Geoff M. Gurr, Robert D. Holt and Douglas A. Landis provided valuable comments on an earlier draft of this chapter. We are grateful to Christoph Bürger, Doreen Gabriel, Indra Roschewitz and Ulrich Thewes for help in the field and processing of landscape data. The work greatly profited from collaboration with Jens Dauber, Tobias Purtauf and Volkmar Wolters within the BIOPLEX project (Biodiversity and spatial complexity in agricultural landscapes under global change), funded by the German Ministry for Research and Education (BMBF). Further support came from the German Science Foundation (Deutsche Forschungsgemeinschaft) and from the German National Academic Foundation (Studienstiftung des deutschen Volkes).
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Introduction
Simply increasing diversity per se in agroecosystems does not guarantee a desirable outcome. It can prove to be a waste of effort, time and money if it does not lead to significantly improved pest control. Worse still, it can even exacerbate some pest problems (Andow and Risch 1985; Collins and Johnson 1985; Sheehan 1986; Baggen and Gurr 1998; Gurr et al. 1998, 2004). Therefore, biological control workers ought ideally to concentrate on manipulating the specific elements of diversity that are most likely to have a positive influence on the survival and repro- duction, and thus the impact, of natural enemies (Landis et al. 2000). Fundamental research into the specific non-pest resource requirements (supplementary foods, prey and hosts, physical refugia) of natural enemies is the key to this. Although research of this kind may be difficult, costly and time-consuming, it should provide information that will at best increase the chances of success of a habitat manipulation program, and will at the very least provide useful insights into natural enemy biology. This chapter considers the approaches available to researchers for improving our understanding of the effects of habitat manipulation on pest–natural enemy interactions. The review is organised as follows:
• the application of selection criteria to insect natural enemies in manipulation programs; • ways of identifying and comparing the limiting resources of natural enemies by using
behavioural, morphological and anatomical studies. Discussion will mainly focus on whether natural enemies demonstrate a need for supplemental food, but we also consider the requirements of natural enemies for alternative hosts/prey and refugia; • how manipulation of such resources might alter the natural enemy’s per capita searching
efficiency;
• how habitat manipulation might alter the abundance of natural enemies by attracting parasitoids and predators to the crop and retaining them there, and by improving natural enemy growth, development, survival and fecundity.
Such work requires knowledge, often of a highly detailed nature, of the life history of the natural enemy. As noted by Gurr et al. (2004), empirically derived life-history information draws from, and also contributes to, ecological theory. It can also be used in parameterising both prospective (pre-manipulation) and retrospective (post-manipulation) biological control models (e.g. see Waage 1990; Kidd and Jervis 2004).