than generalist predators and influenced more locally (Chaplin‐Kramer et al., ).…”

“…Perennial non‐crop structures are important sources of natural enemies in agroecosystems (Bosem Baillod et al., ; Tscharntke et al., ), and therefore have the potential to enhance pest regulation (Rusch et al., ). Yet a growing evidence base suggests that natural habitat is not a panacea to improve biological control (Martin et al., ; Tscharntke et al., ).…”

“…Fahrig et al., ; Palmu, Ekroos, Hanson, Smith, & Hedlund, ). Even fewer do so for the biological control service these organisms provide (Bosem Baillod et al., ; Chaplin‐Kramer & Kremen, ; Holland et al., ), although the mostly positive relationship between enemy diversity and biological control (Altieri, ; Harrison et al., ) can be disrupted by antagonistic species interactions (Letourneau, Jedlicka, Bothwell, & Moreno, ; Straub, Finke, & Snyder, ).…”

Configurational crop heterogeneity increases within‐field plant diversity

“…The few attempts to tease apart the effects of crop heterogeneity components on species diversity have found positive effects of compositional crop heterogeneity, that is, Shannon crop diversity and/or configurational crop heterogeneity, that is, edge density or mean field size, on predatory arthropods (Bertrand, Burel, & Baudry, ; Bosem Baillod, Tscharntke, Clough, & Batáry, ; Fahrig et al, ; Martin et al, ; Palmu, Ekroos, Hanson, Smith, & Hedlund, ), butterflies (Perović et al, ) and wild bees (Hass et al, ). Studies addressing this issue for the diversity of plants within agricultural fields are rarer (but see Fahrig et al, ).…”