Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 232

FIGURE 16.6 Wild mustard (Brassica campestris) forming a barrier around “islands” of cauliflower. The mustard can attract
beneficial insects and retard the movement of herbivorous insect pests to the crop.

adapted flowering plants, including fennel ( Fo e n i c u l u m
vulgare ), yarrow ( Achillea millefolium ), daisy fleabane
( Erigeron annuus ), and butterfly bush ( Buddleia spp.).
Most of these plants were nonnative but not particularly
weedy (an exception is fennel; care should be taken in
using it for such corridors).
Various sampling methods allowed the researchers to
observe the following patterns:

In the other block, these generalist predators
were more evenly distributed.

• The rate of parasitization of leafhopper eggs by

Anagrus epos wasps was roughly the same
throughout both vineyard blocks.

These results showed that the positive effect of the
adjacent riparian forest on the biodiversity of beneficials
was — with the exception of A. epos — amplified by the
flowering plant corridor in block A. For ladybugs and
lacewings, the corridor provided food in the form of aphids
and other homoptera; for hoverflies it supplied nectar and
pollen; for predatory insects such as minute pirate bugs it
offered neutral insect prey. By providing these food
resources, the corridor allowed beneficials to move more
deeply into the vineyard. In island biogeography terms, the
corridor effectively reduced the size of the monoculture
“islands,” facilitating their “colonization” by beneficials.
In addition to demonstrating the applications of island
biogeography theory and the value of diversity, this study
highlights the importance of looking at diversity and eco-
logical processes at the scale of the landscape. Agricul-
tural practices that allow, create, or retain a more diverse
agricultural landscape that includes remnants of natural
vegetation and noncrop areas are to be encouraged for a
variety of reasons, a concept we will explore in more detail
in Chapter 22 (Table 16.7).

• The corridor supported a healthy diversity of

arthropod predators including green lacewings,
minute pirate bugs, big-eyed bugs, damsel bugs,
and several species of hoverflies, ladybugs,
tumbling flower beetles, and spiders.

• Diversity of generalist predators overall was

higher in the vineyard block with the plant cor-
ridor.

• In the vineyard block with the corridor, the

numbers of the two major grape herbivores
present (western grape leafhoppers and western
flower thrips) were lowest near the plant corri-
dor and highest in the central areas. In the other
vineyard block, these herbivores were distrib-
uted evenly throughout the block.

• Most generalist predators showed a density gra-

dient in the block with the corridor, reaching
their greatest numbers near the plant corridor.