Use of sown wildflower strips to enhance natural enemies of agricultural pests

Summary

Intensive agriculture and excessive use of agrochemicals have resulted in an impoverished wildlife in agricultural landscapes, especially in arable landscapes but also in perennial high input crops. The elimination of semi-natural habitats, simplification of crop rotations as well as high input of fertilisers and pesticides is considered to be responsible for the severe decline of biological diversity that has been observed (e.g. Aebischer 1991). These practices can reduce habitat quality and remove the necessary habitat structure that is important to many natural enemies.
Moreover, agricultural landscapes are increasingly being simplified; with natural and semi natural areas fragmented and replaced altogether by large monocultural fields. As a consequence, most of the natural enemies depending on such semi-natural habitats for overwintering (Sotherton 1985; Lys and Nentwig 1994; Pfiffner and Luka 2000) need to disperse further to reach summer feeding habitats like agricultural crops. Fragmentation and loss of suitable habitats has caused natural enemies to decline in species diversity and abundance, and has even resulted in extinctions (Fahrig 1997) and loss of biological control functions (Kruess and Tscharntke 1994). Such landscape-scale aspects in biological control are explored in more detail in chapter 4.
Nowadays, a desirable goal in agricultural landscapes is the enhancement of biotic diversity through the use of sustainable farming methods and the conservation and reestablishment of non-crop habitats. Agri-environmental programs have been established in several European countries (e.g. rural development and set-aside programs). Since 1993, the Swiss government has subsidised low-input and sustainable-farming methods (e.g. low-input integrated crop management, organic farming) and the establishment of non-crop habitats. Farmers are encouraged to increase the amount of these non-crop habitats including low-input habitats in order to reverse the observed decline of farmland fauna and to conserve or improve the functions and services of the agroecosystems.
It has been demonstrated that habitat manipulation of the environment can enhance the survival of natural enemies and thereby improve their efficiency as pest-control agents (Gurr et al. 1998; Landis et al. 2000, Nicholls and Altieri, ch. 3 this volume). Field margins are an important type of habitat that provides refuge and resources for many arthropods. Thus field margins play a key role in maintaining biological diversity on farmland (Fry 1994). In addition, it may be useful to combine these semi-natural habitats with low-input agriculture to enhance effects on fauna diversity and natural pest control (Pfiffner 2000; Pfiffner and Luka 2003).
Some habitat manipulation options are known to improve pest control in adjacent production systems. These include grassy beetle banks (Thomas and Marshall 1999; Collins et al. 2002); weedy strips; set-aside strips and field margins (see overview Marshall and Moonen 2002). This chapter focuses on the use of sown wildflower strips (a synonymous term for weed strips) which may be located in field margins. These are used to augment natural enemies and to increase the diversity of flora and fauna in annual and perennial cropping systems.
One of the first initiatives to implement strip farming with sown wildflower strips in practice was taken by Nentwig (1989). Since 1993 this type of field margin has been encouraged and subsidised in the Swiss agri-environmental programs. Nowadays, more than 3000 ha of wildflower strips (in general with a width of 3–10 m) exist on farms all over Switzerland (Anonymous 2003). This chapter will detail how these strips are composed and how they can be established and managed in practice. Finally, their effects on natural enemies and pest control in annual and perennial systems are discussed.