320 Ecological Restoration and Design
that could then attack natural enemies of pests (e.g. Stephens et al, 1998). Despite
such potential problems, there are many successful instances of biodiversity being
used in agroecosystems to favour natural enemies, suppress pests and, in some cases
at least, reduce crop damage (Barbosa, 1998; Gurr et al, 2000; Landis et al, 2000).
The focus of this review is the use of biodiversity to enhance pest management.
We first present a concise overview of the ways in which this may be achieved. We
then build on this to argue that there is a hierarchy of broader benefits to agricul-
ture and society of enhancing biodiversity on farms. Full recognition of such mul-
ti-function agricultural biodiversity can serve only to encourage appropriate
societal incentive schemes and consequent adoption by farmers.
Enhancing Biodiversity for Biological Control
The degree of complexity involved in increasing biodiversity to enhance pest man-
agement ranges from merely diversifying plant age structure within a monoculture
to the opposite extreme of landscape level diversification including non-crop and
perennial vegetation (Table 17.1).
Often, attempts to diversify agroecosystems remove or reduce the negative
aspects of the features that typify modern, industrialized farming. These features
include the removal or degradation of non-crop habitats such as hedgerows, wood-
land and riparian vegetation; use of monocultures; almost total weed removal from
within and around crops; large field sizes; and tillage operations of varying degrees
of intensity (see Jepson, 1989 and Dent, 1995 for more details).
Diversification within a monoculture
Monocultures dominate modern, industrialized agriculture. Farmers tend to be
risk-averse (Norton, 1976, 1993). This has led to some attempts to enhance pest
management by making only subtle changes to normal management. In a recent
example, strip-cutting of lucerne (Medicago sativa L.) was tested as an alternative
to the normal practice of harvesting entire fields at a time (Hossain et al, 2001). In
this system, natural enemies migrated from harvested strips into adjacent, un-
harvested ones. When these refuges were cut some weeks later, natural enemies
moved into the regrowing strips. Alternating harvests over the course of the hay-
making season (spring–autumn) preserved within-field habitats favourable to nat-
ural enemies and contributed to the suppression of pests (Helicoverpa spp.:
Lepidoptera).
‘Relaxing’ the monoculture
A slightly more complex form of diversification involves growing two or more
varieties or closely related species of crop. This approach is well-known to plant