324 Ecological Restoration and Design
between rows of brassica crops reduced populations of three insect pests (Brevico-
ryne brassicae L., Artogeia rapae (L.) and Erioischia brassicae Bouché). In an experi-
ment involving several living mulches interseeded into cabbage, Andow et al
(1986) showed that living mulches reduced populations of the flea beetle (Phyl-
lotreta cruciferae Goeze) and B. brassicae compared with monoculture plantings of
cabbage. In Hawaii, Hooks and Johnson (2001) interplanted broccoli (Brassica
oleracea L.) with chilli pepper (Capsicum annuum L.) or yellow sweetclover (Melilo-
tus officinalis L.). The latter treatment led to fewer Lepidoptera larvae in the broc-
coli heads compared with the sweet pepper or control treatments and the broccoli
heads did not differ in size. Undersowing with legumes such as clover to improve
soil fertility, is a traditional practice. Its potential for maintaining invertebrate and
bird biodiversity has only more recently been realized (Firbank et al, 1996).
Diversification beyond the field scale
At a greater level of complexity, changes may be made that apply beyond the field
boundary. The work on the effects of adjacent vegetation on rape pollen beetle
from which the opening quotation was taken provides an example (Thies and
Tscharntke, 1999). Oilseed rape crops with simple and structurally poor adjacent
vegetation were more severely damaged by this pest and it suffered lower rates of
parasitism than those observed in crops adjacent to ‘complex, structurally rich ...
large old fallows’.
At a still larger spatial scale, potentially extending beyond the farm boundary,
features such as areas of woodland and hedgerow can have a long-range effect on
rates of parasitism of crop pests. In a study that compared 139 fields in a 3.2km^2
area of ‘complex’ landscape with 61 fields in a similar area of ‘simple’ landscape,
the lepidopteran armyworm, Pseudaletia unipunctata Haworth, was parasitized at
a statistically higher rate in the crops located in the former (Marino and Landis,
1996).
Benefits of Biodiversity – a Hierarchy of Scales
As is clear from the foregoing examples, enhanced agricultural biodiversity can favour
pest management via enhanced biological control or direct (resource concentration
hypothesis) effects on pests. However, benefits beyond this ‘pathosystem’ scale are
also possible and these may be viewed in a hierarchical fashion (Figure 17.2).
Clearly, the effects of biodiversity on pests and their natural enemies have the
potential to benefit the crop. However, crop production may also benefit in other
ways. These effects may extend both spatially to adjacent crops and temporally to
subsequent crops, so increasing the sustainability of the farming system. Benefits
may also influence other, non-crop habitats both on-farm and more extensively,
so lessening the environmental impact of farming and bringing other benefits to