Multi-Function Agricultural Biodiversity 323the field boundary. Many important species are non-aerial so would otherwise be
slow to colonize the entire area of the large fields that are now so common. Beetle
banks generate high densities of polyphagous predators. Modelling and exclusion
work has shown that such natural enemies can reduce aphid densities in cereals
(Chiverton, 1986; Winder et al, 1988; Winder, 1990).
Field margin non-crop vegetation
An extension of this use of non-crop vegetation is its placement adjacent to the
crop within the field margin. Most commonly, nectar- or pollen-rich plants are
used with the former benefiting parasitoids (e.g. Baggen and Gurr, 1998) and the
latter benefiting hoverflies (e.g. Hickman and Wratten, 1996). Importantly, how-
ever, tall boundary vegetation, such as trees, may impede hoverfly dispersal into
nearby areas of crop (Wratten et al, in press). Thus, habitat structure may constrain
the spatial extent of the benefits from adding floral resources to existing bounda-
ries.
Abandoning monoculture
Greater levels of complexity in diversification may be seen when monoculture is
abandoned and intercropping and similar practices, in which one or more addi-
tional crop species are grown within the field, are used. This may take a variety of
forms ranging in complexity from the simple inclusion of a discrete area of a sec-
ondary crop to complex spatial or temporal patterns of polycultures.
An example of a simple departure from monoculture is the use of a lucerne
strip within Australian cotton crops. The effects of this were two-fold. First, the
lucerne is ‘preferred’ over cotton by the green crop mirid (Creontiades dilutus Stal)
so it acts as a decoy or trap crop. Second, lucerne provides a habitat suitable for a
diverse suite of natural enemies and movement of these into the cotton crop dur-
ing a pest outbreak can be achieved by mowing the lucerne or applying an attrac-
tive food spray to the cotton (Mensah and Kahn, 1997).
Dramatic trap-crop results have been obtained by drilling white or black mus-
tard (Sinapis alba (L.) and Pisum sativum (L.)) in the outer few metres of sweet
corn (Zea mays L.) fields in New Zealand (Rea et al, in press). The green vegetable
bug (Nezara viridula L.) normally invades the crop from surrounding vegetation.
In this work, the bugs remained in the mustard, feeding on its developing pods,
and this allowed the sweet corn to reach harvest stage with virtually no damage.
Another approach to conserve and enhance diversity of agroecosystems involves
the use of conservation tillage. Cover crops or existing vegetation can be sup-
pressed with herbicides and crops direct-seeded into the residue. This form of relay
intercropping has increased diversity of generalist predators (Clark et al, 1993).
The use of legume cover crops as living mulches in strip tillage or relay inter-
planted systems is another approach that increases biodiversity in agricultural eco-
systems. Dempster and Coaker (1974) found that the maintenance of clover cover