Contribution of Hedgerows to Biological Control
Of all the non-crop habitats that occur on farmland, hedgerows through their proximity to crops and their diverse yet relatively stable plant and structural complexity (Pollard et al. 1974) offer immense potential to support pest natural enemies. They also act as refugia from adverse agricultural practices (Dainese et al. 2017), thereby aiding recolonisation (Fahrig 1997) and the preservation of gene pools (Loreau et al. 2003). Moreover, they may also facilitate the movement of natural enemies because, in the regions where hedgerows occur, they are the dominant field boundary type, interlacing the landscape and connecting other types of seminatural habitats (Figure 7.1). However, hedgerows are often relatively narrow habitats, occupying a small proportion of the landscape in comparison to, for example, woodland or high-nature-value grassland that can also support natural enemies (Holland et al. 2016). They therefore have the propensity to influence levels of biological control within the adjacent crops and the wider agricultural landscape. Overall, hedgerows and similar semi-natural habitats can help mitigate the negative effects of agricultural intensification and enhance natural enemies (Bianchi et al. 2006; Chaplin-Kramer et al. 2011; Inclan et al. 2016; Morandin et al. 2016).
Hedges also have other agronomic value through the provision of wind shelter and so are frequently planted around wind-sensitive crops, such as orchards. This offers the opportunity to select species that also have the potential to support natural enemies and not the pests (Rieux et al. 1999). There are some limitations, however, to their value. Existing hedgerows are fixed in the landscape, and their benefits are usually distance limited; therefore, only by planting new ones can greater coverage be achieved. Existing hedgerows are often quite old and planted originally to provide stock-proof barriers, fuel, or food, whilst new ones are typically chosen based upon historical species composition and regional considerations. As a consequence, there is little opportunity to manipulate the woody species composition of hedgerows for natural enemies, although the frequency and timing of cutting that determines flower abundance, structure, and physical dimensions can be adjusted.
This chapter will focus predominately on the role of hedges and their associated hedge bases rather than other field margin habitats by:
- considering the advantages and disadvantages of hedgerows in terms of delivering biological control,
- examining the evidence that hedgerows are contributing to biological control by supporting a diverse and abundant natural enemy community and whether this leads to subsequent enhancement of natural enemies within the adjacent fields and further afield across agricultural landscapes,
- reviewing the evidence that hedgerows can reduce pest levels, and
- considering the future of hedgerows and their management including identifying where further research is needed and how agricultural policy may be modified to enhance their role for biological control.